About 100,000 years ago, human evolution has reached the mysterious turn of: the number of our ancestors fell to about 5-10 thousand people who lived in Africa. Later, this small, compared to what it was, the number will be "modern" man, as we know it today, and homo Sapiens. Furthermore, the number of these people will grow rapidly, however, divided mankind into races. Thus, our closest ancestors supplanted earlier, such as the Neanderthals.

The reason for reducing the population of our ancestors and remains a mystery.  Scientists have put forward a variety of suggestions - from gene mutations and climatic hazards to cultural achievements and volcanic eruptions  and now it was suggested that another one of the possible factors: infectious diseases.

In a paper published online June 4, National Academy of Sciences of the USA, described the assumption of the research team from the University of California at San Diego that the inactivation of two genes associated with the immune system may have identified the further development of mankind.  It is awarded to individuals increased protection against some strains of pathogenic bacteria such as E. coli and group B streptococci, which are a common cause of sepsis and meningitis in human fetuses and newborns.

"Within a small, restricted population of one single mutation can have a big effect on its further development" - said the study's lead author Ajit Varki, professor of medicine at the University of California. "We have discovered two genes that are not functional in the human body, but operate in organisms of primates such as chimpanzees. The absence of these genes is almost one hundred percent fatal provided for fetuses and newborns.  Thus, the survival of the human species depended on the resistance of pathogens, or that it is possible gene mutation or elimination of the same target proteins, pathogen used to win the war".

Varki and his colleagues from Japan and Italy suggest just the latest.  In particular, they point to the inactivation of the two receptors Power acid (siglecs) that model the immune response and are part of a large family of genes.  Scientists suggest that they played an important role in human evolution.  Research Dr. Victor Nizeta Varki from the group showed that some infections can be used siglecs to change the nature of the immune response in their favor.  In the latter study, the researchers found that Siglec-13 gene is no longer part of the human genome, although it remains in the genome of chimpanzees, our closest relatives among the animals.  Another gene, Siglec-17, while remaining a part of our genome, however, was substantially modified in the course of evolution, becoming shorter, inactive protein that has no effect on the infection.

Thus, the scientists suggest that it is the impact of modern microbes and genetic mutation, which resulted in the man lost uninterrupted protection from them, have become a key point in our evolution.  In the struggle for survival, which killed a great number of our ancestors, the people got what is now known in biology acquired immunity - the ability to produce antibodies to resist pathogenic microbes.

Scientists were able to study the fossil artifacts, which do indicate that the ancestors of modern humans are faced with the threat of extinction due to infectious diseases in the period 100-200 thousand years ago.  This "selective sweep" devastated the ranks of people, leaving only those who have been some mutations in the genome, which allowed them to have immunity.

However, Varki noted that each stage of evolution - is the result of many different factors.  "Speciation depends on many things.  And we believe that infectious diseases are one of them "- he said.