Earth's fossil record | Evolution | Middle school biology | Khan Academy

• [Instructor] Life on Earth has existed for billions of years. Humans know this, not because we've been around the whole time, but instead, thanks to the discovery of fossils, which tell us about organisms that lived in the distant past. Scientists have found millions of fossils. Some, such as skeletons, document ancient organisms' anatomy or physical features, while others, such as preserved footprints, record organisms' behaviors.

Each fossil that has been found is like a piece of a giant puzzle. To see the bigger picture of the history of life on Earth, scientists have to study the many fossils and see how they all fit together in the larger puzzle. All of the fossil puzzle pieces that have been discovered, along with their relative ages, make up Earth's fossil record. The fossil record is basically a history of life on Earth as documented through fossils.

By studying the fossil record, scientists can see how life on Earth has changed over evolutionary time. The fossil record allows us to see patterns of evolutionary changes that have happened throughout Earth's history. These include an overall increase in organism complexity and diversity and the changes that have occurred along evolutionary lineages, showing us how one group of organisms could have evolved to the next. The fossil record also shows us when certain groups of organisms went extinct.

First, let's take a look at the increase in organism complexity and diversity that is seen in fossil record. Over evolutionary time, organisms have generally become more complicated. Some of the oldest known fossils are formations made by ancient colonies of cyanobacteria. These fossils provide evidence that single-celled organisms were living on Earth roughly around 3.5 billion years ago and that these organisms were relatively simple, similar to the bacteria we have today.

The fossil record then goes on to document major leaps in complexity of life on Earth, with, for example, the large tube-shape cells of Grypania spiralis and the macroscopic organisms in the Francevillian biota. According to scientists, these fossils, which are dated to be roughly from 2 billion years ago, could provide evidence of some of the earliest eukaryotic, or nucleus-containing cells, and of the earliest multicellular organisms.

Not only does the fossil record show an increase in complexity over evolutionary time, but also an increase in the diversity of life on Earth. For example, the Cambrian Explosion describes a multimillion year span of time, beginning about 539 million years ago, in which there was a huge increase in the diversity of animals on Earth. Almost all of the animal lineages we know today actually got started during this period.

All of these increases in complexity and diversity have given rise to the millions of varied organisms that live on Earth today. Next, let's take a look at the changes that can be seen along the evolutionary lineages in the Earth's fossil record. But before we do that, let's quickly look back at how scientists study fossils.

As you may recall, fossils are found in sedimentary rocks, meaning that the deeper the fossil, the older the fossil is likely to be. So based on the layers in which the fossil is found, scientists can put them in order by age. And in doing this, scientists can piece together the evolutionary steps of how one type of organism could have evolved into the next, eventually giving rise to the living organisms we see today.

To give an example of this, did you know that birds are actually direct descendants of a group of dinosaurs called theropods? It may be hard to believe, but the fossil record shows that an organism known as Archaeopteryx existed about 150 million years ago, towards the end of the Dinosaur Age, but before the appearance of modern birds. This organism in particular is interesting because it had some features unique to theropod dinosaurs, such as jaws and sharp teeth, and some features unique to birds, such as feathers.

So Archaeopteryx represents an evolutionary transition between theropod dinosaurs and birds in the fossil record. Finally, let's take a look at the patterns of extinction seen in the fossil record with the help of our extinct friend, the Triceratops. Extinction happens when a species completely dies out.

In the fossil record, extinctions show up as organisms of a certain species being present in older rock layers and then completely disappearing from the newer rock layers. For example, we only see Triceratops fossils in rock layers that are roughly 68 to 66 million years old. After this, we don't find any evidence of Triceratops fossils anywhere, and there certainly aren't any Triceratops living today. The Triceratops is an important example of extinction as they not only went extinct, but they went extinct due to a mass extinction event.

Mass extinction events happened when large-scale environmental change caused many groups of organisms to go extinct around the same time. When we see the triceratops disappear from the fossil record, we also see other types of organisms disappear too, providing evidence for the mass extinction event that led to the disappearance of all non-avian dinosaurs.

And with that, you now know what Earth's fossil record is and the various patterns it can show us. Earth's fossil record is an essential tool that scientists use to piece together the history of life on Earth. And new fossils are continuously being discovered, so the fossil record is more than just a collection of fossils.

It's a window into the mysteries of the past, and it's an ever-growing record of how life on Earth came to be what it is today.