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2.9: Fossil evidence for the history of life on earth - Biology

2.9: Fossil evidence for the history of life on earth - Biology


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The earliest period in Earth’s history is known as the Hadean, after Hades, the Greek god of the dead. Hunters of fossils (paleontologists) do not search for fossils randomly but use geological information to identify outcroppings of sedimentary rocks of the specific age they are studying in order to direct their explorations.

geologists recognized that fossils of specific types were associated with rocks of specific ages. This correlation was so robust that rocks could be accurately dated based on the types of fossils they contained. At the same time, particularly in a world that contains young earth creationists who claim that Earth was formed less than ~10,000 years ago, it is worth remembering both the interconnectedness of the sciences and that geologists do not rely solely on fossils to date rocks. This is in part because many types of rocks do not contain fossils. The non-fossil approach to dating rocks is based on the physics of isotope stability and the chemistry of atomic interactions. It uses the radioactive decay of elements with isotopes with long half-lives, such as 235Ur (uranium) which decays into 207Pb (lead) with a half-life of ~704 million years and 238Ur which decays into 206 Pb with a half-life of ~4.47 billion years. Since these two Pb isotopes appear to be formed only through the decay of Ur, the ratios of Ur and Pb isotopes can be used to estimate the age of a rock, assuming that it originally contained Ur.

In order to use isotope abundance to accurately date rocks, it is critical that all of the atoms in a mineral measured stay there, that none are washed in or away. Since Ur and Pb have different chemical properties, this can be a problem in some types of minerals. That said, with care, and using rocks that contain chemically inert minerals, like zircons, this method can be used to measure the age of rocks to an accuracy of within ~1% or better. These and other types of evidence support James Hutton’s (1726-1797) famous dictum that Earth is ancient, with “no vestige of a beginning, no prospect of an end.”46 We know now, however, that this statement is not accurate; while very, very old, Earth had a beginning, it coalesced around ~5 billion years ago, and it will disappear when the sun expands and engulfs it in about ~5.5 billion years from now.47

Now, back to fossils. There are many types of fossils. Chemical fossils are molecules that, as far as we know, are naturally produced only through biological processes.48 Their presence in ancient rock implies that living organisms were present at the time the rock formed. Chemical fossils first appear in rocks that are between ~3.8 to ~3.5 x 109 years old. What makes chemical fossils problematic is that there may be non-biological but currently undiscovered or unrecognized mechanisms that could have produced them, so we have to be cautious in our conclusions.

Moving from the molecular to the physical, there are what are known as trace fossils. These can be subtle or obvious. Organisms can settle on mud or sand and make impressions. Burrowing and slithering animals make tunnels or disrupt surface layers. Leaves and immotile organisms can leave impressions. Walking animals can leave footprints in sand, mud, or ash. How does this occur? If the ground is covered, compressed, and converted to rock, these various types of impressions can become fossils. Later erosion can then reveal these fossils. For example, if you live near Morrison, Colorado, you can visit the rock outcrop known as Dinosaur Ridge and see trace fossil dinosaur footprints; there may be similar examples near where you live.

We can learn a lot from trace fossils, they can reveal the general shape of an organism or its ability to move or to move in a particular way. To move, an organism must have some kind of muscle or alternative mobility system and probably some kind of nervous system that can integrate information and produce coordinated movements. Movement also suggests that the organisms that made the trace had something like a head and a tail. Tunneling organisms are likely to have had a month to ingest sediment, much like today’s earthworms - they were predators, eating the microbe they found in mud.

In addition to trace fossils, there are also the type of fossils that most people think about, which are known as structural fossils, namely the mineralized remains of the hard parts of organisms such as teeth, scales, shells, or bones. As organisms developed hard parts, fossilization, particularly of organisms living in environments where they could be buried within sediment before being dismembered and destroyed by predators or microbes, became more likely.

Unfortunately for us (as scientists), many and perhaps most types of organisms leave no trace when they die, in part because they live in places where fossilization is rare or impossible. Animals that live in woodlands, for example, rarely leave fossils. The absence of fossils for a particular type of organisms does not imply that these types of organisms do not have a long history, rather it means that the conditions where they lived and died or their body structure is not conducive to fossilization. Many types of living organisms have no fossil record at all, even though, as we will see, there is molecular evidence that they arose tens to hundreds of millions of years ago.


2.9: Fossil evidence for the history of life on earth - Biology

ORIGIN OF LIFE AND THE FOSSIL RECORD

A brief review of salient points regarding the origin of life:

Cosmic calendar: Earth formed 4.6 billion years ago there has been a long time for life to evolve. It took about a billion years to get through the early stages of chemical evolution such that there is some form of self-replicating system (e.g., a primitive living thing in its simplest definition). Miller experiments lead to formation of amino acids under lab conditions simulating a primitive earth atmosphere. Subsequent reactions could produce short polymers of the amino acids. When polymers are heated to 130°C to 180°C and then cooled in water to 25°C - 0°C proteinoid microspheres form. These provide evidence that simple cells could have formed from some of the earliest compounds.

Progress has also been made on the synthesis of nucleic acids. One significant bit of evidence, much further down the line, was the discovery of catalytic RNAs that performed enzyme like functions. This, and other evidence, suggested that RNA may be ancestral and DNA is a derived molecule for the storage of genetic material.

By 3.2 billion years ago, first procaryotes (Bacteria, blue green algae). By 2.5 - 2.0 billion years ago, communities of procaryotes emerge. e.g. Stromatolites as colonies of Blue green algae , formed biosedimentary domes of calcium carbonate = some of the earliest fossils. Photosynthetic bacteria have significant effect on the earth's atmosphere and the subsequent evolution of life. Blue green algae are photosynthetic and produce oxygen as a waste product. This was initially a poisonous molecule (as environment was an anoxic one) Lead to the production of an oxidizing atmosphere.

Large amounts of Oxygen oxidize the vast quantities of dissolved iron in the oceans: i.e., the oceans "rust." This counteracts the poisonous atmosphere problem, but only until the reservoir of iron is depleted and the iron settles out as the banded ironstone formation = layers of iron which form iron ore deposits. Ultimately, with the absence of iron to oxidize, the oxygen builds in the atmosphere and produces an ozone layer. This is a singular event which eukaryotes will ultimately take advantage of in the form of oxidative respiration. Subsequent cellular (at this time = organismal) evolution is contingent on this singular event. If we started earth over again, would this event re-occur? at the same time?, if not would we have evolved.

1.5 Billion years ago, a diverse flora of Eukaryotes present as asexual species. 1.4 By eukaryotic algae present. First metazoans seen in the Ediacara fauna for Australia (680 MyrBP).

Before considering the diversity of fossils we need to think about how representative the fossils are of past life which is largely a function of what gets preserved and where it might get preserved.

