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Fossil Record

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Fossil Record

How did life start on Earth? How did life forms evolve to what we know today? Fossils show how organisms evolved, how new groups of organisms emerged, and how some species became extinct.

In this article, we will discuss the fossil record: what it is, what it says about the evolution of life on Earth, and why it is considered "incomplete" and "biased."

Fossil record definition

Fossils are preserved remnants or traces of organisms from a past geologic age. These are often found in sedimentary rocks.

The fossil record is the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary rock layers called strata (singular: "stratum").

The arrangement of fossils in strata gives us an idea of what organisms existed at what point in geologic time. Other types of fossils like insects preserved in amber and mammals frozen in ice also provide useful information.

Figure 1 below shows some pertinent findings from an excavation site. The image on the left is a stratal pattern on a body of sedimentary rocks; here, we can clearly see the rock layers that indicate different points in geologic time. The image on the upper right shows a surface in one of these layers, while the image on the lower right calls our attention to ammonites in the stratal surface. Ammonites were cephalopods (marine invertebrates) that became extinct around 66 million years ago.

Fossil Record, Ammonite (cephalopod fossils) and stratal pattern on sedimentary rocks, StudySmarterFigure 1. The image on the left is a stratal pattern on a body of sedimentary rocks (facies) in Italy. The image on the upper right is a stratal surface. The image on the lower right shows ammonites found in these facies. Source: Antonov, Public domain, via Wikimedia Commons.

How are fossils dated?

Scientists use the fossil record to figure out when important events took place. They do this by dating rocks and fossils. We will discuss two common methods of determining the age of fossils:

Sedimentary strata

The sequence of sedimentary strata tells us the relative ages of fossils: fossils found in strata approaching the bottom strata are increasingly older; while fossils found in strata approaching the top strata are increasingly younger.

Let's say we identified six strata in an excavation site, which we have labeled strata 1 to 6 from top to bottom. Even without determining the exact age of the fossils, we can infer that a fossil found in stratum 1 is younger than a fossil found in stratum 2. Likewise, a fossil found in stratum 6 is older than a fossil found in stratum 5.

Radiometric dating

Radiometric dating estimates the ages of fossils by measuring the decay of radioactive isotopes.

Decay rates are expressed in “half-life”, which is the time it takes for half of the original isotope to decay into a new isotope. This is done by measuring the number of decayed isotopes in the sample, then determining the ratio between the original and decayed material.

Radiometric dating can also be used to infer the age of fossils by sampling surrounding layers of volcanic rock (Fig. 2). This is because surrounding radioactive isotopes can be trapped when lava cools into volcanic rock. For example, if fossils are sandwiched between two volcanic layers- one estimated to be 530 million years old and the other estimated to be 540 million years old, then the fossils are around 535 million years old.

Fossil record, Relative dating of fossils, StudySmarterFigure 2. Fossils can be dated by sampling surrounding volcanic rocks. Source: Jillcurie, CC BY-SA 3.0, via Wikimedia Commons

Fossil record provides evidence of evolution

Natural selection is a process where individuals with traits that help them survive in their environment are able to reproduce more and pass on those traits. Over time, natural selection leads to a gradual change in the heritable traits of a population of organisms, a process we call evolution.

We can observe these changes in the fossil record. Here we will discuss some examples.

Charles Darwin viewed the fossil record as evidence of evolution

Darwin described evolution as “descent with modification.” This means that different species share a common ancestor, but evolve in different directions.

Darwin used the fossil record to provide evidence of evolution. Specifically, Darwin showed that, at different points in geologic time, different species emerged as the traits of pre-existing species gradually changed. He argued that this "descent with modification" occurs because of natural selection.

Examples of facts scientists have learned about evolution from the fossil record

The fossil record helped scientists trace the evolution of life forms on Earth. In this section, we will discuss the origin of life on earth, the evolution of marine mammals from terrestrial mammals, and the mass extinction of species.

First life on Earth: microbial mats of cyanobacteria

The fossil record shows that 3.5 billion-year-old microbial mats of cyanobacteria that lived in hot springs and hydrothermal vents are the earliest known life forms on Earth. Microbial mats are communities of prokaryotes that are structured as multi-layered sheets. Microbial mats are found in different environments including lagoons, lakes, and tidal flats.

Fossilized microbial mats are called stromatolites. Stromatolites are made up of laminated structures that are formed through the precipitation of minerals by prokaryotes. Figure 3 shows a stromatolite sample from the Paleoarchean of Western Australia, the oldest known fossil occurrence on Earth.

In the first 2 billion years of the Earth, only anaerobic organisms were able to live. Anaerobic organisms are organisms that do not require oxygen to survive and grow. The emergence of cyanobacteria, which are blue-green algae capable of producing oxygen, made it possible for other life forms to evolve on Earth.

Fossil record, Stromatolite sample, StudySmarterFigure 3. This is a stromatolite sample from the Paleoarchean of Western Australia. Source: James St. John, CC BY 2.0, via Wikimedia Commons.

Emergence of cetaceans

The fossil record provides evidence that cetaceans--an order of marine mammals that includes dolphins, porpoises, and whales (Fig. 5)--evolved from terrestrial mammals like hippopotamuses (Fig.4), pigs, and cows. Fossils show that the pelvis and hind limb bones of extinct cetacean ancestors became smaller over time, eventually disappearing completely and developing into flukes and flippers.

