Michael Hammer at the University of Arizona in Tucson and his team analysed the entire genomes of seven individuals from two contemporary Western African Pygmy groups. The researchers identified 265 regions of the Pygmy genomes that may have been acquired through ancient interbreeding with other species, events that could have happened as recently as 9,000 years ago.The article, itself, points out the nature of introgression in hominin evolution, thus:
Introgression, the transfer of genetic material between closely related species through hybridization, is an important and ubiquitous evolutionary force in both plants and animals (Mallet 2007). Although hybrids are often nonviable or infertile, hybridization can be an important driving force for the origin of novel traits and new species (Mallet 2007; Zinner et al. 2011). Within our genus, Homo, there is strong evidence for multiple introgression events between our own species, H. sapiens, and now extinct sister taxa outside Africa (Pääbo 2014). Neanderthal whole-genome sequencing (Green et al. 2010; Prüfer et al. 2014) revealed that Neanderthals contributed an average of ∼2% of the genetic variation of present-day humans living outside of sub-Saharan Africa. This gene flow likely took place 37–86 thousand yr ago (kya), after early modern humans emigrated from Africa and before archaic forms went extinct in Eurasia (Sankararaman et al. 2012), and it may have occurred multiple times (Vernot and Akey 2014, 2015; Kim and Lohmueller 2015).It has been found, for example, that aspects of our immune system owe themselves to Neandertal introgression. This is an often overlooked aspect of how evolution proceeds. There is continual debate about whether or not Neandertals represent a separate species from modern humans but the evidence that they interbred and exchanged genes is pretty good at this point. The authors suggest that introgression of archaic genes occurred continually from the period of one million years down to less than 5 thousand years ago. That says something about the depth of the hybridization as well as the persistence of archaic genes in the modern human genome.