The group traveled thousands of miles across Africa and the Middle East, finally arriving in the dark forests of the New World. They were part of a long-vanished tribe of modern humans, Homo sapiens, who were among the first to invade Europe.
There, these people probably met their distant relatives – Neanderthals.
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first encounter
By 75,000 years ago, and perhaps up to 250,000 years ago, the ancestors of most modern Eurasians first moved out of Africa and into Eurasia. Here modern humans encountered Neanderthals, who shared a common ancestor with modern humans hundreds of thousands of years ago and who had lived on these continents ever since. Many times over thousands of years, the groups interbred.
Related: Can Neanderthals speak?
Sriram Sankararaman, a professor of computer science, human genetics, and computational medicine at UCLA, told Live Science that modern humans originally inherited entire chromosomes from Neanderthals. But from generation to generation, these DNAs have been broken up and stirred up through a process known as genetic recombination.
Neanderthal DNA is generally “toxic” to modern humans, meaning that it was rapidly culled from modern human DNA through evolution. The result, Sankararaman said, was a “desert of Neanderthal DNA,” or a large region of the modern human genome lacking it. For example, scientists believe that a man’s Y chromosome does not contain Neanderthal genes. The Neanderthal type Y genes may have been incompatible with other human genes or may have been lost randomly through a process known as genetic drift.
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In people who have inherited Neanderthal DNA, the X chromosome has far less Neanderthal ancestry than other non-sex chromosomes. This is probably because deleterious or non-functional mutations on the X chromosome are expressed in males. This is because males lack a matching copy of a functional gene to compensate. Therefore, strong evolutionary pressure likely arose to remove such harmful Neanderthal genes from modern Homo X, Emilia Huerta Sánchez, an associate professor of ecology, evolution, and organismal biology at Brown University, told Live Science.
But some Neanderthal DNA remains in the genome because it helped modern humans survive and reproduce. Neanderthal DNA now makes up an average of 2% of the genomes of people outside Africa. But the frequency of Neanderthal DNA encoding beneficial traits could be as high as 80% in some regions of the genome, Akey said.
our appearance
For many people, their Neanderthal heritage manifests itself in a highly visible characteristic: skin color.
The Neanderthal gene mutation on chromosome 9, which affects skin color, is carried by 70% of Europeans today. Another Neanderthal gene mutation found in most East Asians controls keratinocytes, which protect skin from ultraviolet light through a dark pigment called melanin.
Neanderthal genetic mutations are also associated with an increased risk of sunburn in modern humans. Similarly, approximately 66% of Europeans carry Neanderthal alleles associated with an increased risk of childhood tanning and reduced tanning ability.
In some places in our genome we are closer to Neanderthals than humans
Click here for Joshua Akey of Princeton University
Neanderthals spent thousands of years at high latitudes where there was less direct sunlight needed to produce vitamin D. Biological changes in hair and skin may therefore allow modern humans to utilize lower levels of sunlight while still producing the amount of vitamin D they need for health, John Capra, an evolutionary geneticist at Vanderbilt University, told Live Science.
“One of the great things about cross-breeding, which is a very slow process, is that you can introduce large amounts of genetic variation at once, rather than waiting for new beneficial mutations to arise,” Huerta-Sánchez said, which is inherently fast-evolving.
Related: What is the difference between Neanderthals and Homo sapiens?
In addition, our ancestors had to adapt to the cold climate of Eurasia. To do so, they may have acquired Neanderthal genes that affect their facial shape. In a 2023 study, scientists discovered that modern humans inherited genes for tall noses from Neanderthals. The elevated nose may have allowed more cold air to warm up to body temperature in the nose before reaching the lungs, suggested study co-senior author Kaustubh Adhikari, a statistical geneticist at University College London.
The clock that runs our cells
Neanderthal DNA may also have helped Homo sapiens adapt to the large differences in day and night lengths at northern latitudes.
Remaining Neanderthal genes influence the circadian clock, which regulates internal processes such as body temperature and metabolism. For example, some people who wake up early may be grateful to Neanderthal circadian clock genes, Capra et al. found.
