So, to end with the question we began with, Did man evolve from monkeys? No. The concept of biological evolution, that living things share common ancestry, implies that human beings did not descend from monkeys, but shared a common ancestor with them, and shared a common ancestor farther back in time with other mammals, and farther back in time with tetrapods, and farther back in time with fish, and farther back in time with worms, and farther back in time with petunias. We are not descended from petunias, worms, fish, or monkeys, but we shared common ancestors with all of these creatures, and with some more recently than others. The inference of common ancestry helps us make sense of biological variation. We humans are more similar to monkeys than we are to dogs because we shared a common ancestor with monkeys more recently than we shared her a common ancestor with dogs. Humans, dogs, and monkeys are more similar to one another (they are all mammals) than they are to salamanders, because the species that provided the common ancestor of all mammals lived more recently than the species providing the common ancestors of salamanders and mammals. This historical branching relationship of species through time allows us to group species into categories such as primates, mammals, and vertebrates, which allows us to hypothesize about other relationships. Indeed, the theory of evolution, as one famous geneticist put it, is what "makes sense" of biology: "Seen in the light of evolution, biology is, perhaps, the most satisfying science. Without that light it becomes a pile of sundry facts, some of them more or less interesting, but making no comprehensible whole" (Dobzhansky 1973: 129). Evolution tells us why biology is like it is: living things had common ancestors, which makes a comprehensible whole of all those facts and details.
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