Not all ‘imported’ genes were beneficial
Interbreeding with Neanderthals and Denisovans is believed to have introduced many beneficial genes into the modern genome, helping the immune systems of early modern humans to fight pathogens to which they had not previously been exposed. Other ‘imported’ genes include those involved with the production of keratin, a protein that is used in skin, hair and nails, and in East Asian populations, many genes involved with protection from the sun’s UV rays are of Neanderthal origin. It is likely that the transfer of these genes helped early modern humans to adapt to conditions away from their African homeland.
However, a newly-published study suggests that interbreeding with Neanderthals also had a down side. Researchers analysed the electronic health records (EHR) of 28,000 individuals of European origin and integrated the data with high resolution maps of Neanderthal haplotypes across individual modern human genomes. They carried out a large-scale assessment of the functional effects of DNA inherited from Neanderthals on health-related traits in these individuals. Particular use was made of genotype and phenotype data from the Electronic Medical Records and Genomics Network, which is a consortium that links EHR systems combined with patient genetic data from nine sites across the USA.
Genes of Neanderthal origin were found to be associated with smoking addiction, increased risk of depression, incontinence, bladder pain, urinary tract disorders, protein calorie malnutrition, and actinic keratosis (precancerous skin lesions resulting from exposure to the sun). One gene variant was associated with blood coagulation, increasing the risk of strokes. These results follow on from earlier work which implicated increased risk of Crohn’s disease and type 2 diabetes with Neanderthal genes.
Many of these genes would have been advantageous to Neanderthals: for example, the benefits of enhanced blood coagulation would have greatly outweighed the risk of strokes when injuries leading to significant loss of blood were a part of daily life and few people lived past forty. In other cases, genes were probably once advantageous but adverse effects were triggered by the changes in diet following the coming of agriculture in Neolithic times.
Depression can be triggered by disturbed circadian rhythms. It is possible that Neanderthal brain chemistry and skin responses to sunlight were both linked to the lighting conditions and lifestyles of an era when artificial light consisted of torches and camp fires. In which case, the genes might only have become maladaptive with the advent of widespread artificial lighting.
The methodology used by the researchers is likely to provide further insight into the genetic impact of these ancient encounters between Neanderthals and modern humans.
Simonti, C. et al., The phenotypic legacy of admixture between modern humans and Neandertals. Science 351 (6274), 737-741 (2016).