Traditionally evolution was pictured as a straight line, gradually progressing from an ape-like ancestor to modern Homo sapiens. But thanks to next-generation sequencing, studies show that it wasn’t quite so orderly. A recent reports new details about the role of viruses in shaping evolution, in particular viral interactions between modern humans and Neanderthals. Discovery of genetic material from extinct sub-species of early humans also indicate this.

“When modern humans met up with Neanderthals, they infected each other with pathogens that came from their respective environments,” says first author David Enard, assistant professor at evolutionary biology at the University of Arizona. “By inter-breeding, they also passed along genetic adaptations to cope with some of those pathogens.”

It is estimated that modern humans began moving out of Africa and into Eurasia about 70,000 years ago. There they met up with Neanderthals who, along with their own ancestors, had been adapting to that geographic area for hundreds of thousands of years. The Eurasian environment shap­ed Neanderthals’ evolution, including the development of adaptations to viruses and other pathogens that were present there but not in Afr­ica, reports Science News.

The study published n the journal Cell gives new details about the role of adaptive introgression, or hybridisation between species, in human evolution. “Some of the Neanderthals had adaptive mutations that gave them adv­a­n­tages against these pathog­e­ns, and they were able to pass some of these mutati­ons on to modern humans,” explains Enard, who completed the work while he was a postdoctoral researcher at Stanford University. “That’s called positive natural selection – it favours certain individuals that carry these advantageous mutations.”

Enard and Dmitri Petrov, the professor of biology at Stanford University and senior author of the new study, use bio-informatics tools to study global patterns of evolution across tens of thousands of years. Their earlier research focused on how viruses impacted the evolution of humans. Two years ago, they reported that about one-third of protein adaptations since humans split from other great apes was driven by a response to infectious viruses. The new work built on those findings look­ed at which of those adaptations may have come from Neanderthals.

In the current study, the investigators annotated tho­u­s­ands of genes in the hum­an genome that are known to interact with pathogens – more than 4,000 of the 25,000 total genes. “We focused on these genes because the ones that interact with viruses are much more likely to have been involved in adaptation against infectious disease compared with genes that don’t have anything to do with viruses,” Enard says.

They then looked at whet­her there was an enrichment of stretches of Neanderthal DNA in those 4,000 genes. Based on the analysis, Enard and Petrov found strong evidence that adaptive genes that provided resistance against viruses were shared between Neanderthals and modern humans.

“Many Neanderthal sequences have been lost in modern humans, but some stayed and appear to have quickly increased to high frequencies at the time of contact, suggestive of their selective benefits at that time,” Petrov says. Enard says. “This study suggests that one of the roles of those genes was to provide us with some protection against pathogens as we moved into new environments.”

The Indian botanist and scientist CD Varghese, St Th­omas College, Thrissur, agrees with these findings. In an interview given recently at Aluva, Kerala, he held virus is responsible to share the genetic material even between two living human beings.

Thus, the complex and magnificent life is intimately connected with each other, both vertically and horizontally. We are dependent on fellow-human beings, on our ancestors and on other life forms. Truly life is a web of interacting genetic and mimetic interactions.

(The writer is professor of science and religion and author of Death: Live it!)

Kuruvilla Pandikattu