Sleeping around gave early humans immune boost from Neanderthals, Denisovans

[n2doc]

By John Timmer


The ancestry of modern humans has gotten much more interesting in recent years. Completion of the Neanderthal genome came with a bit of a surprise: they may be extinct, but many modern humans carry pieces of their genome, a sign that our ancestors had some rather significant interactions with them. About the same time, we learned that there was another group of archaic humans called the Denisovans present in Asia at the same time; nine months later, we learned that our ancestors had slept with them, too.

The evidence came in the form of DNA that was unevenly distributed within modern human populations. Modern humans had gotten their start in Africa, but Neanderthal or Denisovan DNA is rare or absent in African populations. Instead, it's more common in Asian and European populations, which suggests that interbreeding only took place after modern humans had migrated out of Africa.

What the study didn't touch on was why DNA sequences from archaic humans is still around. Was interbreeding frequent, and the DNA that remains simply present by random chance? Or did specific regions end up providing our ancestors with an evolutionary advantage? A study that will appear in today's issue of Science indicates that, for at least some sequences, the answer is the latter: they're still here because they're useful.

The DNA sequences in question encode a group of related genes called HLA. These genes serve a very specific purpose: they take pieces of proteins from inside the cell and present them on the cell's surface, where the immune system can look them over. If the immune system doesn't like what it sees, it will launch an attack against the cell. In this capacity, the HLA genes are key to determining whether tissues match well enough to allow an organ transplant with a minimal risk of rejection.

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http://arstechnica.com/science/news/2011/08/sleeping-around-gave-early-humans-immune-boost-from-neanderthals-denisovans.ars