February 2001 saw the public release of the human genome, with entire issues of both Nature and Science substantially devoted to discussion of its implications, educational CDs and wall charts. That is if the huge wadge of adverts capitalising on the genome’s release is discounted Pundits have latched onto the fact that humans seem to possess not that many more genes (around 30 000) than grass, a worm or a fruit fly, making comments about how humbling that is. Vastly outnumbering protein-coding genes are “snips” (single nucleotide polymorphisms – SNPs), and humans have around 1.4 million of these and possibly far more in the 3 billion sequences of four nucleotides.
The huge variability of “Snips” holds excellent prospects for deeper understanding of human origins and evolution, previously (and unsatisfactorily) addressed by using DNA in mitochondria and the Y chromosome. Previous means of establishing molecular “distance” to indicate relatedness and the times of divergence from last common ancestors rely on DNA that occurs only once in each cell, and does not undergo division and recombination during sexual reproduction, so that it is passed on in the female or male line of descent. Whereas such haploid material is relatively easy to analyse and interpret, it behaves like a single gene. Differences arise through natural selection or chance events that affect only one item. That makes it possible only to address the history of one variable, rather than that of a whole species or a population – a single thread rather than the multitude that must constitute the signal of real events.
“Snips” potentially can help resolve the out-of-Africa and multiregional hypotheses for the origin and spread of fully modern humans, and even whether we do carry vestiges of other groups of the genus Homo, such as the Neanderthals or various groups of more archaic beings who began to leave Africa for the rest of the Old World around 1.8 Ma ago. In a review of the possibilities, Mark Stoneking of the Max Planck Institute in Leipzig (Stoneking, M. 2001. From the evolutionary past… Nature, v. 409, p 821-822) cautions that much remains to be done before SNPs can really give believable information.