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

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.

Fig. 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.

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.

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. 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 (Fig. 2).

Fig. 2 - Fossils can be dated by sampling surrounding volcanic rocks.

Fossil record provides evidence of 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.

Fig. 3 - This is a stromatolite sample from the Paleoarchean of Western Australia.

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.

Fig. 4-5. Fossils show that the hippopotamus (left) is the closest living relative of the whale (right).

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.