What gets preserved? Hard parts , and other parts that can be mineralized. Sequence of events from death to scavenging to decay to covering with soil. Example from heard of elephants : "wet" stage = two weeks (too much tissue for vultures so many invertebrates helped out). By the end of the third week, Dermestid beetles had removed all the skin and sinew from the bones. Within five weeks the temperature fluctuations caused the bones to crack and flake. Within one year the skeletons were completely disarticulated. Within two years many bones were covered with soil. Current day events can shed light on the fossilization process.

Fossilization : percolation of mineral grains (e.g. calcium carbonate) into interstitial spaces of hard part tissue. In bone the mineral is calcium phosphate which can incorporate fluorine, present in minute amounts in water, into the Calcium Phosphate to produce a crystal more resistant to erosion.

Death assemblage : become fossils at a site away from their actual habitat due to death and transport to an area. Life assemblage : organisms preserved in their natural habitat. Obvious example: If large mammal bones were found scattered among fossil fish, one presumably would not invoke the existence of primitive mammals that walked on lake or ocean floors!

Environments: fossils are generally restricted to areas of deposition. Upland areas less likely to preserve fossils: more erosion. In deserts material is covered by sand and has a good chance of being fossilized. In shallow seas sediment is being deposited and can cover skeletons. Some of the best fossil assemblages are from shallow sea deposits, lake beds, outwash plains from periodic river floodings, etc.

Ediacara fauna (640 MyBP) Many forms that bear some resemblance to modern phyla. Appears as if it were a major "evolutionary experiment" that did not work as it appears that none of their representatives made it into the Cambrian.

Burgess shale (530 MyBP, British Columbian rockies) Discovered in 1909 by Charles Doolittle Walcott: remarkable diversity of many different forms. Some of these are represented today many others are not (about 15-20 distinct, and now extinct, phyla ). e.g. Hallucigenia , Opabinia, Yohoia, Pikaia (first chordate), etc. Nicely illustrate the nature of Contingency (see S. J. Gould, Wonderful Life , 1989, Norton). The " iconography of the cone " led Walcott to erroneous pigeonholing of the Burgess shale organisms into "known" groups. The more appropriate image is " decimation " where only some organisms get through alive and those that do may be simply lucky. Harry Whittington in the 1960s and 1970s with Simon Conway Morris in the mid to late 1970s reanalyzed Walcott's collections. Concluded that there were many unique morphologies so new that they deserve the status of new phyla! . Many of Walcott's classifications were wrong. What would have happened if Pikaia had not made it through the "decimation"? (would you be here reading this? Another example of contingency ).

Other important points in interpreting the fossil record: Dating fossils requires radiometric dating of associated igneous rock . (sedimentary rock is of highly mixed origin). Moreover, fossils and the bed in which they lay have been reworked and redeposited. Careful stratigraphy and analyses of surrounding strata must be done to provide meaningful data about the relative and absolute ages of fossils. Gaps in the record . The nature of the fossilization process almost assures that there will be gaps in the fossil record. We have to live with it.

What do we know about fossil organisms? Certain associated information allows informed speculation about the biology of fossil organisms. Large dinosaurs that left tracks without tail dragging marks suggest an active lifestyle ? (other fossil remains do show clear evidence of tail dragging and footprints). Other assemblages show fossil bones of adults associated with nest sites and eggs: suggests parental care ? Simple footprints may seem like a cute form of fossil evidence. Actually a lot can be learned about the organisms: one can corroborate estimates of the animal's size one can measure distance between prints and obtain information about gait, travel speeds, etc. these interpretations further dictate a host of different physiological processes that might be able to sustain such a manner of locomotion. These types of issues are the main point of this lecture: from a small amount of fossil information, certain biological interpretations are implied simply by the necessary biological attributes that go along with a given footprint size, shape, etc.


Fossil

Mary Anning
The 19th-century British fossil collector Mary Anning proved you don't have to be a paleontologist to contribute to science. Anning was one of the first people to collect, display, and correctly identify the fossils of ichthyosaurs, plesiosaurs, and pterosaurs. Her contributions to the understanding of Jurassic life were so impressive that in 2010, Anning was named among the ten British women who have most influenced the history of science.

Microfossils
Even though most of us have only seen dinosaur fossils in museums, most fossils are not that big. Some of them are so small, you can't see them without a microscope.

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

translucent, yellow-orange material made of the resin of ancient trees. Amber is sometimes considered a gemstone.

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

preserved evidence of what was once the body of an ancient organism, such as bones or teeth.

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

remnant, impression, or trace of an ancient organism.

to become a solid mineral.

air containing a large amount of water vapor.

tropical ecosystem filled with trees and underbrush.

language of ancient Rome and the Roman Empire.

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

fossil that is large enough to be seen and analyzed without a microscope.

one of many extinct species of large animals related to elephants, with long, curved tusks. The last mammoths became extinct about 5,000 years ago.

extinct shark that lived between 25 million and 1.5 million years ago.

fossil that can only be seen and analyzed with a microscope, such as a grain of pollen or a single bacterium.

inorganic material that has a characteristic chemical composition and specific crystal structure.

living or once-living thing.

person who studies fossils and life from early geologic periods.

powdery material produced by plants, each grain of which contains a male gamete capable of fertilizing a female ovule.

materials left from a dead or absent organism.

clear, sticky substance produced by some plants.

solid material transported and deposited by water, ice, and wind.

to slowly flow through a border.

hard outer covering of an animal.

dark, sticky petroleum product created from the decomposition of organic material such as wood.

preserved evidence of the presence or behavior of an ancient organism, such as tracks, feces, or burrows.

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Related Resources

Fossil Record

The fossil record helps paleontologists, archaeologists, and geologists place important events and species in the appropriate geologic era. It is based on the Law of Superposition which states that in undisturbed rock sequences the bottom layers are older than the top layers. Therefore, some discovered fossils are able to be dated according to the strata, a distinct layer of rock, that they are found in. Another common way that fossils are dated, is through radiocarbon dating. The development of this type of dating, in the 1950s, transformed paleontology and enhanced the accuracy of the fossil record. With every new fossil discovery, our understanding of the environment in a particular time becomes richer. Use these resources to teach middle schoolers more about the fossil record and radiocarbon dating.

Extinction

Extinction is the complete disappearance of a species from Earth. Species go extinct every year, but historically the average rate of extinction has been very slow with a few exceptions. The fossil record reveals five uniquely large mass extinction events during which significant events such as asteroid strikes and volcanic eruptions caused widespread extinctions over relatively short periods of time. Some scientists think we might have entered our sixth mass extinction event driven largely by human activity. Our planet is dependent on an interconnected system. If we lose one species, how does that impact the whole system? What if we lose hundreds? Help your students understand the gravity of extinction with these classroom resources.

Dinosaurs

Dinosaurs gambol and charge through our imagination as scaly reptilian creatures with menacing teeth, claws, spikes, and hammering, bony bulbs. They roamed Earth roughly 175 million years ago, and most were wiped out by an extinction event roughly 65 million years ago. Thanks to ongoing scientific research, we continue to revise our theories about how dinosaurs evolved, what they ate, and how they moved through their environments. Read about the latest discovery in National Geographic&rsquos Science article: Bizarre Spinosaurus Makes History as First Known Swimming Dinosaur.

