  On the other hand, however, there is evidence that FOXP2 is particularly implicated in vocal learning and expression. First, it is highly expressed in songbirds that modify their innate vocal repertoires: in canaries it is expressed seasonally, when adult birds modify their songs (Teramitsu et al. 2004) and in zebra finches, it is expressed more at the time when young birds learn their songs (Haessler et al. 2004). In addition, there is evidence that the variant of the FOXP2 gene that is present in humans has undergone strong positive selection in the hominid line (Enard et al 2002; Zhang et al. 2002). The protein produced by human FOXP2 differs in just three out of its 715 constituent amino acids from that of the mouse, and a recent analysis (Zhang et al. 2003) indicates that two of these differences are unique to the hominid lineage. According to Enard et al 2002, the fact that these two differences are fixed in the human genome, whereas no fixed substitutions occurred in the lineage of our closest relatives, the chimpanzees, suggests that those changes were strongly selected for in our lineage; Enard et al.  put the date of fixation of these changes in the human population at around 200,000 years ago. This date accords well with at least some estimates of the emergence of modern human language, suggesting that the vocal capacities underwritten by FOXP2 — and impaired in those lacking the gene — are after all critical to language competence.