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Doses of Denisovan and Neanderthal DNA may have provided immunity to many modern humans.
Not much remains of the Denisovans—only a little girl’s finger and a couple of big teeth from a cave in Siberia. We don’t even know what Denisovans looked like. But some of our ancestors did. And so did some Neanderthals.
Genomic sequencing of Neanderthals and the more recently discovered Denisovans1 has suggested that each of these groups of humans is represented in the genomes of many living humans. According to earlier estimates, interbreeding of more modern types of humans with Neanderthals has left a Neanderthal presence in the DNA of 1–4% of modern Eurasians. Mixing with Denisovans in the past appears to have left a footprint in the DNA of 4–6% of Melanesian people. (Melanesians are native to islands in and around Papua New Guinea, Fiji, and the Solomons.) A more focused bit of detective work, however, has shown a much more widespread impact in a key area of the genome of very many humans.
When the Denisovans and the Neanderthals dropped off the map, the immune memory stored in their DNA did not.
When the Denisovans and the Neanderthals dropped off the map, the immune memory stored in their DNA did not. Researchers in Dr. Peter Parham’s lab at Stanford decided to pick up where the Max Planck Institute’s Svante Paabo—who sequenced the archaic human DNA—left off. Parham’s team decided to search for a rare genetic variation in the HLA-B portion of the human genome.
HLA is a group of genes which contains the database used by the immune system. HLA genes supply the information that allows the immune system to recognize foreign invaders. When exposed to dangerous pathogens, people whose HLA types contain the information to fight those particular pathogens most effectively have the best chance of combating the infection. There are many variations for the genes in the HLA complex, so the chance of two people who are not identical twins having identical HLA sequences is practically zero.
The Stanford group found that Denisovan DNA contains the elusive HLA-B*73 allele. Nowadays, the rare gene variant HLA-B*73 is primarily found in about 5% of the population of western Asia.2 Continuing the search for other HLA matches, the team found additional Denisovan and Neanderthal footprints in other parts of the HLA complex.
“Certain traits coming from these archaic humans have become the dominant form,” said Parham. “The likely interpretation was that these HLA class variants provided an advantage to modern humans and so rose to high frequencies.”3 HLA genes are on the front line of the body’s immunological defenses. For that reason, variations of HLA types should be extremely responsive to natural selection.
Parham speculates that “cross breeding wasn't just a random event that happened. . . It gave something useful to the gene pool of the modern human.”4 Denisovan- and Neanderthal-derived portions of the HLA have become prevalent in many modern populations, estimated to be present in over 50% of Europeans, over 70% of Asians, and over 95% of some Melanesian groups.5 If these estimates are accurate, then the immunity provided by those gene types at some point in time may well have offered a survival advantage and increased in certain populations through natural selection or other mechanisms.
This summer a group of human origins researchers attended a symposium near the Denisova Cave to sort out the mystery all this genetic information has created.
This summer a group of human origins researchers attended a symposium near the Denisova Cave to sort out the mystery all this genetic information has created. They debated the various models for human evolution and population dispersion. Hoping more data will be forthcoming, they concluded, “Genomic data have already shown that our ancestors mingled with archaic humans, who may have given us valuable immune cell types. But it's not clear when and where this happened.”6
From a biblical perspective, there really is no mystery. After the rebellion at the tower of Babel,7 people groups dispersed and migrated to diverse places. Isolation of groups resulted in the formation of some distinctive characteristics as genetic diversity within populations decreased. Variations of humans in the fossil record—such as Neanderthal and Denisovan and Homo erectus—as well as in living populations today are ultimately traceable to that dispersion.
As Stanford’s Abi-Rached notes, “For small migrating populations, admixture with archaic human could restore HLA diversity following population bottleneck, and also provide a rapid way to acquire new, advantageous HLA variants already adapted to local pathogens.”8 And that’s not evolution. That’s just genetics, natural selection, and the biblical history of the world.
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