Paleontology

Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock.

Fossil Impressions

Students make molds and casts of objects to make their own fossils.

Sediment Fossil Surprise

Students analyze illustrations to understand how a fossil forms. Then they make a model of fossils found in sediment layers and eat it.

Related Resources

Fossil Record

The fossil record helps paleontologists, archaeologists, and geologists place important events and species in the appropriate geologic era. It is based on the Law of Superposition which states that in undisturbed rock sequences the bottom layers are older than the top layers. Therefore, some discovered fossils are able to be dated according to the strata, a distinct layer of rock, that they are found in. Another common way that fossils are dated, is through radiocarbon dating. The development of this type of dating, in the 1950s, transformed paleontology and enhanced the accuracy of the fossil record. With every new fossil discovery, our understanding of the environment in a particular time becomes richer. Use these resources to teach middle schoolers more about the fossil record and radiocarbon dating.

Extinction

Extinction is the complete disappearance of a species from Earth. Species go extinct every year, but historically the average rate of extinction has been very slow with a few exceptions. The fossil record reveals five uniquely large mass extinction events during which significant events such as asteroid strikes and volcanic eruptions caused widespread extinctions over relatively short periods of time. Some scientists think we might have entered our sixth mass extinction event driven largely by human activity. Our planet is dependent on an interconnected system. If we lose one species, how does that impact the whole system? What if we lose hundreds? Help your students understand the gravity of extinction with these classroom resources.

Dinosaurs

Dinosaurs gambol and charge through our imagination as scaly reptilian creatures with menacing teeth, claws, spikes, and hammering, bony bulbs. They roamed Earth roughly 175 million years ago, and most were wiped out by an extinction event roughly 65 million years ago. Thanks to ongoing scientific research, we continue to revise our theories about how dinosaurs evolved, what they ate, and how they moved through their environments. Read about the latest discovery in National Geographic&rsquos Science article: Bizarre Spinosaurus Makes History as First Known Swimming Dinosaur.

Paleontology

Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock.

Fossil Impressions

Students make molds and casts of objects to make their own fossils.

Sediment Fossil Surprise

Students analyze illustrations to understand how a fossil forms. Then they make a model of fossils found in sediment layers and eat it.


Observing fossils

Examine fossils at a museum or fossil site or look at photographs of fossils.

Materials

Some fossil sites have already been described.

These websites provide a list of museums that contain fossils:

A list of fossil sites around the world is given below. Identify the ones that are within South Africa:

If you are unable to visit the fossil sites or museums, the following website gives photographs and explanations of the major fossils that have shaped our understanding of the history of life:

Instructions

Travel to your nearest museum, fossil site or the website listed and observe any fossils on display. Find out how they have been preserved, describe the key features of each fossil, how they were dated and what they tell us about our past.

Activity: Observing fossils

Learners can be taken to museums or fossil sites or look at photographs of fossils to examine various fossils.

Some fossil sites have already been described.

These websites provide a list of museums that contain fossils:

A list of fossil sites around the world is given below. Identify the ones that are within South Africa:

If you are unable to visit the fossil sites or museums, the following website gives photographs and explanations of the major fossils that have shaped our understanding of the history of life:

Depending on how you are going to do this activity with your learners, they will need to find out how they have been preserved, describe the key features of each fossil, how they were dated and what they tell us about our past.

This can be presented in a research report format, or may simply be done as a class exercise. Formal assessment is not necessary.


FOSSIL SUCCESSION

Scientific theories are continually being corrected and improved, because theory must always account for known facts and observations. Therefore, as new knowledge is gained, a theory may change. Application of theory allows us to develop new plants that resist disease, to transplant kidneys, to find oil, and to establish the age of our Earth. Darwin's theory of evolution has been refined and modified continuously as new information has accumulated. All of the new information has supported Darwin's basic concept--that living beings have changed through time and older species are ancestors of younger ones.


A species is the most basic unit of classification for living things. This group of fossil clams shows likely ancestor-descendant relationships at the species level. These fossils from the Mid-Atlantic States show the way species can change through time. Notice how the shape of the posterior (rear) end of these clams becomes more rounded in the younger species, and the area where the two shells are held together (ligamental cavity) gets larger. Paleontologists pay particular attention to the shape of the shells and the details of the anatomy preserved as markings on the shells.

Numbers in the left-hand column refer to the following geologic time segments: 1, Pliocene 2, Miocene 3, Oligocene 4, Eocene 5, Paleocene 6, Late Cretaceous.

Figure courtesy of G. Lynn Wingard.

The Law of Fossil Succession is very important to geologists who need to know the ages of the rocks they are studying. The fossils present in a rock exposure or in a core hole can be used to determine the ages of rocks very precisely. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called index fossils .

Today the animals and plants that live in the ocean are very different from those that live on land, and the animals and plants that live in one part of the ocean or on one part of the land are very different from those in other parts. Similarly, fossil animals and plants from different environments are different. It becomes a challenge to recognize rocks of the same age when one rock was deposited on land and another was deposited in the deep ocean. Scientists must study the fossils from a variety of environments to build a complete picture of the animals and plants that were living at a particular time in the past.

The study of fossils and the rocks that contain them occurs both out of doors and in the laboratory. The field work can take place anywhere in the world. In the laboratory, rock saws, dental drills, pneumatic chisels, inorganic and organic acids, and other mechanical and chemical procedures may be used to prepare samples for study. Preparation may take days, weeks, or months--large dinosaurs may take years to prepare. Once the fossils are freed from the rock, they can be studied and interpreted. In addition, the rock itself provides much useful information about the environment in which it and the fossils were formed.


Evolution and the Fossil Record

This chapter was first shared with the public on January 6, 2020. It was last updated on on January 6, 2020.

Chapter citation:

J.R. Hendricks and B.S. Lieberman. 2019. Evolution and the Fossil Record. In: The Digital Encyclopedia of Ancient Life. https://www.digitalatlasofancientlife.org/learn/evolution/

Chapter contents:

Charles Darwin concluded the first edition of his famous book On the Origin of Species by Means of Natural Selection with one of the loveliest passages ever written by a scientist:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

What led him to this stunning conclusion, that natural forces are responsible for life's great present and ancient diversity of lifeforms? What is the evidence that evolution has produced new species and has honed them to survive in the environments in which they live? How does evolution work and how does understanding of the fossil record inform our understanding of evolutionary processes? This chapter will explore these and other questions.

What is evolution?

In a biological context, the word evolution is best defined by three words: descent with modification. This means that all life forms have descended from an ancient, shared common ancestor and have changed over time to survive in the environments in which they live.

Other definitions exist and are valid, though are often too narrowly defined. If you have taken an introductory biology class, you may have learned that evolution means “changes in gene frequencies within a population.” Our definition above is broader and also more general. In particular, it focuses on a pattern of change (descent with modification), rather than the mechanisms or processes that are responsible for that change. Distinguishing between patterns and processes will come up again in our later discussion of "macroevolution."

