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Travel back in evolutionary time to explore the relationship between humans and Neanderthals By Courtney Kousser
The origin of human life remains a mystery. Echoes of the past reverberate within modern human DNA, reminiscent of the paths taken by ancestral humans. Tens of thousands of years ago, Homo sapiens were just one of several hominid species, which included Neanderthals, Denisovans, and the tiny Homo floresiensis (nicknamed “the Little Lady of Flores” and “the Hobbit”). They all appeared to co-exist at some point in history and whispers of their existence and their impact on contemporary life remains imprinted within the human genome.
Modern humans left Africa some 60,000 years ago, with one group heading East and the other migrating West, toward Europe. There they met Neanderthals, another species of “great ape” that left Africa about 150,000 years earlier. The two species began interbreeding, and today most people of non-African descent are between 1–5% Neanderthal, based on genetic sequencing. This finding dashed previous assumptions that Homo sapiens and Homo neanderthalensis did not interbreed.
Nearly half of Neanderthal genes found within modern human DNA influence hair colour and skin pigmentation, with specific genes associated with increased susceptibility to sunburn and lower ability to tan. This makes sense, as Neanderthals evolved in Europe with lower sun-exposure; here, paler skin allowed for enhanced vitamin D synthesis. Neanderthal genes are also correlated with a variety of hair colours, mainly light hair, but are never associated with red hair, making this a uniquely human characteristic.
Some Neanderthal genes are associated with sleeping patterns, influencing people who prefer to stay awake late, have day-time naps and are even linked to narcolepsy. There is also a strong correlation with depression and mood disorders. Like skin and hair pigmentation, sleeping schedules and mood disorders are associated with sunlight and circadian rhythms, further highlighting the influence of climate in Neanderthal evolution. Certain physical diseases are linked to the presence of Neanderthal DNA, such as blood clotting disorders, cardiovascular disease, and Crohn’s disease. There is a gene of Neanderthal origin that results in a 25–50% increased risk of developing Type II diabetes. Ancestral Homo sapiens also acquired sexually transmitted diseases, specifically human papillomavirus (HPV), from interbreeding with Neanderthals. Infectious diseases were traded, as Homo sapiens passed along herpes virus, tapeworms, tuberculosis, and Helicobacter pylori (the bacterium causing stomach ulcers) to the Neanderthals. While these associations are fascinating, it is important to note that the presence of Neanderthal genes does not necessarily mean a person will develop these diseases. There are thousands of factors influencing whether a particular gene is expressed, and there are countless other causes of these diseases.
On a positive note, humans inherited key aspects of the innate immune system from Neanderthals, including a cluster of genes (specifically, Toll-like receptors 1, 6, and 10) that are essential for fighting invading microorganisms. Acquiring these genes likely played an important role in quickly adapting to new environmental challenges as ancestral Homo sapiens migrated. However, these genes are also partly responsible for the over-activity of immune responses, leading to hay-fever and other allergies.
Interestingly, couplings between Neanderthals and Homo sapiens seem to have always resulted in less fertile, or even sterile males. Neanderthal genes were originally passed through the female lineage only, and no Neanderthal genes are ever found in male testes or on the X-chromosome. This highlights that while breeding between these species occurred, the relationships “were at the edge of biological compatibility,” as described by David Reich from Harvard Medical School.
Humans are truly a melting pot, with ancestral Homo sapiens migrating around the world, absorbing other hominid communities and possibly playing a role in their demise. Who we are today is influenced by where, and with whom, our ancestors have been.
From Issue 16