Genetic differences between humans and great apes.

The remarkable similarity among the genomes of humans and the African great apes could warrant their classification together as a single genus. However, whereas there are many similarities in the biology, life history, and behavior of humans and great apes, there are also many striking differences that need to be explained. The complete sequencing of the human genome creates an opportunity to ask which genes are involved in those differences. A logical approach would be to use the chimpanzee genome for comparison and the other great ape genomes for confirmation. Until such a great ape genome project can become reality, the next best approach must be educated guesses of where the genetic differences may lie and a careful analysis of differences that we do know about. Our group recently discovered a human-specific inactivating mutation in the CMP-sialic acid hydroxylase gene, which results in the loss of expression of a common mammalian cell-surface sugar throughout all cells in the human body. We are currently investigating the implications of this difference for a variety of issues relevant to humans, ranging from pathogen susceptibility to brain development. Evaluating the uniqueness of this finding has also led us to explore the existing literature on the broader issue of genetic differences between humans and great apes. The aim of this brief review is to consider a listing of currently known genetic differences between humans and great apes and to suggest avenues for future research. The differences reported between human and great ape genomes include cytogenetic differences, differences in the type and number of repetitive genomic DNA and transposable elements, abundance and distribution of endogenous retroviruses, the presence and extent of allelic polymorphisms, specific gene inactivation events, gene sequence differences, gene duplications, single nucleotide polymorphisms, gene expression differences, and messenger RNA splicing variations. Evaluation of the reported findings in all these categories indicates that the CMP-sialic hydroxylase mutation is the only one that has so far been shown to result in a global biochemical and structural difference between humans and great apes. Several of the other known genetic dissimilarities deserve more exploration at the functional level. Among the areas of focus for the future should be genes affecting development, mental maturation, reproductive biology, and other aspects of life history. The approaches taken should include both going from the genome up to the adaptive potential of the organisms and going from novel adaptive regimes down to the relevant repercussions in the genome. Also, as much as we desire a simple genetic explanation for the human phenomenon, it is much more probable that our evolution occurred in multiple genetic steps, many of which must have left detectable footprints in our genomes. Ultimately, we need to know the exact number of genetic steps, the order in which they occurred, and the temporal, spatial, environmental, and cultural contexts that determined their impact on human evolution.