Further, this broadly-focused definition of evolution allows consideration of not only genetic changes within populations, but also both larger and smaller scale evolutionary changes. Distinctions between phenomena occurring at different scales will become important later in the chapter when we discuss “hierarchies.”

What does evolution seek to explain?

Scientists who study evolution seek to explain two fundamental observations of nature.

First, they seek to explain the “ goodness of fit ” of organisms to the environments in which they live. A less fancy way of saying this is that scientists want to explain why organisms seem to be very good at doing what they do. Consider, for example, a squirrel. Its chisel-like teeth easily open the shells of hard acorns, its tail provides great balance as it runs along telephone wires, and its brownish-gray fur provides it ideal camouflage against tree trunks. Early naturalists attributed such goodness of fit to divine design. In fact, they carefully explored nature in order to discover evidence of God’s creation. It is from this school of study (known as Natural Theology) that Darwin’s idea of natural selection as an explanation for “goodness of fit”–better known today as adaptation –was first developed (we explore this in detail in the Natural Selection page of this chapter).

Second, scientists who study evolution seek to explain another fundamental observation of the living world: the great number of different species, or biodiversity. In short, why are there so many different kinds of plants, animals, and other lifeforms, and how did they come to be? We explore both the nature of species and the process responsible for their generation (speciation) later in this chapter.

Is evolution a fact or a theory?

Okay, what we actually mean is that it is both, and hopefully a bit more context will be useful. It turns out that whether we refer to evolution as a “fact” or a “theory” depends on if we’re speaking in a strict scientific sense or in the colloquial usage of these terms.

A “theory,” in scientific parlance, is best thought of as some extremely well-supported body of knowledge which can explain the behavior of, or relationships among, certain objects in the universe. One example of a scientific theory is the "germ theory of disease." This is the theory that some germs make humans sick. We take this as an obvious fact now, but just several hundred years ago, many attributed disease to evil spirits and other causes that seem strange to us now. Another example of a scientific theory is plate tectonics, which is the idea that the surface of the world is divided into a series of plates that interact at their edges, causing the formations of mountains and volcanoes, as well as triggering earthquakes. (It is probably worth noting, again, that not so long ago in human history, volcanic eruptions and earthquakes were sometimes attributed to supernatural forces). So, you can see that the scientific usage of the word "theory” is very different from its usage in day-to-day discourse, where it indicates a hunch or poorly formed idea (e.g., that the Cleveland Browns are going to win the Super Bowl next year).

To the general public, the term “fact” is often invoked to indicate something is absolutely and indisputably true. For example, brick walls are solid and that is a fact: even if you try to imagine that they don’t exist, that won’t help you if you decide to run into one (don't do it it will hurt).

Portion of a Roman city wall built in the present location of the Tower of London. This wall was constructed around 200 A.D. Photograph by Jonathan R. Hendricks.

In a scientific sense, however, we should recognize that at the smallest scale, brick walls are made up of atoms, and atoms in turn are primarily comprised of empty space. Further, the sub-atomic particles inside of them only have a probability of being at a particular place at a particular time. On top of that, much of the universe—perhaps 90%–consists of dark matter, which we can’t even detect and only have the vaguest understanding of.

Only by using very powerful instrumentation can we come close to glimpsing the underlying infinitesimal materials that comprise physical objects (like a brick wall). Yet, our interpretation of these observations is at least partly based on theory (in this case, atomic theory). That is why scientists are sometimes reluctant to think of the existence of “facts” in an absolute sense (because facts depend on theory, which provides context for understanding).

To cut to the chase—again vis a vis the theoretical nature of atoms and the sub-atomic particles they contain—theory posits a great degree of empty space inside of atoms. Within this space, sub-atomic particles have different probabilities of occupying a very small part of that space. These particles make up only 10% of the matter of the universe (so, the brick wall in the photograph above is mostly empty space, but don't run into it!). Note that this applies to both the outside and inside of the brick wall, which brings to mind Groucho Marx’s brilliant statement that “outside of a dog, a book is a man’s best friend, but inside of a dog it’s too dark to read.”

But all of this theory slams literally and figuratively into the reality that although you could “test the theory that the brick wall exists,” the chance that you could reject the theory of the wall’s existence is very small.

The chance that the wall in the photograph above does not exist is infinitesimally small. You could test this theory by running head first into it, but it is probably not worth the inevitable pain and suffering that would ensue immediately after making contact with it. Because of this, we can probably accept the existence of brick walls as facts, even though our understanding of their underlying atomic nature is largely theoretical. Because we can accept that brick walls exist, we are free to make other discoveries about them, including about when they were built and why. We argue that people would be most fully served if they don’t try to get the concepts of “theory” and “fact” too mixed up, and further if they avoid running into brick walls in order to test the existence of matter.

Additionally, we note that “theory” and “fact” aren’t the only terms that have different scientific and colloquial meanings. Consider the meanings of the word “random.” In day-to-day discourse, saying someone is acting “completely random” implies that we have no idea what this person is doing and what they are going to do next. By contrast, in a scientific sense, random behavior is in fact entirely predictable. Indeed, this is why casinos know that they’ll always win money.

Randomness speaks to odds of probability and casinos stack the odds of probability in their favor so that, while you may occasionally hit the jackpot, in the long term you will always lose.

In the context of evolution, whether we use the term “fact” or “theory” also depends on whether we are speaking about whether evolution has happened, the pattern of evolution, or the various individual processes that cause it to happen. Later in this chapter, we will get into much more detail about the distinction between the patterns and processes of evolution. But it is worth recognizing at the very outset that the evidence that life has evolved is overwhelming and is just as robust as the data supporting the existence of brick walls. Trying to falsify evolution at this point would be like trying to run through a brick wall in order to disprove atomic theory (don't do it it will hurt).

The evolutionary pattern of descent with modification produces a genealogy, just as there is a pattern of descent and genealogy within individual human families. Evolution is so overwhelmingly supported by so many different lines of evidence that we can treat it as a “fact” (at least in the general parlance of the term), just as we treat the notion that the different planets revolve around the sun in elliptical orbits as a “fact.”

"Solar System 101" by National Geographic (YouTube).

Although the notion that life has evolved is so well supported that we can treat it as a fact, there are various ideas about particular aspects of evolution that should be characterized as theories. These all focus on identifying the different mechanisms of evolution (i.e., how evolution works) and how observed evolutionary patterns are shaped by those processes.

Some of these theories will be the focus of subsequent sections of this chapter. For instance, there is a theory of "species selection” and scientists actively research and debate how frequently it happens and whether it can explain various macroevolutionary patterns in the history of life. There are even debates about how species selection should be defined, and one’s perspective on these definitions has bearing on how one should go about testing this theory.

