Until Peter Parham from Stanford University (California, USA) took a closer look at human genes, it was not known whether interbreeding with hominins made any difference to human evolution. The recently published Neanderthal genome offered proof that Homo sapiens bred with Neanderthals after leaving Africa.
     At the Royal Society in London, Peter Parham presented his findings at a talk on Human evolution, he focused on the human leukocyte antigens (HLAs), a family of about 200 genes essential to the human immune system, and compared these to HLAs of Neanderthals and Denisovians. Findings indicate that non-African humans picked up new alleles (forms of a gene) and particular alleles found in Neanderthals and Denisovians are common to modern Europeans and Asians but are not found in modern Africans.
     The first modern humans to leave Africa would have been ill-equipped to deal with unfamiliar illnesses. As Neanderthals and Denisovians had lived outside Africa for over 200,000 years prior to meeting Homo sapiens, they would have had plenty of time to develop tolerance to local diseases, thus, by interbreeding, Homo sapiens would have increased their rates of survival. The rates of alleles shared with other hominins increases the further east across the world, and this could be that those migrating North would have faced fewer diseases than those traveling to the tropics of south-east Asia says Chris Stringer from the Natural History Museum, London.

Edited from NewScientist (16 June 2011)