Evolution, in the sense of common descent, is not a theory of similarity. Linnaeus, Cuvier, and Agassiz knew all about similarity, yet they denied common descent. Evolution is a theory of transformation.
Here’s an instructive parallel. Lead (Pb) and gold (Au) share many similarities: Au and Pb are metals. Au and Pb are dense, with very similar atomic masses. Au and Pb are highly ductile.
And so on. It doesn’t take much to imagine that, with the right chemical methods, one could transform lead into gold. After all, given their numerous similarities, lead and gold must lie on a pathway of natural transformation from one metal to the other. The alchemists certainly believed as much.
Yet as far as we know, there exists no chemical pathway from lead to gold. One requires the assistance of a particle accelerator, which would only be employed in any event once a physical theory of the elements was in hand, telling us that chemical transformations of the sort we had imagined actually were impossible.
The question of interest for evolution is not the genetic similarity of, let’s say, chimps and humans. Rather, it is to explain why these two species are so different, and how those differences — the transformations — came to be from a common ancestor.
University of North Carolina biological anthropologist Jonathan Marks put the point beautifully in a 1993 talk to the American Association for the Advancement of Science in Boston:
If the overall biology of the animals tells you that they are very different, and the genetics tells you that they are nearly identical, it follows that the genetic comparison is telling you something relatively trivial about the overall biology.
More from Jonathan Marks (from a 2009 paper, “What is the viewpoint of hemoglobin, and does it matter?“):
It is not that difficult to tell a human from an ape, after all. The human is the one walking, talking, sweating, praying, building, reading, trading, crying, dancing, writing, cooking, joking, working, decorating, shaving, driving a car, or playing football. Quite literally, from the top of our head (where the hair is continually growing, unlike gorillas) to the tips of our toes (the stoutest of which is non-opposable), one can tell the human part from the ape part quite readily if one knows what to look for. Our eye-whites, small canine teeth, evaporative heat loss, short arms and long legs, breasts, knees, and of course, our cognitive communication abilities and the productive anatomies of our tongue and throat are all dead giveaways. However, they are not readily apparent in a genetic comparison.
A great site to explore for specific anatomical, developmental, and behavioral differences is the MOCA database; MOCA stands for Matrix of Comparative Anthropogeny, part of the CARTA program. Look around in the Absolute and Relative Differences between chimps and humans.
Image: Alchemist Micha? S?dziw�j, by Jan Matejko [Public domain], via Wikimedia Commons.