Another theory focuses on the relative role that competition has played in producing macroevolutionary patterns. Charles Darwin and 20th century paleontologist George Simpson theorized that competition was extremely important in shaping observed evolutionary patterns. By contrast, other theories—such as the Turnover Pulses theory, developed by Elisabeth Vrba—posit a much more prominent role for the physical environment in driving macroevolutionary patterns. These are examples of the types of theories that are tested by evolutionary biologists and paleontologists throughout their careers. By contrast, no legitimate scientist is still testing the theory that evolution happened. That’s something that hasn't been worth testing since the mid-1800's.

What is the evidence for evolution?

Just as Darwin said that “natural selection is daily and hourly scrutinizing” organisms, so can we say that biologists are daily and hourly scrutinizing populations of organisms—running the gamut from bacterial strains to corn, fruit flies, and mice—in laboratories and observing how they evolve in response to changes in their environments or manipulation of their genetics. This is work that has been proceeding apace for more than 100 years, and that makes it possible to very nicely understand how organisms evolve (especially in laboratory settings) on time scales of weeks to decades.

The fruit fly Drosophila melanogaster, a common subject of genetic experiments in the lab. The fly is feeding upon a piece of banana. Image by Sanjay Acharya (Wikipedia/Wikimedia Commons Creative Commons Attribution-Share Alike 4.0 International license).

A similar range of studies are also being conducted on natural populations of organisms in the wild, including the famous example of the Galapagos finches (or, Darwin's finches) and their evolutionary responses to changes in environmental conditions (especially in terms of the shapes and sizes of their beaks, as illustrated below). The video below discusses a long term project that has been studying the evolution of the Galapagos finches in the field.

Drawings of different species of Galapogos finches from Darwin's (1845) book, Voyage of the Beagle (Wikipedia/Wikimedia Commons public domain).

"Galapagos Finch Evolution" by HHMI BioInteractive Video (YouTube).

Other examples of modern-day evolution include the repeated evolution of antibiotic resistance in various bacterial populations, the declining average size of many fish species as larger fish are preferentially caught by humans, and numerous examples of plant and animal domestication (see the next section for additional information on such artificial selection ).

One might remark that each of these types of studies provide examples involving small-scale evolutionary changes (e.g., changes within a species) and of course that is true. But there is also abundant evidence of large-scale evolutionary transitions. These come from two sets of sources: 1) the fossil record and 2) phylogenetic analyses of the body parts (morphology) and DNA of modern species.

The fossil record is replete with numerous examples of evolutionary transitions that have occurred, both in recent geological history and the distant past. One of the best examples comes from our own lineage, the hominids. The fossil record of the past 6 million years reveals the transition from chimp-like species with smaller average brain sizes to species that are increasingly human-like in appearance (i.e., bipedal), with larger average brain sizes (see also the speciation section of this chapter).

"Great Transitions: The Origin of Humans" by HHMI BioInteractive Video (YouTube).

The traditional, iconic view of human evolution is one of a monkey turning into an ape, which then turns into a caveman, and then a human. This view is everywhere in popular culture. As evidence, take a moment to do a Google Image search of the word "evolution." You will see countless examples of this. The same thing is shown in the evolution of Homer Simpson video below.

"The Simpsons Homer Evolution" from the television show, The Simpsons (YouTube).

This is not how evolution works, however. Evolution is a branching process, driven largely by geographic isolation of populations, which we cover later in this chapter in the section on speciation.

It is important to understand that humans did not evolve directly from the modern day chimpanzee. Rather we share a common ancestor with them that lived 5-6 million years ago, probably in Africa. This is depicted by the phylogenetic tree of relationships (or, cladogram) shown below. (Visit the DEAL pages on phylogenetics to learn how to read and interpret these trees.)

A phylogenetic tree depicting the relationships between a gorilla, a chimpanzee, and a human (19th century paleontologist Mary Anning portrait public domain). Image by Jonathan R. Hendricks.

While chimps are our closest living relatives (this is well established by comparisons of the DNA of each species), we are far more closely related to species of ancient hominids that are now completely extinct. If any of those hominid species were still alive, we would say that they are our closest living relatives, not the chimpanzee.

Phylogenetic tree depicting the relationships between gorillas, chimpanzees, humans (depicted by 19th century paleontologist Mary Anning portrait public domain), and human-like relatives. Image by Jonathan R. Hendricks.

Another great example of a large-scale transition preserved in the fossil record is the initial transition of the vertebrates onto land during the Devonian period (about 375 million years ago). This transition is recorded by the discovery of fossils like Tiktaalik, which bears features of both fish and four-legged land animals (tetrapods). As we would predict, this transition occurred long before the origin of hominids, as hominids are the evolutionary descendants of these earliest land-dwelling vertebrates.

"The Origin of Four-Legged Animals" by HHMI BioInteractive Video (YouTube).

Not only did vertebrates move from the sea to the land, but there were also transitions from land back into the sea that happened hundreds of millions of years after the aforementioned Devonian origin of land vertebrates. These too are very well preserved in the fossil record. One example is the origin of whales from their land-dwelling ancestors.

When we compare the DNA of modern whales (including dolphins and orcas) to all other living mammals, we find that they share the greatest similarity with hippos.

Image from Tsagkogeorga et al. (2015 in Royal Society Open Science). Original caption: " Evolutionary relationships among laurasiatherian mammals as used in molecular evolution analyses. The four clades tested for divergent selection are shown in colour and numbered in uppercase: (I) Whippomorpha (Hippopotamidae + Cetacea) (II) Cetacea (III) Mysticeti and (IV) Odontoceti. Branches tested for positive selection are numbered in lowercase: (i) Whippomorpha (Hippopotamidae + Cetacea) (ii) Cetacea (iii) Mysticeti (iv) Odontoceti and (v) hippo." Creative Commons Attribution 4.0 International license.

Obviously, modern hippos and whales do not look very similar to each other. Moreover, whales did not evolve directly from hippos (just as humans did not evolve directly from chimpanzees). Instead, whales and hippos share a common ancestor that lived tens of millions of year ago. Some of the descendants of this common ancestor are highlighted in the video below. Note how these descendants become more-and-more whale-like over time, acquiring features that improved their ability to more efficiently move through the water (streamlined body, flippers, etc.).

"When Whales Walked" by PBS Eons (YouTube).

Although we could provide many more examples (e.g., the evolution of birds from theropod dinosaurs), the last one we will mention is the origin of arthropods. Arthropods are the phylum of animals that includes insects, spiders, centipedes, crustaceans, and other similar creatures (all of which have segmented external skeletons and jointed appendages). The origin and diversification of arthropods is very well recorded in the fossil record. In combination with DNA evidence, the fossil record shows us that arthropods are most closely related to “wormy” animals that molt their exoskeletons as they grow (all arthropods also molt). Examples of these "worms" include priapulids and kinorhynchs.

Left: the priapulid worm Priapulus caudatus image by Shunkina Ksenia (Wikipedia/Wikimedia Commons Creative Commons Attribution 3.0 Unported license). Right: drawing of a kinorhynch, from the Encyclopedia Britannica (11th edition, 1911) (Wikimedia Commons public domain).

