The Ape’s Reflection? 371species of chimpanzee, genetically closer than just about any other three species of animals
on the planet. For example, the genetic similarity between lions and tigers is only 95 percent,
yet they can even interbreed in zoos. Without seeing the differences in our bodies and behav-
ior, the conclusion is unavoidable: we are very closely related.
What do we make of this? Although there are many different ideas suggesting what
is going on here, the basic idea is that the 1–2 percent of the genome that differentiates us
from chimps must be the regulatory genes that turn on and turn off the structural genes
(which make up most of the 97.6 percent that is the same). We have the genes for most
parts of the ape body, and the monkey body too, and every once in a while there is a genetic
mistake or atavism, and humans express the long-repressed genes that we still carry to
make a tail (fig. 15.9).
(A)
FIGURE 15.9. Every once in a while, a human is born with an atavistic tail, a throwback to our evolutionary
past, when the regulation that normally shuts down our genes for tails fails to operate. The human tail comes
complete with fully developed vertebrae, muscles, and other features of animal tails. (A) X-ray of a human
with well-developed tail vertebrae. (B) Image of two humans with fully developed tails. (From Bar-Maor
et al. 1980; used with permission of the Journal of Bone and Joint Surgery)
(B)