Humans and chimpanzees have been diverging for around 5 million
years, during which time they've acquired major anatomical and
cognitive differences, but New York–based researchers report
this week in Building on an earlier study, Cornell University's
Rasmus Nielsen and colleagues compared over 13,000 annotated
human genes and their chimp equivalents, using the ratio of
synonymous to nonsynonymous mutations as evidence of positive
selection. Their aim, Nielsen said, was to determine "which
genes have been changing as humans and chimpanzees have evolved
into their modern forms." The team subjected orthologous
human–chimp gene pairs from published human data and polymerase
chain reaction data obtained from a single male chimpanzee to
likelihood analysis, which revealed any differences in mutation
type ratio. They excluded genes with fewer than three nucleotide
differences. Nielsen's team made several surprising discoveries,
including increased positive selection on the X chromosome and a
relative lack of positi