Fossil specimens from the Cambrian period have transitional features that help to bridge the gap between these living worm-like animals and groups of modern arthropods. Examples of these Cambrian animals include strange creatures like Anomalocaris and Hallucigenia.

Left: reconstruction of Anomalocaris canadensis image by Tobu Tamura (Wikipedia/Wikimedia Commons Creative Commons Attribution-Share Alike 4.0 International license). Right: reconstruction of Hallucigenia sparsa image by Jose Manuel Canete (Wikipedia Wikipedia/Wikimedia Commons Creative Commons Attribution-Share Alike 4.0 International license).

Tardigrades (or, if you prefer, water bears or moss piglets) are living animals that also show features of both worms like priapulids and also arthropods like crustaceans and insects. They are most remarkable for being able to survive extreme environmental conditions.

"Meet the tardigrade, the toughest animal on Earth" by Thomas Boothby and TED-Ed (YouTube).

The best approach for documenting and understanding the evolutionary connections between major groups of plants and animals is to combine information from both modern and fossil organisms in a phylogenetic context. Learn more about how phylogenetic trees are built and read here. Most of the organismal chapters of this textbook are arranged in a phylogenetic context to help you understand the relationships among particular groups, as well as the evolution of the features that define those groups.

Did Darwin discover evolution?

Darwin, who wrote On the Origin of Species, is certainly the individual most associated with the idea of evolution and most people assume that he is the first person to suggest that species have evolved over time. This is a major misconception.

Left: a marble statue of Charles Darwin on display at the Natural History Museum, London. Right: title page of the first edition of Darwin's On the Origin of Species (Wikipedia/Wikimedia Commons public domain). Image by Jonathan R. Hendricks.

Although Darwin certainly was a great thinker who contributed in very important ways to both biology and geology, he did not “discover” evolution (we will focus on what some of his crucial contributions were in the next section of this chapter).

In fact, for many centuries before Darwin, philosophers and naturalists speculated on the nature of species and whether they are static or instead change over time. For instance, ancient Greek philosophers like Anaximander and Empedocles were active in the 4 th century B.C., and they reflected upon possible natural (i.e., not supernatural) explanations for the origins of species, though today we find their purported mechanisms for such origins strange. (They range from humans developing inside fish to a view that the different body parts of organisms naturally assembled—that is, random heads connected to headless bodies—until "perfect combinations were achieved" (Mayr, 1982, p. 302). Weird, but it was a beginning.

Left: Tile mosaic of Anaximander (Wikipedia/Wikimedia Commons). Right: Empedocles (Wikipedia/Wikimedia Commons public domain).

Ideas regarding the origins of species may extend back even earlier, but we can’t say for sure. The reason we know about Anaximander and Empedocles is that they wrote their musings down and these writings survive to the present day. Furthermore, ideas on evolution were not just developed in Europe. Several Native American tribes have traditions that extend back many hundreds of years, possibly even thousands of years, that postulate that life forms have changed through time, and that present-day animals are the modified descendants of ancient ones.

It is clear though that there was a very long-time interval, stretching from the end of the Roman Republic in the first century B.C. to the growth of the enlightenment in the 17 th century, when the concept of evolution received scant attention in Europe and elsewhere. The idea of evolution—and, for that matter, most other non-Biblical explanations for natural phenomena—were considered taboo for more than 1,000 years because learned individuals (those who today we might call philosophers or scientists) feared challenging the hegemonic dogma of the religious leaders of the Dark and Middle Ages.

The scene "She's a witch!" from the movie Monty Python and the Holy Grail (YouTube), which we presume at least semi-accurately portrays life in Europe life during the Dark Ages.

We enjoy studying the fossil record and evolution, but admit that the possibility of getting hideously tortured and or executed for even discussing it would be a pretty strong inducement for us to keep our research results to ourselves. We thus hesitate to too strongly chastise scientific practitioners from previous eras, although those who threatened to mete out such punishments are certainly worthy of excoriation.

In the late 17th century, Isaac Newton demonstrated that the physical properties and motions of the universe (from microscopic particles to celestial bodies) could be explained by natural laws and forces, rather than Divine micromanagement. This event, sometimes called the Newtonian Revolution, began to free science from the shackles of dogma and set the stage for the rise of biology in the late part of the 18 th century and the early part of the 19 th century. During this interval, numerous papers and books about the living world were published, and many scholarly societies were forming and having meetings, describing various lines of evidence that indicated that life had changed over time and that species might not be static. These events all happened before Darwin’s (1859) publication of “On the Origin of Species” and they influenced both him and other great scientists of the 19 th century.

Perhaps first and foremost among these were the numerous works by the French naturalists Georges-Luis Leclerc, Compte de Buffon (1707-1788 published between the 1750’s and 1780’s) and Jean-Baptiste, Chevalier de Lamarck (1744-1829 published between 1801 and the 1820’s).

Left: portrait of Buffon by artist François-Herbert Drouais (Wikipedia/Wikimedia Commons public domain). Right: portrait of Lamarck by artist Charles Thévenin (Wikipedia/Wikimedia Commons public domain).

Each author outlined excellent evidence for evolution, although in Buffon’s many works it was at times equivocal and contradictory, as he occasionally asserted that the diversity of life had been produced by a creator. We can state with confidence, however, that by 1801 Lamarck was a fully-fledged evolutionist who had documented that evolution had occurred and proposed mechanisms for that pattern. Lamarck is often mentioned only in introductory courses as a biologist who got the mechanism for evolution wrong (one component of which was inheritance of acquired characteristics , which was often taught using giraffes as an example). While we know now that characters acquired in life (e.g., big muscles developed by weightlifting) are not passed on to offspring (babies are not born with big muscles, regardless of how ripped their parents are), Lamarck was way ahead of his contemporaries in understanding the biological world. Most importantly, Lamarck convinced many naturalists that evolution was true. Among these was Robert Grant, who became one of Darwin's most important teachers.

There are many important differences between Lamarck’s views on the processes most important for driving evolutionary change and those of Darwin, but each author held that there was a pattern of evolution: that all life was descended from a shared common ancestor. (Note that Darwin primarily described evolution as “descent with modification” [which is the definition we use above]. The only time that he used a word related to “evolution” in the entirety of On the Origin of Species was the last word of the book see this passage at the top of this page.)

In the next section, we will explore Darwin's mechanism for evolutionary change (natural selection), as well as some related concepts that pertain to explaining the forms of organisms.


The End-Ediacaran Extinction

However, about 541 million years ago, most of the Ediacaran creatures disappeared, signaling a major environmental change that Douglas Erwin and other scientists are still working to understand. Evolving animal body plans, feeding relationships, and environmental engineering may have played a role.

Burrows found in the fossil record, dating to the end of the Ediacaran, reveal that worm-like animals had begun to excavate the ocean bottom. These early environmental engineers disturbed and maybe aerated the sediment, disrupting conditions for other Ediacaran animals. As environmental conditions deteriorated for some animals, they improved for others, potentially catalyzing a change-over in species.


