370 Evolution? The Fossils Say YES!
The Third Chimpanzee
The next time you visit a zoo, make a point of walking by the ape cages. Imagine that
the apes had lost most of their hair, and imagine a cage nearby holding some unfor-
tunate people who had no clothes and couldn’t speak but were otherwise normal.
Now try guessing how similar those apes are to us in their genes. For instance, would
you guess that a chimpanzee shares 10 percent, 50 percent, or 99 percent of its genetic
program with humans?
—Jared Diamond, The Third ChimpanzeeIf the fossil record of human evolution was not proof enough, the clinching evidence is
found in every cell in your body. When molecular biologists first began comparing the
molecules from apes, monkeys, and humans back in the 1960s, they made a startling dis-
covery: we are extremely similar to chimps and gorillas at the molecular level. The first
evidence of this came when Vince Sarich’s famous experiments showed by the relatively
crude method of comparing immune responses that there was little difference between the
genes of chimps and humans. Then the DNA-DNA hybridization method came along and
got better results still. This technique took the DNA of chimps and humans and separated
the strands in a solution by warming it up. As the solution cooled down, the individual
unpaired strands of DNA link up again, with some combining a strand of chimp and a
strand of human DNA. When you heat up that hybrid DNA again, it unzips and separates.
The more genes the two strands have in common and the more similar they are, the harder
it is for them to unzip, so that the temperature of separation is directly proportional to the
number of genes in common.
When these results were first published by Sibley and Ahlquist in 1984, they caused a
shock in the scientific community. It turns out that human and chimpanzee DNA is 97.6 percent
identical! Less than 3 percent of our DNA is different from that of a chimpanzee. Likewise,
our DNA shares about 96 percent of its genes with gorillas, and it is 94.7 percent identical
with that of gibbons, 91.1 percent identical with that of rhesus monkeys (an Old World
monkey, of family Cercopithecidae), 84.2 percent identical with that of the capuchin mon-
key (a New World monkey, of family Cebidae), but only 58 percent identical with that of
a primitive “prosimian” such as a galago. Not only did the sequence of similarity match
exactly with the branching sequence of primates (fig. 15.4), which made sense, but the real
shock was just how similar our genes are to those of most of the great apes. Molecular
biologists pointed out that the genetic similarity between humans and chimps was closer
than between any other two species they had studied, such as two closely related species
of rats or two kinds of frogs. Since these experiments, the actual sequence of the mitochon-
drial DNA and the nuclear DNA have been determined for both chimps and humans, and
the results are the same (within a fraction of a percent). There’s no two ways about it. The
chimps are our “kissing cousins” among the animal kingdom, just a few genes removed
from being fully human.
Jared Diamond puts it all in perspective with an interesting analogy. Suppose you were
a molecular biologist from another planet and you were given just DNA samples of humans
and the two chimpanzee species (the common chimp, Pan troglodytes, and the pygmy chimp
or bonobo, Pan paniscus), plus a sampling of DNA from the primates and other groups of
animals. You sequence the genes, plot the results, and conclude that humans are just a third