Scientists reported that they were able to extract the oldest known Homo sapiens DNA from human remains found in Europe, and this information helps flesh out the overall history of the human species, beginning with Neanderthals.
Ancient genomes sequenced from 13 bone fragments found in a cave beneath a medieval castle in Ranis, Germany, belonged to six people, including a mother, daughter and distant relatives, who lived in the region about 45,000 years ago, according to a study published Dec. 12 in the journal Nature.
Signs of Neanderthal ancestry have been found in genomes. The researchers found that ancestors of early humans living in and around Ranis likely met and had children with
Neanderthals some 80 generations earlier, or 1,500 years earlier, although this interaction did not necessarily occur in the same place.
Scientists have known that early humans interbred with Neanderthals since the first Neanderthal genome was sequenced in 2010, a sensational discovery that left a genetic legacy traceable to this day.
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However, when exactly, how often and where this pivotal and mysterious event in the history of mankind took place, it was difficult to find out. Researchers believe that interspecies relations arose somewhere in the Middle East, when part of Homo sapiens
left Africa and encountered the Neanderthals, who had lived in Eurasia for 250,000 years.
A more detailed description of the study into the origins of Neanderthals, was published Dec. 12 in the journal Science. The scientific paper, which analyzes information from the genomes of 59 ancient humans and 275 present-day humans, confirmed a more precise chronology, finding that most of the Neanderthal ancestors of modern humans can be attributed to "a single, shared extended period of gene flow."
"We are much more similar than we are different from each other," Priya Murjani, an associate professor of molecular and cell biology at the University of California, Berkeley, said at a press briefing, author of the study published in the journal Science. "The differences that we imagined to be very large between these groups actually turned out to be very small genetically. They appear to have mixed with each other over a long period of time and lived side by side."
The study revealed a pivotal period that began about 50,500 years ago and ended about 43,500 years ago - shortly before the extinct Neanderthals began disappearing from the archaeological record. During those 7,000 years, early humans encountered Neanderthals and had children on a fairly regular basis. The peak of activity occurred 47,000 years ago.
The study showed how some genetic traits inherited from Neanderthal ancestors, which make up 1 to 3% of the modern human genome, have changed over time.
Some, such as those related to the immune system, were beneficial to humans during the last ice age when temperatures were much lower, and continue to be beneficial today.
According to evolutionary geneticist Tony Capra, a professor of epidemiology and biostatistics at the Bachar Institute for Computational Health Sciences at the University of California, San Francisco, the two studies provide "significant confidence" that there was a process by which humans and Neanderthals exchanged genes, called introgression by geneticists.
"Genetic data pertaining to this crucial period of our evolution are very rare," says Capra, who was not involved in the study. "These studies emphasize that the availability of even a few ancient genomes provides a powerful perspective that has allowed the authors to refine our understanding of human migration and Neanderthal introgression."
The scientists, who were working on two research projects, decided to publish their work simultaneously when they realized they had reached similar conclusions separately.
How Neanderthal ancestry affected human genes
The study, published in the journal Science, found that genetic variants inherited from Neanderthal ancestors are unevenly distributed across the human genome.
Some regions, which scientists call "archaic deserts," are devoid of Neanderthal genes. These deserts probably evolved rapidly after the two groups interbred, within 100 generations, perhaps because they resulted in birth defects or diseases that would have affected the offspring's chances of survival.
"This suggests that hybrid individuals with Neanderthal DNA in these regions were significantly less fit, which was likely associated with severe disease, mortality or infertility," Capra says.
In particular, the X chromosome was archaic. According to Capra, the influence of disease-causing Neanderthal variants may have been stronger on the X chromosome, perhaps because it is present in two copies in females and only one copy in males.
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"There are also many genes in the X chromosome that are associated with male fertility when altered, so it has been suggested that part of this effect may have resulted from introgression leading to sterility in male hybrids," he says.
The Neanderthal gene variants most commonly found in ancient and modern Homo sapiens genomes are associated with traits and functions such as immune function, skin pigmentation, and metabolism, with the frequency of some of them increasing over time.
"Neanderthals lived outside of Africa in the harsh climates of the Ice Age and were adapted to the climate and pathogens in that environment. When modern humans left Africa and interbred with Neanderthals, some individuals inherited Neanderthal genes that presumably allowed them to better adapt and survive in that environment," says Leonardo Yasi, co-author of the paper in Science and a doctoral student at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
The people who lived in Ranis had 2.9% Neanderthal ancestry, just like most modern humans.
The new chronology allows scientists to better understand when humans left Africa and migrated around the world. According to the authors of the study, the main wave of migration from Africa was completed 43,500 years ago, as most modern humans outside Africa have Neanderthal ancestors originating from this period.
However, there is still much scientists don't know. It is unclear why people in East Asia today have more Neanderthal ancestors than Europeans, and why there is little evidence of Homo sapiens DNA in the genomes of Neanderthals from this period.
Although the genomes sequenced from humans from Ranis are the oldest Homo sapiens genomes, scientists have previously extracted and analyzed DNA from Neanderthal remains dating back 400,000 years.
A lost branch of the human family tree
The people who called the cave at Ranis their home were among the first Homo sapiens to live in Europe.
These early Europeans numbered several hundred people, including a woman who lived 230 kilometers from Zlaty Kun in the Czech Republic. DNA from her skull was sequenced, and the scientists who published the study in Nature were able to link her to people from Ranis.
According to the study, these people had dark skin, dark hair and brown eyes, possibly indicating their relatively recent arrival from
Africa. Scientists continue to study remains from the site to get an idea of their diet and lifestyle.
This family group was part of a primitive population that eventually died out, leaving no trace of ancestors in living humans today. According to Johannes Krause, director of archaeogenetics at the Max Planck Institute for Evolutionary Anthropology, other lineages of ancient humans also died out about 40,000 years ago and disappeared just like the Neanderthals.
These extinctions may indicate that Homo sapiens played no role in the demise of Homo neanderthalensis. "We saw that the story of human survival is not always a success story," says Krause, senior author of the study in Nature.