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Paleontology

Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock.

Biology, Ecology, Geology, Geography, Social Studies, World History

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Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock. Paleontologists use fossil remains to understand different aspects of extinct and living organisms. Individual fossils may contain information about an organism&rsquos life and environment. Much like the rings of a tree, for example, each ring on the surface of an oyster shell denotes one year of its life. Studying oyster fossils can help paleontologists discover how long the oyster lived, and in what conditions. If the climate was favorable for the oyster, the oyster probably grew more quickly and the rings would be thicker. If the oyster struggled for survival, the rings would be thinner. Thinner rings would indicate an environment not favorable to organisms like the oyster&mdashtoo warm or too cold for the oyster, for example, or lacking nutrients necessary for them to grow.

Some fossils show how an organism lived. Amber, for instance, is hardened, fossilized tree resin. At times, the sticky resin has dripped down a tree trunk, trapping air bubbles, as well as small insects and some organisms as large as frogs and lizards. Paleontologists study amber, called &ldquofossil resin,&rdquo to observe these complete specimens. Amber can preserve tissue as delicate as dragonfly wings. Some ants were trapped in amber while eating leaves, allowing scientists to know exactly what they ate, and how they ate it. Even the air bubbles trapped in amber are valuable to paleontologists. By analyzing the chemistry of the air, scientists can tell if there was a volcanic eruption or other atmospheric changes nearby.

The behavior of organisms can also be deduced from fossil evidence. Paleontologists suggest that hadrosaurs, duck-billed dinosaurs, lived in large herds, for instance. They made this hypothesis after observing evidence of social behavior,including a single site with approximately 10,000 skeletons.

Fossils can also provide evidence of the evolutionary history of organisms. Paleontologists infer that whales evolved from land-dwelling animals, for instance. Fossils of extinct animals closely related to whales have front limbs like paddles, similar to front legs. They even have tiny back limbs. Although the front limbs of these fossil animals are in some ways similar to legs, in other ways they also show strong similarities to the fins of modern whales.

Subdisciplines of Paleontology

The field of paleontology has many subdisciplines. A subdiscipline is a specialized field of study within a broader subject or discipline. In the case of paleontology, subdisciplines can focus on a specific fossil type or a specific aspect of the globe, such as its climate.

One important subdiscipline is vertebrate paleontology, the study of fossils of animals with backbones. Vertebrate paleontologists have discovered and reconstructed the skeletons of dinosaurs, turtles, cats, and many other animals to show how they lived and their evolutionary history.

Using fossil evidence, vertebrate paleontologists deduced that pterosaurs, a group of flying reptiles, could fly by flapping their wings, as opposed to just gliding. Reconstructed skeletons of pterosaurs have hollow and light bones like modern birds.

One type of pterosaur, Quetzalcoatlus, is considered one of the largest flying creatures in history. It had a wingspan of 11 meters (36 feet). Paleontologists have competing theories about if and how Quetzalcoatlus flew. Some paleontologists argue it was too heavy to fly at all. Others maintain it could distribute its weight well enough to soar slowly. Still other scientists say Quetzalcoatlus was muscular enough to fly quickly over short distances. These theories demonstrate how vertebrate paleontologists can interpret fossil evidence differently.

Invertebrate paleontologists examine the fossils of animals without backbones&mdashmollusks, corals, arthropods like crabs and shrimp, echinoderms like sand dollars and sea stars, sponges, and worms. Unlike vertebrates, invertebrates do not have bones&mdashthey do leave behind evidence of their existence in the form of fossilized shells and exoskeletons, impressions of their soft body parts, and tracks from their movement along the ground or ocean floor.

Invertebrate fossils are especially important to the study and reconstruction of prehistoric aquatic environments. For example, large communities of 200-million-year-old invertebrate marine fossils found in the deserts of Nevada, in the United States, tell us that certain areas of the state were covered by water during that period of time.

Paleobotanists study the fossils of ancient plants. These fossils can be impressions of plants left on rock surfaces, or they can be parts of the plants themselves, such as leaves and seeds, that have been preserved by rock material. These fossils help us understand the evolution and diversity of plants, in addition to being a key part of the reconstruction of ancient environments and climates, subdisciplines known as paleoecology (the study of ancient environments) and paleoclimatology (the study of ancient climates).

At a small site in the Patagonia region of Argentina, paleobotanists discovered the fossils of more than 100 plant species that date back about 52 million years. Prior to this discovery, many scientists said South America&rsquos biological diversity is a result of glaciers breaking up the continent into isolated ecosystem "islands" two million years ago. The Patagonia leaf fossils may disprove this theory. Paleobotanists now have evidence that the continent&rsquos diversity of plant species was present 50 million years before the end of the last Ice Age.

Some plant fossils are found in hard lumps called coal balls. Coal, a fossil fuel, is formed from the remains of decomposed plants. Coal balls are also formed from the plant remains of forests and swamps, but these materials did not turn into coal. They slowly petrified, or were replaced by rock. Coal balls, found in or near coal deposits, preserve evidence of the different plants that formed the coal, making them important for studying ancient environments, and for understanding a major energy source.

Micropaleontology is the study of fossils of microscopic organisms, such as protists, algae, tiny crustaceans, and pollen. Micropaleontologists use powerful electron microscopes to study microfossils that are generally smaller than four millimeters (0.16 inches). Microfossil species tend to be short-lived and abundant where they are found, which makes them helpful for identifying rock layers that are the same age, a process known as biostratigraphy. The chemical makeup of some microfossils can be used to learn about the environment when the organism was alive, making them important for paleoclimatology.

Micropaleontologists study shells from deep-sea microorganisms in order to understand how Earth&rsquos climate has changed. Shells accumulate on the ocean floor after the organisms die. Because the organisms draw the elements for their shells from the ocean water around them, the composition of the shells reflects the current composition of the ocean.By chemically analyzing the shells, paleontologists can determine the amount of oxygen, carbon, and other life-sustaining nutrients in the ocean when the shells developed. They can then compare shells from one period of time to another, or from one geographic area to another. Differences in the chemical composition of the ocean can be good indicators of differences in climate.

Micropaleontologists often study the oldest fossils on Earth. The oldest fossils are of cyanobacteria, sometimes called blue-green algae or pond scum. Cyanobacteria grew in shallow oceans when Earth was still cooling, billions of years ago. Fossils formed by cyanobacteria are called stromatolites. The oldest fossils on Earth are stromatolites discovered in western Australia that are 3.5 billion years old.

History of Paleontology

Throughout human history, fossils have been used, studied, and understood in different ways. Early civilizations used fossils for decorative or religious purposes, but did not always understand where they came from.

Although some ancient Greek and Roman scientists recognized that fossils were the remains of life forms, many early scholars believed fossils were evidence of mythological creatures such as dragons. From the Middle Ages until the early 1700s, fossils were widely regarded as works of the devil or of a higher power. Many people believed the remains had special curative or destructive powers. Many scholars also believed that fossils were remains left by Noah's flood and other disasters documented in the Hebrew holy book.

