تأثیر DNA نئاندرتال بر ریخت و رفتار انسانهای مدرن
Neanderthal DNA Influences the Looks and Behavior of Modern Humans
Neanderthals are the closest evolutionary relatives identified to date of all people living today. They are so close to us that some people — those of European and Asian heritage — retain a fair amount of DNA from these big-brained, big-headed hominids who once dominated much of the world.
Now new research finds that Neanderthals are even more with us than previously suspected. A paper published in the journal Science finds that individuals whose primary heritage lies outside of Africa possess 10–۲۰ percent more Neanderthal DNA than was reported earlier, with probable influences on their appearance, behavior, health, and even habits, such as smoking.
East Asians were found to carry somewhat more of this DNA, 2.3–۲.۶ percent, than people now living in Western Eurasia, 1.8–۲.۴ percent.
“There are two hypotheses that can explain this difference,” lead author Kay Prüfer, of the Max Planck Institute for Evolutionary Anthropology, told Seeker. “Either Europeans possess ancestry from a modern human group with little Neanderthal admixture, or East Asians had additional admixture with Neanderthals.”
For now, the answer remains a mystery.
Less mysterious is fossil evidence showing that a related group of Neanderthals lived in Vindija Cave, Croatia, around 52,000 years ago — just 12,000 years before Neanderthals as a distinct type of human are thought to have died out in Europe. Prüfer and her team sequenced the genome of one of the Vindija’s females. The achievement marks only the second time that a Neanderthal genome has been sequenced in detail.
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Prüfer and her team compared the newly generated sequence to that of the earlier detailed one for what is known as the Altai Neanderthal, whose remains were found in Denisova Cave in the Altai Mountains of southern Siberia. The researchers also compared these two sequences to those of still other known Neanderthals, such as the Mezmaiskaya Neanderthal of southern Russia.
“What we see is that the admixing Neanderthals were more closely related to Vindija and Mezmaiskaya compared to the Altai individual, and that they have a last common ancestor with Vindija and Mezmaiskaya sometime between 80,000 and 150,000 years before the present,” Prüfer said.
People of European and Asian heritage today therefore retain DNA from a population of still-unknown Neanderthals who are ancestral to the identified ones from Croatia and southern Russia.
The genomes of both the Vindija and Altai Neanderthals provide evidence that Neanderthals lived in small and isolated populations of no more than about 3,000 individuals per region. Climate and available resources likely contributed to keeping their numbers low.
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The Altai genome suggested that the individual’s parents were half-siblings, which led scientists to suspect that extreme inbreeding may have been ubiquitous among Neanderthals. While the Vindija female shared a maternal ancestor with two of the three other Neanderthals found in the Croatian cave, she did not show evidence of significant inbreeding.
Prüfer and her team additionally identified a wealth of new gene variants in the Neanderthal genome that are influential in people today of European and Asian heritage. The variants are related to blood levels of LDL cholesterol and vitamin D, as well as eating disorders, body fat accumulation, rheumatoid arthritis, schizophrenia, and the response to anti-psychotic drugs.
In most cases, the researchers know only of the association with the identified gene variants, and not what the genes might specifically do. But it is established, Prüfer noted, that the “LDL cholesterol variant from Neanderthals is associated with a lower level, that is, in the direction of reduced chance of heart disease.”
A second new paper concerning Neanderthals, published this week in the American Journal of Human Genetics, finds that their genetic influence in living populations also extends to skin tone, hair color, sleep patterns, mood, and a person’s smoking status.
“What was somewhat surprising is that we observe multiple different Neanderthal alleles contributing to skin and hair tones,” Janet Kelso, a computational biologist who co-authored the study with her colleague Michael Dannemann, told Seeker.
“Some Neanderthal alleles are associated with lighter tones and others with darker skin tones, and some with lighter and others with darker hair colors,” she added. “This may indicate that Neanderthals themselves were variable in these traits.”
The skin tones, she added, ranged from very fair to dark olive.
The origins of red hair still remain a mystery, she indicated, as a clear link to Neanderthal ancestry could not be established.
“If variants contributing to red hair were present in Neanderthals, they were probably not common,” Kelso said.
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She and Dannemann, both from the Max Planck Institute for Evolutionary Anthropology, made these determinations and more after looking for known Neanderthal genetic variants in the UK Biobank, a database of information on 112,000 UK individuals that includes genetic data, along with details related to their physical appearance, diet, sun exposure, behavior and disease.
Per the other study led by Prüfer, Kelso and Dannemann could only identify associations between Neanderthal genetic variants and traits of people today, as opposed to determining what these variants actually did in Neanderthals, and how they precisely function now in their distant living relatives.
Of her own research, Kelso said, “We cannot infer from this study that Neanderthals smoked, or suffered from other addictions or mood and sleep disorders.”
“What we instead learn,” she continued, “is that Neanderthal DNA that is present in people today has detectable effects on behavior. This suggests that Neanderthal DNA influences the brain in ways that affect these behaviors.”
“It’s very important to point out that we can’t start blaming Neanderthals for these traits,” she quickly added. “Behavioral traits such as sleep, mood and addiction behavior are complex, and there are many different parts of our genomes that contribute to them.”
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As for why the percentage of Neanderthal DNA in some modern humans still appears to be so low, Kelso explained that there was selection against such genes in early modern human populations. In other words, Neanderthal DNA has been purged from modern humans over time.
People of African heritage also retain DNA from as-of-yet unidentified early hominids. Other recent studies have found that Homo sapiens was not the only hominid in Africa around 350,000 years ago. Homo naledi was likely there too, along with possibly still other archaic human species. How they interacted, or not, remains a mystery for now.
Another unknown is whether or not the ancestors of Neanderthals evolved in Africa. Prüfer said that the new studies “do not help with this question.”
But research in this field does suggest that each person alive today possess a unique genome tied to his or her particular ancestry — both recent and going back tens of thousands of years. Beyond environmental influences, these DNA signatures underlie the very essence of people, affecting everything from their appearance to their health.