Recent human exons are evolving much faster than ancient exons. (A) The synonymous and nonsynonymous substitution rates, Ks (open squares) and Ka (filled diamonds), respectively, between human and chimpanzee exon pairs and the Ka/Ks ratio (dotted line) are shown for each evolutionary group. The number of coding exons used to calculate the Ka and Ks were 67,476, 8,801, 14,080, 730, and 509 for the five evolutionary groups, respectively. (B) Recent human exons have a higher SNP density compared with ancient exons. SNPs from human dbSNP database were mapped to human exons in different evolutionary groups, and the density of SNPs was measured as described in Methods.