Some ancient scientists did understand what fossils were, and were able to formulate complex hypotheses based on fossil evidence. Greek biologist Xenophanes discovered seashells on land, and deduced that the land was once a seafloor. Remarkably, Chinese scientist Shen Kuo was able to use fossilized bamboo to form a theory of climate change.

The formal science of paleontology&mdashfossil collection and description&mdashbegan in the 1700s, a period of time known as the Age of Enlightenment. Scientists began to describe and map rock formations and classify fossils. Geologists discovered that rock layers were the product of long periods of sediment buildup, rather than the result of single events or catastrophes. In the early 1800s, Georges Cuvier and William Smith, considered the pioneers of paleontology, found that rock layers in different areas could be compared and matched on the basis of their fossils.

Later that century, the works of Charles Lyell and Charles Darwin strongly influenced how society understood the history of Earth and its organisms. Lyell&rsquos Principles of Geology stated that the fossils in one rock layer were similar, but fossils in other rock layers were different. This sequence could be used to show relationships between similar rock layers separated by great distances. Fossils discovered in South America may have more in common with fossils from Africa than fossils from different rock layers nearby.

Darwin&rsquos On The Origin of Species observed somewhat similar sequencing in the living world. Darwin suggested that new species evolve over time. New fossil discoveries supported Darwin&rsquos theory that creatures living in the distant past were different from, yet sometimes interconnected with, those living today. This theory allowed paleontologists to study living organisms for clues to understanding fossil evidence. The Archaeopteryx, for example, had wings like a bird, but had other features (such as teeth) typical of a type of dinosaur called a theropod. Now regarded as a very early bird, Archaeopteryx retains more similarities to theropods than does any modern bird. Studying the physical features of Archaeopteryx is an example of how paleontologists and other scientists establish a sequence, or ordering, of when one species evolved relative to another.

The dating of rock layers and fossils was revolutionized after the discovery of radioactivity in the late 1800s. Using a process known as radiometric dating, scientists can determine the age of a rock layer by examining how certain atoms in the rock have changed since the rock formed. As atoms change, they emit different levels of radioactivity. Changes in radioactivity are standard and can be accurately measured in units of time.

By measuring radioactive material in an ancient sample and comparing it to a current sample, scientists can calculate how much time has passed. Radiometric dating allows ages to be assigned to rock layers, which can then be used to determine the ages of fossils.

Paleontologists used radiometric dating to study the fossilized eggshells of Genyornis, an extinct bird from Australia. They discovered that Genyornis became extinct between 40,000 and 50,000 years ago. Fossil evidence from plants and other organisms in the region shows that there was abundant food for the large, flightless bird at the time of its extinction. Climate changes were too slow to explain the relatively quick extinction.

By studying human fossils and ancient Australian cave paintings that were dated to the same time period, paleontologists hypothesized that human beings&mdashthe earliest people to inhabit Australia&mdashmay have contributed to the extinction of Genyornis.

Paleontology Today

Modern paleontologists have a variety of tools that help them discover, examine, and describe fossils. Electron microscopes allow paleontologists to study the tiniest details of the smallest fossils. X-ray machines and CT scanners reveal fossils' internal structures. Advanced computer programs can analyze fossil data, reconstruct skeletons, and visualize the bodies and movements of extinct organisms.

Paleontologists and biologists used a CT scan to study the preserved body of a baby mammoth discovered in Siberia in 2007. A CT scanner allows scientists to construct 3-D representations of the bones and tissue of the organism. Using this technology, scientists were able to see that the baby mammoth had healthy teeth, bones, and muscle tissue. However, the animal&rsquos lungs and trunk were full of mud and debris. This suggested to scientists that the animal was healthy, but most likely suffocated in a muddy river or lake.

Scientists can even extract genetic material from bones and tissues.

Paleontologists made a remarkable genetic discovery when the bones of a Tyrannosaurus rex were broken during an excavation in the 1990s. Soft tissue was discovered inside the bones. Soft tissue is the actual connective tissue of an organism, such as muscle, fat, and blood. Soft tissue is rarely preserved during fossilization. Paleontologists usually must rely on fossilized remains&mdashrocks. Paleontologists now hope to use this rare discovery of 68-million-year-old tissue to study the biology and possibly even the DNA of the T. rex.

Even with all these advancements, paleontologists still make important discoveries by using simple tools and basic techniques in the field.

The National Geographic Society supports field work in paleontology throughout the world. Emerging Explorer Zeresenay "Zeray" Alemseged conducts studies in northern Ethiopia. There, Alemseged and his colleagues unearth and study fossils that contribute to the understanding of human evolution.

Emerging Explorer Bolortsetseg Minjin is a paleontologist who has found fossils of dinosaurs, ancient mammals, and even corals in the Gobi Desert of Mongolia. She also works to teach Mongolian students about the dinosaurs in their backyard, and is hoping to establish a paleontology museum in the country.

Many dig sites offer visitors the chance to watch paleontologists work in the field, including the following U.S. sites: Gray Fossil Site in Gray, Tennessee the La Brea Tar Pits in Los Angeles, California and the Ashfall Fossil Beds in Royal, Nebraska.

Photograph by Robert Sisson

Evolutionary Biology
Many paleontologists are also evolutionary biologists. Evolutionary biology is the study of the origin, development, and changes (evolution) in species over time. Other scientists that contribute to evolutionary biology are geologists and geneticists.

Soaking Up History
The oldest fossils ever discovered are stromatolites, the remains of ancient cyanobacteria, or blue-green algae. The oldest animal fossils ever discovered are sponges. Prehistoric sponges have been discovered on the Arabian Peninsula and Australia.

Fossils and Myths
Ancient cultures did not always understand what fossils were, and adapted their discovery to fit with myths and stories.

China is rich in dinosaur fossils. Dinosaurs are ancient reptiles whose bones share characteristics with both reptiles and birds. Ancient Chinese people often interpreted dinosaur skeletons as the remains of flying dragons!

Fossilized remains of dwarf elephants have been found on several Mediterranean islands. Dwarf elephants grew to only 2 meters (6 feet) tall. Their skulls are about the same size as a human skull, with a large hole in the middle where the living animal's trunk is. In the ancient Mediterranean cultures of Greece and Rome, the remains of dwarf elephants were often interpreted as the remains of cyclopes, a type of feared, one-eyed giant.

Mary Anning
The 19th-century British fossil collector Mary Anning proved you don't have to be a paleontologist to contribute to science. Anning was one of the first people to collect, display, and correctly identify the fossils of ichthyosaurs, plesiosaurs, and pterosaurs. Her contributions to the understanding of Jurassic life were so impressive that in 2010, Anning was named among the ten British women who have most influenced the history of science.


Watch the video: Fossils u0026 Evidence For Evolution. Evolution. Biology. FuseSchool (July 2022).


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