With all of the technology available to scientists today, there are many ways to support the Theory of Evolution with evidence. DNA similarities between species, knowledge of developmental biology, and other evidence for microevolution is abundant. However, scientists haven't always had the capabilities to examine these types of evidence. So how did they support the Theory before these discoveries?
The main way scientists have supported the Theory of Evolution throughout history is by using anatomical similarities between organisms. Showing how body parts of one species resemble the body parts of another species, as well as accumulating adaptations until structures become more similar on unrelated species are some ways evolution is backed up by anatomical evidence. Of course, there is always finding traces of long extinct organisms that can also give a good picture of how a species changed over time.
Traces of life from the past are called fossils. How do fossils lend evidence in support of the Theory of Evolution? Bones, teeth, shells, imprints, or even entirely preserved organisms can paint a picture of what life was in time periods from long ago. Not only does it give us clues to organisms that are long extinct, it also can show intermediate forms of species as they underwent speciation.
Scientists can use information from the fossils to place the intermediate forms in the right place. They may use relative dating and radiometric or absolute dating to find the age of the fossil. This can help fill in gaps to the knowledge of how a species changed from one time period to another throughout the Geologic Time Scale.While some opponents of evolution say that the fossil record is actually evidence of no evolution because there are “missing links” in the fossil record, it does not mean evolution is untrue. Fossils are very hard to create and circumstances need to be just right in order for a dead or decaying organism to become a fossil. There most likely are also many undiscovered fossils that could fill in some of the gaps.
Even though a dolphin and a shark look very similar in body shape, size, color, and fin location, they are not very closely related on the phylogenetic tree of life. Dolphins are actually much more closely related to humans than they are sharks. So why do they look so much alike if they are not related?
The answer lies within evolution. Species adapt to their environments in order to fill a vacant niche. Since sharks and dolphins live in the water in similar climates and areas, they have a similar niche which needs to be filled by something in that area. Unrelated species that live in similar environments and have the same type of responsibilities in their ecosystems tend to accumulate adaptations that add up to make them resemble each other.
These types of analogous structures do not prove species are related, but rather they support the Theory of Evolution by show how species do build up adaptations in order to fit into their environments. That is a driving force behind speciation, or a change in species over time. This, by definition, is biological evolution.
If the aim is to figure out how closely two species are related on the phylogenetic tree of life, then homologous structures need to be examined. As mentioned above, sharks and dolphins are not closely related. However, dolphins and humans are. One piece of evidence that supports the idea that dolphins and humans come from a common ancestor is their limbs.
Dolphins have front flippers that help reduce friction in water as they swim. However, by looking at the bones within the flipper, it is easy to see how similar in structure it is to the human arm. This is one of the ways scientists use to classify organisms into phylogenetic groups that branch off from a common ancestor.
Some parts in or on an organism's body no longer have any apparent use. These are leftovers from a previous form of the species before speciation occurred. The species apparently accumulated several adaptations that made the extra part no longer useful. Over time, the part stopped functioning, but did not completely disappear.
The no longer useful parts are called vestigial structures and humans have several of them including a tailbone that does not have a tail connected to it, and an organ called an appendix that has no apparent function and can be removed. At some point during evolution, these body parts were no longer necessary for survival and they disappeared or stopped functioning. Vestigial structures are like fossils within an organism's body that give clues to past forms of the species.