Several studies of genetic signatures of selection in humans have found something going on with olfactory receptor genes, which has been difficult to interpret. Why would there be strong selection for smelling related traits in the lineage leading to humans (smelling rotting meat, tracking animals??).
In this paper, the authors look at OR (olfactory receptor) genes in humans, chimps and macaques, and finds that:
The pseudogenization rates and the numbers of genes affected by positive selection are also similar between humans and chimpanzees.and
...approximately 25% of their functional gene repertoires are species specific due to massive gene losses. These findings suggest that the tempo of evolution of OR [olfactory receptor] genes is similar between humans and chimpanzees, but the OR gene repertoires are quite different between them.Similar Numbers but Different Repertoires of Olfactory Receptor Genes in Humans and Chimpanzees
Yasuhiro Go, and Yoshihito Niimura
Molecular Biology and Evolution 2008 25(9):1897-1907
Abstract: Animals recognize their external world through the detection of tens of thousands of chemical odorants. Olfactory receptor (OR) genes encode proteins for detecting odorant molecules and form the largest multigene family in mammals. It is known that humans have fewer OR genes and a higher fraction of OR pseudogenes than mice or dogs. To investigate whether these features are human specific or common to all higher primates, we identified nearly complete sets of OR genes from the chimpanzee and macaque genomes and compared them with the human OR genes. In contrast to previous studies, here we show that the number of OR genes (810) and the fraction of pseudogenes (51%) in chimpanzees are very similar to those in humans, though macaques have considerably fewer OR genes. The pseudogenization rates and the numbers of genes affected by positive selection are also similar between humans and chimpanzees. Moreover, the most recent common ancestor between humans and chimpanzees had a larger number of functional OR genes (>500) and a lower fraction of pseudogenes (41%) than its descendents, suggesting that the OR gene repertoires are in a phase of deterioration in both lineages. Interestingly, despite the close evolutionary relationship between the 2 species, approximately 25% of their functional gene repertoires are species specific due to massive gene losses. These findings suggest that the tempo of evolution of OR genes is similar between humans and chimpanzees, but the OR gene repertoires are quite different between them. This difference might be responsible for the species-specific ability of odor perception.