Is Neanderthal gene introgression into Homo sapiens genome an adaptive anti-inflammatory phenomenon that increases depression risk?

Genes of Neanderthal ancestry are believed to have become incorporated in the modern Homo sapiens genome via hybridisation and introgression. Although the majority have been eliminated from the population by natural selection due to Dobzhansky–Muller incompatibilities, some of them nevertheless remain, suggesting they have been selected for and have some adaptive value. The current work examines hypotheses explaining the emergence of depressive symptoms and disorders from an evolutionary standpoint. Neither the incentive hypothesis nor any social hypothesis (social position hypothesis, attachment hypothesis, social navigation hypothesis) accommodates any evidence of archaic introgression. However, the immunological hypothesis, corroborated by a considerable body of research, treats depressive symptoms as part of immunologic response. According to the hypothesis, infections have placed a considerable selective pressure on humans. Upon arrival in Eurasia from Africa, Homo sapiens was confronted with unknown pathogenic microorganisms. In contrast, the Neanderthals populating Eurasia had already been adapting to them for millennia. Introgression of Neanderthal man alleles of genes associated with the immunological response has already been demonstrated in Homo sapiens, and may well increase the fitness of newcomers. Such inclusion of genes connected with depressive symptoms may explain why archaic alleles are still present in the gene pool of modern humans.