LONDON (Dispatches) -- One day about 70,000 to 60,000 years ago a wave of Homo sapiens trekked out of Africa and survived. If you are not African, you descended from these people.
As this group unhurriedly ambled along, over generations, it spent 15,000 to 20,000 years on the Persian plateau. In that time, their red blood cell systems underwent rapid adaptation, suggests a new paper published last week by Stéphane Mazières of Aix Marseille University and colleagues in the Nature journal Scientific Reports.
This is not the first paper to propose that our ancestors exiting Africa ground to a standstill on the Persian or Iranian plateau. We know modern humans were wandering out of Africa at least 200,000 years ago; human remains have been found in Arabia from 85,000 years ago; and there are findings that support a major wave of migration around 70,000 to 60,000 years ago.
The question is about that last wave; where it went; and how it mutated. Because, the modern human lineages who colonized Europe got there less than 45,000 years ago, recent research has shown. Modern human lineages who reached Europe earlier apparently all went extinct.
Hence the notion that in the gap from 70-60,000 to 45,000 years ago, the humans were happily dwelling on the hospitable Persian (Iranian) plateau, where the conditions were splendid. While they were there, we learn - their red cell blood group systems mutated. The changes may have helped them conquer Eurasia while all other hominid species, including other modern humans who got there earlier, died out.
What was the change? New genetic variations of the rhesus or Rh factor, the team deduced based on genetic analysis of 22 ancient modern humans and 14 Neanderthals who lived in Eurasia from 120,000 to 20,000 years ago.
The rhesus factor is named for the monkey species exploited in researching proteins that may exist on the surface of red blood cells, which have become known as Rhesus or Rh factors. Some of us are Rh positive – our red blood cells sport these proteins. Some of us are Rh negative. Positive is the most common condition but Rh negative does not imply sickliness.
Trouble ensues when a mother and her fetus have different Rh types: the first birth should be fine but will expose the mother to the child’s blood system. The mother will become sensitized to the child’s blood group, and her immune system will attack subsequent fetuses.
This gives rise to the thought that when Homo sapiens and Neanderthals and/or Denisovans mated, which they did, many potential offspring may have been doomed because of Rh incompatibility.
How do we know the new Rh variants emerged after the group left Africa, but before they reached Europe?
Reason one: These variants, called RHD and RHCE, do not exist in Africans, only in non-Africans (about 40% of Eurasians have them, Mazières adds).
Theoretically these variants could have emerged in Africa early on, and subsequently disappeared there while persisting in the population that exited. That is not the most parsimonious explanation, Mazières and the team explain.
Reason two: The variants didn’t exist in European Neanderthals or Denisovans. The humans didn’t “obtain” these variants by mixing with these species.
Three: These variants did exist in some of the modern humans who were discovered in Europe from about 45,000 years ago, who had originated in the group mucking about the Persian plateau.
A previous study led by Leonardo Vallini showed that the postulated Persian plateau group produced three pulses of migration into Eurasia, Mazières explains by email. Two of these pulses were more than 45,000 years ago and are represented by modern human remains found at Zlaty Kun in the Czech Republic, Oase in Romania, Bacho Kiro in Bulgaria, Tian Yuan in China and in a few spots in Siberia.
The Siberian early modern humans had the new Rh gene, Mazières says. The others didn’t.
“The novel form of the RH gene is found in Ust’Ishim, Sunghir, and Kostenki, so in three individuals that expanded in at least two distinct waves from the Persian Plateau,” he sums up that chapter.
After about 38,000 years ago there was a third expansion from the Persian plateau, Mazières says. It produced modern humans whose remains have been found from Siberia to Belgium.
As for the hiatus on the plateau, it is supported not only by the genetic evidence but by the emergence of new stone tool technologies, he adds. And that period is long enough for the people to differentiate.
Mazières and the team support their thesis by noting that populations who occupied the Beringia land bridge between Siberia and Alaska starting 20,000 years ago stayed there for thousands of years and differentiated before walking onto the Americas. (Possibly with dogs in tow, by the way). That explains how gene variants unique to their Native Americans descendants emerged, the team says.
And possibly, the team suggests, these new blood groups that sprang up on the Persian plateau gave the humans an adaptive advantage that helped them colonize Eurasia. They underwent other adaptations, too – “They developed new genetic lineages,” Mazières says. “From the entire genome point of view, not only red cell genes – they acquired new mutations and spread them all over Eurasia.”
Certainly their arrival and spread coincided with the final extinction of all other human types there (as well as other modern human lineages); gone were the Neanderthals and the Denisovans and the diminutive peoples of southeast Asia. And maybe even to erectus himself, who had survived in the jungles of Java until at least 108,000 years ago.
The paleogenomic analysis didn’t go back as far as the earliest postulated mating between modern humans and Neanderthals, which was as much as a quarter million years ago. The analysis starts with Neanderthals from about 120,000 years ago.
Intriguingly, blood group diversity in the Neanderthals does not seem to have changed from 120,000 years ago until they went extinct about 40,000 years ago. There is little point in speculating about why mutations didn’t happen but the team says it could attest to inbreeding.
Separate work has indicated that the other human species lived in small bands. Inbreeding has been speculated as their population sizes shrank, and also based on fossil evidence. For instance, a 2013 paper detected that the parents of a Neanderthal woman in Siberia had been half-siblings and that their ancestors had been, shall we say, close.
Further support for the theory that the modern humans suddenly developed new Rh groups on the Persian plateau may be found in Ust’-Ishim, one of the modern humans who lived in Siberia 45,000 years ago. He did have the new-fangled blood system – as well as some Rh genes that no longer exist.
What have we in Ust-Ishim? A leg bone who represents a man who represents the “Persian plateau population” who reached Europe early, and whose line died out with the Neanderthals and Denisovans. His is a lost lineage. Only later lineages survived.
So what have we? About 40% of non-Africans today have blood group system genes that do not appear in Africans. These Rh genes apparently evolved while our species was squatting on the Persian plateau for 15-20,000 years – that’s long enough.
Some members of this Persian group who went to Europe earlier than others had these new Rh genes. They also had some Rh genes that are lost to posterity because these lineages died out.
Meanwhile, the new paper shows that the Neanderthals had a unique Rh blood group – that still exists today in a handful of Oceanian humans. They would have obtained these genes through ancient introgressions, the team says.
In other words a Neanderthal Rh gene entered the human gene pool before the peopling of Oceania which means, more than 50,000 years ago. Likely, Mazières theorizes – that contribution arose from mating between Neanderthals and humans in the Near East around 100,000 years ago. There is proof of their meeting in Israel. Then a branch of humans, after that rest stop on the plateau, continued on and went right, likely walking along the southern rim of the Eurasian continent.
Today Rh incompatibility can be treated. Then it may have contributed to the other species’ extinction as modern humans spread while their populations dwindled. Human and Neanderthal and Denisovan Rh factors were by and large incompatible and they may have had nobody to left to mate with us.