Neanderthals and Humans
Homo neanderthalensis interbred with modern humans 50,000 years ago. Approximately 2.5% of the DNA of many individuals living today came from Neanderthals.
The only members of the Homo genus roaming the world today are humans, but 100,000 years ago Homo sapiens and Homo neanderthalensis were entwined in an interspecies love affair. Based on the perception of what Neanderthals supposedly looked like, with sloping foreheads, squashed noses, and a nasal voice, it is hard to believe modern humans ever found them alluring. But recent genetic evidence indicates that the two species did indeed fall in love and have fertile offspring. After a 4-year effort led by Svante Paabo and numerous research groups around the world, scientists published the complete Neanderthal genome in 2014 (Prüfer et al. 2014). Researchers compared it with contemporary human DNA and found that on average 2.5% of H. sapiens DNA is of Neanderthal origin. Because we all do not share the same genes, 20% of the Neanderthal genome can be found distributed in humans around the world today (Green et al. 2008). H. neanderthalensis, a population of humans who once ruled over Europe and Asia, went extinct around 40,000 years ago – but not before a little promiscuity that left foreign DNA in most of us years later.
Interactions with Modern Humans
Our Homo sapiens ancestors were never alone on the Earth. While we have been the singular species of the Homo genus for the past 10,000 years, the first humans evolved in East Africa approximately 2.5 million years ago. They were descendants of an earlier hominid called Australopithecus, who walked upright but had small brains (Their brains were not much larger than apes, usually 500 cc or less. In comparison, Homo brains are generally larger than 600 cc and modern humans average at 1350 cc.). Researchers have discovered as many as 27 unique human species, though paleoanthropologists still argue as to where to draw the line of speciation (Darwin was not the first but maybe the most well-known scientist who grappled with the definition of a species. He said, “I look at the term ‘species’ as one arbitrarily given, for the sake of convenience….” Years later Ernst Mayr introduced the biological species concept – species are groups of individuals that are reproductively compatible. However, this definition fails with all asexual organisms or populations that are only geographically isolated. While slightly embarrassing that scientists cannot agree on this basic concept, there are at least 20 valid published definitions and the debate does not appear to be slowing down. In light of current genetic data, some geneticists classify Neanderthals as a subspecies of humans). Some bands of these individuals left their homelands to travel north and settle in Northern Africa, Europe, and Asia. As different adaptations are needed to survive the cold forests in Siberia or hot temperatures of the Sahara Desert, human populations evolved and diversified. H. neanderthalensis lived and evolved in Europe for about 250,000 years before early H. sapiens moved into their lands (Higham et al. 2014). Our ancestors proliferated in their new environment, and the saga of the Neanderthals ended 10,000 years later when the species disappeared.
For decades, two competing theories were employed to explain the evolution of contemporary humans. One theory proposes that modern humans evolved in unison all over the world. When H. sapiens moved into new lands, they bred with the early humans in Eurasia, and humanity evolved through recurrent gene flow (Wolpoff et al. 2000). In the alternative version, H. sapiens emigrated out of Africa and replaced all other hominins they encountered (Stringer 2003). In this tale of incompatibility and competition, H. sapiens had little sexual interest in other humans and wiped out all populations they encountered. The theory gained support in the 1980s after genetic studies revealed a recent origin of contemporary humans in Africa around 200,000 years ago (Allan Wilson, a geneticist at UC Berkeley, and his team used mitochondrial DNA to estimate how long human populations have been diverging from each other. All of the unique mitochondrial genomes today descended from a woman who lived in Africa around 200K years ago. Likening her to the first women from The Book of Genesis, scientists named her “mitochondrial Eve.”). Current information on the Neanderthal components in our genome suggests a third theory should be added to the mix: H. sapiens moved out of Africa around 100,000 years ago, and a few instances of interbreeding altered their genome. Though the receding chin and stocky frame of a Neanderthal may not be attractive to humans today, maybe being an individual adorned in feathers, with skin thicker than a rhino’s, was just the prettiest thing our ancestors had even seen.
