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.
References:
Simonti, C. et al., The phenotypic legacy of admixture
between modern humans and Neandertals. Science 351 (6274),
737-741 (2016).
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