The waning presence of Neanderthal genetic material in modern humans has been explained by a new study.
The research, led by Ivan Juric and colleagues at the University of California, Davis refers to the removal of weak and deleterious Neanderthal genes from the humans during the process of natural selection as the core reason for that, despite interbreeding.
Neanderthal Genes And Gene Selection
The study published in PLOS Genetics on Nov. 8 notes that genetic content of Neanderthals has been reduced to less than 4 percent in the genomes of modern non-African people. The work aligns the loss of Neanderthal content in modern humans with a unique model in estimating the level of natural selection.
Neanderthal alleles that were removed from the human genome had mild negative effects. Unlike humans, gene variations persisted in Neanderthals as their population size was smaller when compared to humans.
The study says that alleles of Neanderthals, after moving to the human genome, had a catharsis of sorts and natural selection led to their selective depletion.
Juric explained the significance of the study as a key effort at explaining the interplay of forces in gene selection. Furthermore, it supplements the already existing explorations on that front by Kelley Harris and Rasmus Nielsen in discussing the contribution of Neanderthals to human genomes.
"For a while now we have known that humans and Neanderthals hybridized ... We wanted to better understand the causes of this loss," Juric said.
The study shines as a debut attempt that demonstrates the power of natural selection in genetic engineering and in brushing aside the natural expectations stemming from genetic mixing that follows interbreeding.
There had been many studies that showed hybridization with Neanderthals leading to loss of gene variants from the modern human population. Juric claimed the new study stands out by proving those variants were lost after hybridization because of purging by natural selection.
Previous Theories Contested
The new study subtly highlights the role of population size in deciding evolutionary factors that enhance our understanding of the extinct close relative of humans. It also emphasizes the role of natural selection in catalyzing the loss of latent Neanderthal elements.
The paper also rebuts the conventional idea of "survival of the fittest" by showing that there was no ground to believe that Neanderthals were genetically inferior to modern humans and failed to survive.
Graham Coop, senior author of the paper, points to the slender population size of Neanderthals mixing with a large group of modern humans as the reason for the gene erosion.
It may be noted that slightly harmful genes may survive in a common, small and inbred group. But when mixed with a larger genome sample, purging by more beneficial variants follows.