Charles Darwin seemingly had a little faith in the fossil record as a source of evidence for his theory. It was too incomplete. However, in the last 150 years, new findings and ways of analyzing old fossils have revealed many pieces of evidence in support of evolution. Fossils show how much, or how little, organisms have changed over time. One of the problems with the fossil record is that it contains gaps. Not all organisms fossilize well, to become a fossil is a difficult thing; when an organism dies usually the soft parts (flesh) will rot away, and for many organisms the ‘soft part’ is the whole thing! So most of the fossils we find are shells, bones and woody tissues and there will be many fossils that have been destroyed by the movements of the Earth, or simply not yet been discovered.
For an organism to be fossilized, the remains usually need to be covered by sediment soon after death. Sediments are different things including mud, sand, silt, and even ash from a volcano. Over time, minerals in the sediment seep into the remains. The remains become fossilized. Fossilization usually occurs in organisms with hard, bony body parts, such as skeletons, teeth, or shells. Soft-bodied organisms, such as worms, are rarely fossilized.Over time, more sediment will cover the remains of the animal or plant and the pressure caused by this will start to turn the sediment into rock.
Over millions of years, the deeply buried fossil will start to rise to the Earth’s surface. The fossil will rise to the surface because of things like earthquakes and other things that can cause rocks deep in the Earth to move. Eventually, once the fossil comes to the surface, the rock surrounding it will start to wear away and then the fossil can be seen.
There are several types of fossils that can be identified. We can divide them in to four major categories. The type called ‘trace fossils’ shows the evidence of animal activity. Those fossils such as footprints, coprolites, burrows are used study animal behavior. ‘Mold fossils’ show an impression of the animal or plant. Once the animal or plant dissolved away only the mark is left. ‘Cast fossils’ shows the positive image of the organism protruding from the rock. Mostly it gives a 3D image with bones. Finally the ‘true form ‘fossils contain true animal or plant. They are often made when dead bodies are trapped in ice, tar or amber. All original features such as color, skin and fur are intact.
One of the few animals with a fairly complete evolutionary record is the horse. All the main stages of the evolution of the horse have been preserved in fossil form. Over 60 million years, the horse evolved from a dog-sized creature that lived in rainforests into an animal adapted to living on the plains and standing up to two meters high. In the process its multi-toed feet, that were adapted for walking across the forest floor, evolved into single-toed hooves more suited for running over open country.
Paleobiologists are able to use similar characteristics of any fossil to trace common ancestry and look at how different groups are related to each other. Then scientists can reveal how closely relate an extinct species is to a living. Perhaps the most convincing evidence of evolution is transitional fossil which show the intermediate states between an ancestral form and that of its descendants. The evolution of tetrapods (animals with four legs) from fish, the evolution of birds from dinosaurs, and the evolution of humans from a more ‘ape-like’ ancestor are just a few nice examples from among the many that can be found.