According to the latest issue of Genetics, the Neanderthal genome included harmful mutations that made them about 40% less reproductively sound than modern humans. Studies have found that while a lot of that burned has been bred out, Non-African humans did inherit some of these genes. This is because Non-African humans mated with Neanderthals. The results
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"Neanderthals are fascinating to geneticists because they provide an opportunity to study what happens when two groups of humans evolve independently for a long time--and then come back together," says study leader Kelley Harris, of Stanford University. "Our results suggest that inheriting Neanderthal DNA came at a cost."
Previous studies showed Eurasian hominids were more likely to be inbred, making them less genetically diverse than we are today. The population of Neanderthals was small, making mating amount family members more common - and it isn't like they knew the dangers.
Somewhere between 50,000 and 100,000 years ago, "anatomically modern" humans left Africa and moved to where Neanderthals were. They interbred, mingling the distinct genomes. This means that even though it isn't an even contribution, a small fraction of the genome of non-African populations has Neanderthal in them.
"Whenever geneticists find a non-random arrangement like that, we look for the evolutionary forces that caused it," says Harris.
Harris and Rasmus Nielsen hypothesized that there was natural selection involved. In some small populations, like with the Neanderthals, natural selection isn't as effective because it has the chance to outside influence. This allows for the harmful mutations to persist, rather than stronger genes taking over.
The pair used computer programs to simulate the mutation during the evolution of Neanderthals and estimated how humans were impacted by the influx of genetic variants.
The results showed that there were mild (but still harmful) effects. According to estimates, it made Neanderthals 40% less fit than human, meaning that they were 40% less likely to reproduce.
The research also shows that humans and Neanderthals mixed more freely than we ever thought before. In fact, Neanderthal sequences make up about 2% of the genome. This is probably down from the 10% at the time of interbreeding, so the number will likely continue to go down unless Neanderthals somehow come back.
As for what the Neanderthal genome has done, it looks like non-Africans may have a 1% lower reproductive fitness rate. There were also changes to skin color and immunity.
"Genetic rescue is designed to move gene variants from an outbred population to an inbred population," says Harris. "Our results suggest managers must ensure that this movement only goes one way; otherwise harmful mutations from the inbred population may lower the fitness of the outbred group."