WHEN the first modern humans left Africa they were ill-equipped
to cope with unfamiliar diseases. But by interbreeding with the
local hominins, it seems they picked up genes that protected
them and helped them eventually spread across the planet. The
publication of the Neanderthal genome last year offered proof
that Homo sapiens bred with Neanderthals after leaving Africa.
There is also evidence that suggests they enjoyed intimate
relations with other hominins including the Denisovans, a
species identified last year from a Siberian fossil. But what
wasn't known is whether the interbreeding made any difference to
their evolution. To find out Peter Parham of Stanford University
in California took a closer look at the genes they picked up
along the way. He focused on human leukocyte antigens (HLAs), a
family of about 200 genes that is essential to our immune
system. It also contains some of the most variable human genes:
hundreds of versions - or alleles - exist of each gene in the
population, allowing our bodie