DNA not only contains the blueprint for life but also a glimpse into the past. Frequently, certain populations of individuals around the globe will accumulate mutations in their genome. Most of these are neutral and cause no harm but can be used as markers for scientists to determine where and when a person’s ancestors came from. Based on the mutation rate in humans, interbreeding events took place around 50,000–60,000 years ago (Kuhlwilm et al. 2016). Whether these acts of copulation were clandestine meetings or commonplace is still a mystery, enough hybrid children were born within H. sapiens populations to allow Neanderthal alleles into the human gene pool. H. sapiens still living in Africa avoided meeting H. neanderthalensis and are the only individuals today without any foreign DNA in their genomes. Contemporary East Asians inherited around 1.4% and Europeans about 1.2% of their genome from our hominin relatives. This Neanderthal DNA is clustered in our genome; some parts are bare, while other sections contain as much as 60% foreign genetic material (Vernot and Akey 2014). Like interbreeding that takes place in other vertebrates and plants, this hybridization happened in a patchy fashion. Advantageous H. neanderthalensis genes, like ones tied to the immune system, hair, and resilience for cold weather, survived through time and fixed themselves in our genome. Little Orphan Annie might have been singing a different tune if she knew that the freckles sprinkled on her nose are due to interbreeding – almost 70% of Europeans carry the Neanderthal version of a gene called BNC2 which influences skin pigmentation (Sankararaman et al. 2014). Fair skin and freckles helped H. sapiens efficiently synthesize vitamin D from low sunlight in northern latitudes.
Some Neanderthal-derived genes may have helped us in our evolutionary journey, but the vast majority were duds. When Paabo and his team were analyzing the H. sapiens genome for Neanderthal DNA, they found significantly less of it in functional versus nonfunctional parts of our genome (Prüfer et al. 2014). Only 1% of our DNA codes for proteins, and the majority of Neanderthal DNA is found in the spaces between our genes, where it has no biological effect. In the estimated 2,000 generations since modern humans and Neanderthals mated, natural selection has been hard at work, removing any of their deleterious alleles from our genome. The variants that escaped this purging also may not always have been a disadvantage. For example, Neanderthal DNA is linked with a blood clot disorder caused by the over-activation of the SLEP genes. In prehistoric times, wounds that closed quickly might have been advantageous when running away from a saber-toothed tiger but now can lead to strokes and heart attacks (Simonti et al. 2016).
Our current understanding of our distant relatives is that they were a sophisticated species who had large brains, used tools, and buried their dead. Scientists do not have a definitive reason for the Neanderthals’s extinction but do know that their entire population was struggling 40,000 years ago (Galván et al. 2014). Archaeological sites indicate that they lived in small bands and rarely interacted with people outside these troops. As men and women continued to mate with their relatives, deleterious genetic mutations remained in their gene pool, and the Neanderthals were reduced to low numbers. Traditional theories speculate that their small population was not able to compete with H. sapiens for resources, either through direct warfare or falling prey to evolutionary pressures from the changing climate. Whatever the reason, in the war between the two of human evolution, Neanderthals were cast as the losers, but not before their love affairs left their faint mark on our genome.
Homo neanderthalensis, called Neanderthals, walked the Earth around 200,000 to 30,000 years ago. They closely resembled Homo sapiens, though with a prominent brow, larger brain, and stockier build. Like other members from the Homo genus, they originated in Africa and migrated to Eurasia. Some scientists estimate that, at their peak, their population was as large as 70,000 individuals. In 2017, researchers found that modern human groups who originated outside of Africa contain around 2.5% Neanderthal DNA. This lends credence to the theory that humans and Neanderthals interbred and produced offspring. While the Neanderthals lived for several hundred thousand years, they died out about 30,000 years ago, from either warfare with modern humans, disease, or absorption into the larger human population.
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