How do anatomical and DNA similarities in vertebrates show scientist that evolution has occurred?

Answer 1

Anatomical similarities:

Homologous organs are probably the most obvious sign of evolution. Homologous organs are body parts which, despite having almost identical structures, have evolved to fulfill different function for each animal.

A human arm consists of fingers, wrist bones, a forearm and a humerus. If you look at the construction of a cat's foot, you will notice it is made up of identical bones, only of different sizes to suit the cat.

A bat's wing also has fingers, a forearm and upper arm, although the fingers are much longer in comparison to it's arm due to the need for relatively large wings.

Perhaps most surprisingly of all, a whale has all the same bones embedded in it's front flippers, showing that it is descended from a common ancestor with all of the human, cat, and bat species.

A whale, rather oddly, also has leg bones. These are vestigial organs - remnant body parts from an ancestor which used them which have no function in the modern organism. Nevertheless, it shows the whale to be descended from a land-living, terrestrial species which used legs.

Same goes for the human appendix. In herbivores, the appendix serves to digest the cellulose in grass and leaves. Since cellulose no longer features in the human diet, the appendix is completely useless. It serves as evidence for an ancestor which did use to eat leaves. (It remains because there is no evolutionary pressure to be rid of it.)

If evolution hadn't occurred, a) why would these seemingly dissimilar animals have such similar anatomies, and b) why would animals have body parts they don't even make use of.

Answer 2

It is not so much the similarities that led scientists to the conclusion of common descent, but *nested hierarchies*. Nested hierarchies are a pattern formed by the similarities *and* the differences combined.

One can use combinations of traits to define groups of animals. If one does this comprehensively, for as many traits as possible, then one finds that a pattern of sets within sets within sets emerges.

For instance, all vertebrates are animals, but not all animals are vertebrates. All tetrapods are vertebrates, but not all vertebrates are tetrapods. All mammals are tetrapods, but not all tetrapods are mammals. All primates are mammals, but not all mammals are primates. All apes are primates, but not all primates are apes. All humans are apes, but not all apes are human (though most of them are, these days: the human species is a *very* successful kind of ape).

These nested hierarchies are the most telling clue as to the origin of modern lifeforms: this pattern can only be explained by common descent.