The interbreeding between Neanderthals and modern humans at various points in evolutionary history has left genetic traces in the genomes of contemporary human populations.
Early modern human populations migrated from Africa into Eurasia, where they encountered Neanderthals, who had previously adapted to the colder European climate. These interactions, occurring over millennia, resulted in the transfer of genetic material between the two groups. Today, these ancient genetic contributions influence various aspects of human biology, ranging from physical characteristics to disease susceptibility.
Genetic Integration and Retention
Ancestors of most modern Eurasians first ventured out of Africa between 75,000 and 250,000 years ago, encountering Neanderthals, with whom they shared a common ancestor hundreds of thousands of years prior. Initially, modern humans acquired entire chromosomes from Neanderthals. Over subsequent generations, genetic recombination fragmented these DNA segments.
Much of the inherited Neanderthal DNA was generally considered deleterious and was systematically removed from the modern human genome through natural selection, resulting in 'deserts' of Neanderthal DNA. For example, the Y chromosome in males and the X chromosome show significantly reduced Neanderthal ancestry compared to non-sex chromosomes. This reduction may be attributed to genetic incompatibility or strong selective pressures against harmful mutations.
Conversely, certain beneficial Neanderthal DNA segments persisted. On average, Neanderthal DNA constitutes approximately 2% of the genomes of individuals outside Africa. However, in specific genomic regions, the frequency of beneficial Neanderthal DNA can be as high as 80%.
Influence on Physical Appearance
Neanderthal genetic variants contribute to physical traits in modern humans. A variant on chromosome 9, influencing skin color, is present in 70% of Europeans. Another variant, common in East Asians, regulates keratinocytes, which protect skin from ultraviolet radiation through melanin production. Some Neanderthal gene variants are also associated with an increased risk of sunburn, with approximately 66% of Europeans carrying an allele linked to heightened childhood sunburn susceptibility and reduced tanning ability. These genetic adaptations may have facilitated modern humans' ability to produce sufficient vitamin D in regions with less direct sunlight.
Furthermore, Neanderthal genes may have influenced facial morphology, such as the development of taller noses, which could have assisted in warming inhaled cold air to body temperature, aiding adaptation to colder Eurasian environments.
Impact on Circadian Rhythms
Neanderthal DNA also affects the human circadian clock, which regulates internal biological processes like body temperature and metabolism. Research indicates that certain circadian clock genes inherited from Neanderthals may contribute to the 'early riser' chronotype. This genetic contribution is hypothesized to have supported adaptation to the more pronounced seasonal variations in day and night length at higher northern latitudes.
Contributions to the Immune System
Many strongly retained Neanderthal genes are linked to immune function. By the time Homo sapiens arrived in Europe, Neanderthals had developed immune adaptations to region-specific pathogens. Interbreeding provided modern humans with an accelerated acquisition of these pathogen-fighting genes. These Neanderthal DNA segments, already adapted to local pathogens, increased in frequency within modern human populations due to natural selection. For example, a 2018 study indicated that modern humans inherited Neanderthal DNA that confers protection against RNA viruses, a group that includes influenza, HIV, and hepatitis C.
Potential Negative Genetic Effects
While some Neanderthal genes proved beneficial, others may have adverse effects in contemporary contexts. Neanderthal genes exhibit limited expression in the human brain, suggesting strong evolutionary selection against them. Some Neanderthal genes have been associated with an increased risk of mood disorders, such as depression, and with brain signaling pathways linked to nicotine addiction.
Immune-related Neanderthal genes, while protective against pathogens, may also predispose individuals to allergic diseases and autoimmune conditions like Graves' disease, rheumatoid arthritis, and Dupuytren's contracture (known as 'Viking disease'). A specific Neanderthal gene variant on chromosome 3 has been linked to an increased risk of severe COVID-19, prevalent in half of South Asians and one-sixth of Europeans. However, other Neanderthal genes are associated with a reduced risk of severe COVID-19. It is important to note that complex traits and diseases, such as heart disease and cancer, are influenced by numerous genes and environmental factors, making direct attribution to Neanderthal DNA challenging.
Future Research Directions
Over extended periods, some Neanderthal DNA fragments will continue to be eliminated from human genomes, while others will become permanently integrated. Ongoing research aims to further elucidate the influence of Neanderthal DNA on human biology. Advanced genomic technologies, including CRISPR and gene editing, are expected to play a significant role in understanding the biological mechanisms through which Neanderthal sequences contribute to human traits and diseases. This understanding could potentially inform the development of treatments for certain conditions. Additionally, scientists are investigating the influence of modern human DNA on Neanderthals and employing artificial intelligence methods to reconstruct a more detailed picture of Neanderthal characteristics. Studying the fate of these ancient DNA segments provides insights into functionally important regions of the human genome during a critical period of environmental adaptation.