This may have helped our ancestors adapt to shorter winter days far from the equator, Capra said.
“It doesn’t seem like it’s important to be a morning person,” Capra says. “It shows how inherently flexible clocks are and how well they can adapt to changes in seasonal light-dark cycles,” he says.
our internal defense
Many of the strongly conserved Neanderthal genes are linked to immune function.
By the time Homo sapiens arrived in Europe, Neanderthals had already been battling infectious diseases endemic to Eurasia for hundreds of thousands of years. By interbreeding with Neanderthals, modern humans were instantly injected with genes that fight these infectious diseases.
“Neanderthal DNA, particularly immune system DNA, that was already adapted to pathogens with which Neanderthals had long lived together began to rise in frequency under natural selection in modern human populations,” David Ennard, assistant professor of ecology and evolutionary biology at the University of Arizona, told Live Science.
Although many of the ancestral pathogens that sickened ancient humans have been lost over time, some of the Neanderthal genes that helped fight them off still work against modern pathogens. For example, a 2018 study by Ennard and colleagues revealed that modern humans inherited Neanderthal DNA that helps them fight RNA viruses, a group that includes today’s influenza, HIV, hepatitis C, and more.
RELATED: 10 unexpected ways Neanderthal DNA affects our health
The dark side of Neanderthal DNA
Some Neanderthal genes that once helped our ancestors may be harmful in the modern world.
In most cases, Neanderthal genes are not strongly expressed in the brain, suggesting that Neanderthals were strongly selected during evolution. Neanderthal genes are thought to be linked to mood disorders such as depression and to brain signaling pathways that make people more susceptible to nicotine addiction.
And Neanderthal immunity may have a downside. In 2016, scientists discovered that a Neanderthal gene that stimulates the immune system to fight pathogens may make people more susceptible to allergic diseases. Additionally, Neanderthal genes have been linked to an increased risk of developing autoimmune diseases such as Graves’ disease, which is caused by an overactive thyroid gland. There’s even rheumatoid arthritis, which causes inflammation of the joints, and “Viking disease,” which causes one or more fingers to become bent or stiff.
One genetic mutation in Neanderthals may have made them more likely to develop a severe case of coronavirus infection. This mutation is located on chromosome 3 and is found in half of South Asians and one-sixth of Europeans. But the picture is complicated because other Neanderthal genes, carried by up to half of people in Eurasia and the Americas, are associated with a lower risk of severe COVID-19 infection.
“Unfortunately, we don’t have a disease or even a common trait that we can say, ‘Oh, that’s because of Neanderthal DNA,'” Capra said.
This is especially true for some of the biggest health diseases, such as heart disease and cancer, where dozens or hundreds of genes and countless environmental factors influence disease risk.
What lies ahead?
So how long will these long-lost hominid traces remain in our genome? Over hundreds of thousands of years, some of these Neanderthal fragments will be gradually eliminated from our genome. Others will be firmly integrated, Akey said.
In the meantime, there is still much to learn about how Neanderthals left their mark on us.
“The ability to leverage new genomic technologies like CRISPR and gene editing will play an important role in understanding the actual underlying biology of how Neanderthal sequences contribute to human traits and diseases,” Akey said.
Deciphering what these genes actually do could help develop treatments for specific conditions, he said.
And gene flow was not one-way. Scientists are also trying to understand how modern human DNA influenced Neanderthals and applying artificial intelligence (AI) techniques to ancient genomes to create a more detailed picture of what our long-lost cousins looked like.
Elucidating the role of Neanderthal DNA in our genomes will not only help us understand our health. Sankararaman says these pieces of DNA could provide clues about what makes us unique.
“Neanderthal DNA entered the genome at a critical time in human history,” Sankararaman said.
“By studying the fate of these DNA fragments, we hope to understand what the functionally important regions of the genome were over this period,” he said.
Editor’s note: This article was originally published in March 2024.
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