Modern-day humans contain up to 4% of NeanderthalDNA in their genome. They shared a common ancestor with Neanderthals up until about half a million years ago. Humans and Neanderthals started to interbreed tens of thousands of years ago, producing human-Neanderthal hybrids. Over time, the prevalence of Neanderthal DNA has steadily decreased over human generations until it has reached its current amount.
The reasons as to why scientists believe this interbreeding occurred and why Neanderthals do not exist to this day is because of the small population sizes of Neanderthals. Because they originally tended to live in harsher climates than humans, population sizes were smaller to cope with the extreme conditions. This resulted in a higher amount of interbreeding in Neanderthal populations, leading to less gene variation and a lower chance of natural selection.
The scientists of this study used mathematical equations to predict how allele frequencies of Neanderthal DNA inthe human genome changed over time. They suspect that while the chosen alleles may be neutral in Neanderthals, they may pose a greater threat to hybrids, resulting in high mortality rates. Therefore, offspring of any surviving hybrids would have a greater chance of survival if the hybrid parent mated with a human, decreasing the amount of harmful Neanderthal DNA in the offspring. This repetition of surviving hybrids mating with humans, and the resulting offspring also mating with humans, lowered the frequency of Neanderthal alleles in generations over time. When the hybrids reproduced with humans, there was greater gene flow and any disadvantages from the Neanderthal DNA could be effectively bred out over time. The breeding habits and survival rates of different generations of hybrids and their offspring would result in the varying degrees of prevalence of Neanderthal DNA in modern day humans. This study is important in understanding the origins of modern-day humans and how we came to be.