Neanderthals and modern humans diverged from a common ancestor approximately half a million years ago. Neanderthals lived in colder climates in Eurasia, so they evolved barrel chests, large skulls and strong hands. In Africa, modern humans evolved with short faces, a prominent chin, and slender limbs. Then, almost 50,000 years ago, the two species interbred and modern humans spread out of Africa. The theory of interbreeding has been up for debate; today, up to four percent of the genes of non-Africans are Neanderthal in origin, which could have influenced a range of traits. Questions that many people pose is “why did a Neanderthal-human hybrid not predominate? The answer is that in small populations, natural selection is less effective and Neanderthals have a very small population over a vast time period, hundreds of thousands of years. Since Neanderthals lived in harsh conditions, they have a very small population according to a biology professor at University of California Davis, Graham Cook.
Neanderthals were more inbred than modern humans, so there were more mutations that had an adverse effect on their populations including increasing an offspring’s risk of disease. In 2014, a professor from Harvard, David Reich, and his group found that Neanderthal DNA is usually located far away from important genes in the human genome, which provided the first building block of evidence that natural selection was working against Neanderthal DNA. At first they thought infertility was the explanation for this pattern, but in a different study it was concluded that differences in population size was a better explanation. In another study that was done in April of 2016, the authors started with historical estimates of Neanderthal and human population sizes. From the data, they were able to conclude that Neanderthals most likely had a greater chance of passing down genes that were somewhat more disadvantageous. In the original study that this article focused on, the simulations showed that the early hybrids would have been much less fit than humans, which would have decreased the amount of Neanderthal DNA around modern human genes. This new theory is so crazy to read about because we are always taught about how similar we are to Neanderthals and how important they were to our evolution, but now we are learning more about the genome and it is basically saying that those genes are there but they are less important than our genes.