XPost: sci.archaeology

https://www.pnas.org/doi/epdf/10.1073/pnas.2502158122

Abstract
Light eyes, hair, and skins probably evolved several
times as Homo sapiens dispersedfrom Africa. In areas
with lower UV radiation, light pigmentation alleles
increased in frequency because of their adaptive
advantage and of other contingent factors such as
migration and drift. However, the tempo and mode of
their spread is not known. Phenotypic inference from
ancient DNA is complicated, both because these
traits are polygenic and because of low sequence depth.
We evaluated the effects of the latter by randomly
removing reads in three high-coverage ancient samples,
the Paleolithic Ust’- Ishim from Russia, the Mesolithic
SF12 from Sweden, and the Neolithic I5077 from current
Croatia. We could thus compare three approaches to
pigmentation inference, concluding that for suboptimal
levels of coverage (<8×), a probabilistic method
estimating genotype likelihoods leads to the most
robust predictions. We then applied that protocol to
348 ancient genomes from Eurasia, describing how skin,
eye, and haircolor evolved over the past 45,000 y.
The shift toward lighter pigmentations turned out to
be all but linear in time and place, and slower than
expected, with half of the individuals showing dark
or intermediate skin colors well into the Bronze and
Iron ages. We also observed a peak of light eye
pigmentation in Mesolithic times, and an accelerated
change during the spread of Neolithic farmers over
Western Eurasia, although localized processes of gene
flow and admixture, or lack thereof, also played a
significant role.

"By a probabilistic approach, we showed that eye,
hair, and skin color changed substantially through
time in Eurasia. It was reasonable to imagine that
the first hunting-gathering settlers, who came from
warmer climates, had mostly dark pigmentation. We
are now showing that their phenotypes persisted up
to the Iron age. We found the earliest instance of
light skin color in the Swedish Mesolithic, but it
comes from only one sample in >50. Things changed
afterward, but very slowly, so that only in the
Bronze Age did the frequency of light skins equal
that of dark skins in Europe; during much of
prehistory, most Europeans were dark-skinned. A
similar trend, with dark pigmentation long
coexisting with an increasing, yet relatively small
proportion of lighter traits, is observed for hair
and eye color, although there was a temporary peak
of light eye frequency in the Mesolithic period,
when we inferred light pigmentation for 11 out of
35 samples."

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