By comparing the human genome to the genomes of simpler organisms such as the fly and the worm, scientists are also seeing with unprecedented detail how just a few genetic innovations helped launch early vertebrates ahead of the biological pack hundreds of millions of years ago. In one case, a few genes that probably helped flies and worms fight off invading microbes apparently copied and recopied and rearranged themselves inside newly evolving backboned creatures to create what is today the army of genes that together encode the human immune system, with its remarkable ability to recognize and attack a huge variety of invaders. Similar duplications of genes led to the development of the blood clotting system, a complicated but crucial breakthrough for vertebrates. A third series of duplications gave rise to the spectrum of hormones and other molecules that today allow human cells to communicate with one another and coordinate their activities with exquisite precision. And yet another set of duplicated genes gave rise to the vertebrate nervous system and brain, the capstone of mammalian complexity.
Some human genes seem to have been ripped off wholesale from simpler organisms.
By scanning the human genome for DNA sequences nearly identical to those in microbes, scientists found more than 200 genes whose origins can be traced to ancient infecting bacteria. One, for example, controls production of a chemical called monoamine oxidase, which the body today uses to regulate brain chemicals affecting mood and is the target of a class of antidepressants called MAO inhibitors. For all the evolutionary creativity that led to the human race, however, there is shockingly little variation from person to person, genome scientists said. Individuals around the world all are about 99.9 percent genetically identical, an indication of how recently the human species arose and how little time it has had to diversify. Celera's analysis, which included genes from five people who identified themselves as Caucasian, Hispanic Mexican, Asian Chinese or African American, indicates that the number of harmless variations nestled in the human genome varies by race.