Scientists say that our genes contain the hints of an evolutionary tug of war that took place in the wombs of our ancestors, balancing the drive to bigger brains with the need for a strong immune system

The push and pull of these genetic variants apparently became more pronounced after pre-humans branched off from the ancestors of chimpanzees, according to biologists Peter Parham of Stanford University and Ashley Moffett of the University of Cambridge. The two biologists focus on how particular types of white blood cells, known as natural killer cells, work in the human immune system. In addition to fighting infections and tumors, natural killer cells help regulate the growth of the placenta during pregnancy. Humans are unique among primates in having two variants of the genes that control the receptors for natural killer cells. "B haplotypes are favored during reproduction. A haplotypes are more specialized toward defending against infections," Parham explained. "These are subtle effects. On average, if you're an individual that has two A haplotypes and no B haplotype, you're going to have a slightly more robust immune system in terms of dealing with disease." Having two B haplotypes, in contrast, would allow for a more robust placenta. That would provide the fetus in the womb with more of the nutrients needed to grow a bigger brain. "In the course of human evolution, you had the evolution of these B haplotypes, which really did enable the brain to get bigger. ... There are correlations between the size of the brain of the baby and these genetic factors," Parham said. A detailed analysis of human genetic diversity suggests that the genes for the B haplotype emerged in the time frame lasting from about 7 million years ago to 1.7 million years ago. That would cover a period starting with the divergence of human and chimp ancestors, and ending with the human migration out of Africa. The A-vs.-B breakdown is found in all present-day human populations, suggesting that both variants were important to have for different situations. Parham and Moffett speculate that the A variant was important when a population was facing a disease epidemic, while the B variant became important for brain-building once the epidemic passed.