Gene losses during human origins.
Pseudogenization is a widespread phenomenon in genome evolution, and it has been proposed to serve as an engine of evolutionary change, especially during human origins (the "less-is-more" hypothesis). However, there has been no comprehensive analysis of human-specific pseudogenes. Furthermore, it is unclear whether pseudogenization itself can be selectively favored and thus play an active role in human evolution. Here we conduct a comparative genomic analysis and a literature survey to identify 80 nonprocessed pseudogenes that were inactivated in the human lineage after its separation from the chimpanzee lineage. Many functions are involved among these genes, with chemoreception and immune response being outstandingly overrepresented, suggesting potential species-specific features in these aspects of human physiology. To explore the possibility of adaptive pseudogenization, we focus on CASPASE12, a cysteinyl aspartate proteinase participating in inflammatory and innate immune response to endotoxins. We provide population genetic evidence that the nearly complete fixation of a null allele at CASPASE12 has been driven by positive selection, probably because the null allele confers protection from severe sepsis. We estimate that the selective advantage of the null allele is about 0.9% and the pseudogenization started shortly before the out-of-Africa migration of modern humans. Interestingly, two other genes related to sepsis were also pseudogenized in humans, possibly by selection. These adaptive gene losses might have occurred because of changes in our environment or genetic background that altered the threat from or response to sepsis. The identification and analysis of human-specific pseudogenes open the door for understanding the roles of gene losses in human origins, and the demonstration that gene loss itself can be adaptive supports and extends the "less-is-more" hypothesis.