Fossil record, image of hippopotamus, StudySmarter

Fossil record, image of whale, StudySmarter

Figures 4-5. Fossils show that the hippopotamus (left) is the closest living relative of the whale (right). Source (left): Diego Delso, CC BY-SA 4.0, via Wikimedia Commons. Source (right): Gabriel Barathieu, CC BY-SA 2.0, via Wikimedia Commons.

Mass extinctions

There are five strata in the fossil record where there seems to be a sudden and dramatic disappearance of species, which indicates that there have been at least five mass extinctions to date. Mass extinction is an event wherein over half the extant species disappear worldwide. It is believed that the sixth mass extinction—referred to as the Anthropocene period—has already begun as a result of human activities.

Alongside evidence of mass extinctions, the fossil record also shows how much time it took for biodiversity--the total variation of life--to recover. The fossil record indicates that the longest biodiversity recovery took about 30 million years. This information helps scientists predict contemporary extinction rates and come up with possible conservation measures to prevent human-caused extinction.

Fossil record incomplete and biased

While the fossil record provides us with important data, we need to keep in mind that it is incomplete for the following reasons:

  • Many organisms were not preserved as fossils because they did not die under the right conditions for fossilization. In fact, fossilization is so rare that scientists believe that only around 0.001% of all animal species that have ever existed have become fossils.

  • Even if fossils were formed, many were destroyed by geologic events.

  • Even if fossils survived those geologic events, many fossils have yet to be discovered.

For these reasons, the fossil record is biased towards species with the following characteristics:

  • Species that existed for a long time.

  • Species that were abundant in environments where scavengers could not take or destroy their remains.

  • Species that had hard shells, bones, teeth, or other parts that kept their remains from being destroyed after death.

The fossil record is incomplete and biased, yet crucial in our understanding of evolution. To fill gaps in information, scientists continue to search for fossils as well as other evidence of evolution including molecular data.

Fossil Record - Key takeaways

  • The fossil record is the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary rock layers called strata.
  • Sedimentary strata and radiometric dating are two common methods of determining the age of fossils. The sequence of sedimentary strata tells us the relative ages of fossils.
  • Radiometric dating estimates the ages of fossils by measuring the decay of radioactive isotopes.
  • Darwin used the fossil record to provide evidence of evolution. He showed that, at different points in geologic time, different species emerged as the features of pre-existing species gradually changed.
  • While the fossil record provides us with important data, we need to keep in mind that it is incomplete and biased because fossilization rarely happens.

Frequently Asked Questions about Fossil Record

The fossil record is the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary rock layers called strataThe arrangement of fossils in strata gives us an idea of what organisms existed at what point in geologic time.

The fossil record is the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary rock layers called strataThe arrangement of fossils in strata gives us an idea of what organisms existed at what point in geologic time.

The fossil record is incomplete for the following reasons:

  • Many organisms were not preserved as fossils because they did not die under the right conditions for fossilization. 
  • Even if fossils were formed, many were destroyed by geologic events.
  • Even if fossils survived those geologic events, many fossils have yet to be discovered.

Darwin used the fossil record to provide evidence of evolution. Specifically, Darwin showed that, at different points in geologic time, different species emerged as the traits of pre-existing species gradually changed. He argued that this "descent with modification" occurs because of natural selection.

Examples of what scientists have learned from the fossil record include the origin of life on Earth, the evolution or marine mammals from terrestrial mammals, and the mass extinction of species.

Final Fossil Record Quiz

Question

What are layers of sedimentary rocks called?

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Answer

Strata

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Question

What does the position of fossils in the sequence of sedimentary strata tell us?

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Answer

The relative age of fossils

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Question

What is the fossil record?

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Answer

The fossil record is the documentation of the history of life on Earth based primarily on the sequence of fossils in sedimentary strata.

Show question

Question

How does radiometric dating measure the age of fossils?

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Answer

Radiometric dating estimates the ages of fossils by measuring the decay of radioactive isotopes. 

Show question

Question

How are decay rates usually expressed?

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Answer

Half life

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Question

What is a half-life?

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Answer

The time it takes for half of the original isotope to decay into a new isotope

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Question

How is the decay rate of an isotope calculated?

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Answer

The decay rate of an isotope is calculated by measuring the number of decayed isotopes in the sample then determining the ratio between the original and decayed material.

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Question

How can radiometric dating be used to infer the age of fossils?

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Answer

Radiometric dating can also be used to infer the age of fossils by sampling surrounding layers of volcanic rock.

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Question

How did Darwin use the fossil record as evidence for evolution?

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Answer

Darwin showed that at different points in geologic time, different species emerged as the traits of pre-existing species gradually changed.

Show question

Question

What are the earliest life forms on Earth?

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Answer

The fossil record shows that 3.5 billion-year-old microbial mats of cyanobacteria that lived in hot springs and hydrothermal vents are the earliest life forms on Earth.

Show question

Question

What did the microbial mats produce that made it possible for other life forms to emerge?

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Answer

The microbial mats produced oxygen which allowed other life forms to emerge.

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Question

What is a mass extinction?

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Answer

Mass extinction is an event where over half of the extant species seem to have suddenly disappeared.

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Question

Why is the fossil record incomplete?

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Answer

Many organisms were not preserved as fossils because they did not die under the right conditions for fossilization, while those preserved as fossils may have been destroyed or are yet to be discovered.

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Question

What does the fossil record document?

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Answer

The fossil record documents the history of life on Earth.

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Question

What are fossilized microbial mats called?

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Answer

Fossilized microbial mats are called stromatolites.

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