The often-mentioned fact that humans and chimpanzees are 99.9 percent identical in their DNA is hard to accept for some people, who can't comprehend how we could share so much of our basic genetic endowment even with the most humanlike ape. Yet this genetic similarity is very real, and it dramatically shows how parsimonious natural selection can be -- it reuses genes and structures that have worked well in the past.
It was also mind-boggling when, in 1987, British researchers demonstrated that a human gene could be inserted into the cells of a lowly yeast -- and it functioned perfectly well. In this landmark experiment, researchers Paul Nurse and Melanie G. Lee showed that the gene in question, one that controlled the division of cells, was extremely similar despite the fact that yeast and the distant ancestors of humans diverged about 1 billion years ago.
The Human Genome Project is revealing many dramatic examples of how genes have been "conserved" throughout evolution -- that is, genes that perform certain functions in lower animals have been maintained even in the human DNA script, though sometimes the genes have been modified for more complex functions.
This thread of genetic similarity connects us and the roughly 10 million other species in the modern world to the entire history of life, back to a single common ancestor more than 3.5 billion years ago. And the evolutionary view of a single (and very ancient) origin of life is supported at the deepest level imaginable: the very nature of the DNA code in which the instructions of genes and chromosomes are written. In all living organisms, the instructions for reproducing and operating the individual is encoded in a chemical language with four letters -- A, C, T, and G, the initials of four chemicals. Combinations of three of these letters specify each of the amino acids that the cell uses in building proteins.
Biologically and chemically, there is no reason why this particular genetic code, rather than any of millions or billions of others, should exist, scientists assert. Yet every species on Earth carries a genetic code that is, for all intents and purposes, identical and universal. The only scientific explanation for this situation is that the genetic code was the result of a single historic accident. That is, this code was the one carried by the single ancestor of life and all of its descendents, including us.