A team led by Stanford University School of Medicine in California, USA, has recently carried out a study which suggests that some inter breeding amongst our prehistoric ancestors may have lead to improvements in our immune system. Their studies were based on an immune systen class 1 gene known as HLA (human leucocyte antigen) and a particular variant called HLA-B*73, which is found in modern humans.
     To understand the significance of their findings we must first look at ancient migration patterns. Between 130,000 and 30,000 years ago Eurasia was occupied by Neanderthals and a lesser known species, the Danisovians (the first remains of which were only recently discovered in 2008 in Siberia). Analysis of their remains shows that they had the HLA immune gene in question. Then, approximately 65,000 years ago, the ancestors of modern Europeans migrated north, out of Africa and into Eurasia.
     The gene was not present in the immigrants, as can be attested to by analysis of the remains of those who stayed behind. So a conclusion can quickly be drawn that modern Europeans obtained their immune gene by inter breeding with the then native population of Neanderthals. In fact, Europeans have gained nearly 50% of their HLA genes from this inter breeding.
     The data obtained is quite raw and further research will be needed before any definitive conclusions can be reached. An early sceptic of the findings is John Hawks, assistant professor of anthropology at the University of Wisconsin-Madison, USA, who is quoted as saying "It is difficult to align ancient genes in this part of the genome. Also, we don't know what the value of these genes really was, although we can hypothesise that they are related to the disease environment in some way".

Edited from Popular Archaeology (25 August 2011) BBC News (26 August 2011) Discover Magazine (27 August 2011)