Evolution is a scientific theory that explains the gradual changes in living organisms over time. It is widely accepted among scientists as the best explanation for the diversity of life on Earth. While there are many pieces of evidence that support the theory of evolution, two particularly compelling examples are the fossil record and comparative anatomy.
The Fossil Record
The fossil record is a collection of all the fossils that have been discovered and studied by scientists. These fossils provide a window into the past, allowing us to see what organisms were like millions of years ago.
One of the most compelling pieces of evidence for evolution is the pattern of fossil succession. This refers to the fact that older rocks contain fossils of simpler organisms, while younger rocks contain fossils of more complex organisms. This pattern suggests that species have evolved over time, with new and more complex species emerging from older and simpler ones.
For example: The oldest known fossils are simple single-celled organisms such as bacteria and algae. As we move up through the geological layers, we see more complex organisms such as sponges, jellyfish, and worms. Later still we find fish, amphibians, reptiles, mammals and eventually humans.
Another important aspect of the fossil record is transitional forms – fossils that show characteristics intermediate between two major groups of organisms. For example, Tiktaalik is a transitional form between fish and amphibians. It has features like gills, scales, and fins found in fish but also limbs with wrists and digits found in amphibians.
- Key Point: The pattern of fossil succession suggests that species have evolved over time.
- Key Point: Transitional forms provide evidence for how major groups of organisms are related.
Comparative anatomy is another area where we can see evidence for evolution. Organisms that share a common ancestor tend to have similar physical structures, even if they have evolved to perform different functions. For example, the limbs of vertebrates (animals with backbones) have a similar bone structure, even though they may be adapted for different purposes such as running in cheetahs or flying in bats.
For example: The forelimbs of humans, whales, bats and birds all have the same basic bone structure – one long bone (the humerus), two forearm bones (the radius and ulna), several wrist bones and five digits. However, these limbs are adapted for very different functions such as grasping in humans, swimming in whales and flying in bats and birds.
Comparative anatomy also provides evidence for vestigial structures – body parts that are no longer used but have been retained from earlier ancestors. For example, the wings of flightless birds like ostriches are vestigial structures – they no longer serve a function but are remnants of the wings of their flying ancestors.
- Key Point: Similarities in physical structures between organisms suggest that they share a common ancestor.
- Key Point: Vestigial structures provide evidence for evolution by showing how structures can become redundant over time.
The theory of evolution is supported by a vast array of evidence from many different fields. The fossil record and comparative anatomy are just two examples of compelling pieces of evidence that support this theory. By studying these areas and others like molecular biology and biogeography, scientists continue to refine our understanding of how life on Earth has evolved over time.