Y chromosomes of Neanderthals and Denisovans now sequenced
Neanderthals have adopted male sex chromosome from modern humans

Date:
September 24, 2020
Source:
Max-Planck-Gesellschaft
Summary:
An international research team led by Martin Petr and Janet Kelso of
the Max Planck Institute for Evolutionary Anthropology in Leipzig,
Germany, has determined Y chromosome sequences of three Neandertals
and two Denisovans. These Y chromosomes provide new insights into
the relationships and population histories of archaic and modern
humans, including new evidence for ancient gene flow from early
modern humans into Neandertals.



FULL STORY ==========================================================================
In 1997, the very first Neanderthal DNA sequence -- just a small part of
the mitochondrial genome -- was determined from an individual discovered
in the Neander Valley, Germany, in 1856. Since then, improvements in
molecular techniques have enabled scientists at the Max Planck Institute
for Evolutionary Anthropology to determine high quality sequences of
the autosomal genomes of several Neanderthals, and led to the discovery
of an entirely new group of extinct humans, the Denisovans, who were
relatives of the Neanderthals in Asia.


========================================================================== However, because all specimens well-preserved enough to yield sufficient amounts of DNA have been from female individuals, comprehensive studies
of the Y chromosomes of Neanderthals and Denisovans have not yet been
possible. Unlike the rest of the autosomal genome, which represents
a rich tapestry of thousands of genealogies of any individual's
ancestors, Y chromosomes have a peculiar mode of inheritance -- they
are passed exclusively from father to son. Y chromosomes, and also the maternally-inherited mitochondrial DNA, have been extremely valuable
for studying human history.

New method to identify Y chromosome molecules In this study, the
researchers identified three male Neanderthals and two Denisovans that
were potentially suitable for DNA analysis, and developed an approach to
fish out human Y chromosome molecules from the large amounts of microbial
DNA that typically contaminate ancient bones and teeth. This allowed
them to reconstruct the Y chromosome sequences of these individuals,
which would not have been possible using conventional approaches.

By comparing the archaic human Y chromosomes to each other and to the Y chromosomes of people living today, the team found that Neanderthal and
modern human Y chromosomes are more similar to one another than they
are to Denisovan Y chromosomes. "This was quite a surprise to us. We
know from studying their autosomal DNA that Neanderthals and Denisovans
were closely related and that humans living today are their more distant evolutionary cousins. Before we first looked at the data, we expected that their Y chromosomes would show a similar picture," says Martin Petr, the
lead author of the study. The researchers also calculated that the most
recent common ancestor of Neanderthal and modern human Y chromosomes lived around 370,000 years ago, much more recently than previously thought.

It is by now well established that all people with non-African ancestry
carry a small amount of Neanderthal DNA as a result of interbreeding
between Neanderthals and modern humans approximately 50,000-70,000 years
ago, quite shortly after modern humans migrated out of Africa and started spreading around the world. However, whether Neanderthals might also
carry some modern human DNA has been a matter of some debate.

These Y chromosome sequences now provide new evidence that Neanderthals
and early modern humans met and exchanged genes before the major
out of Africa migration -- potentially as early as 370,000 years
ago and certainly more than 100,000 years ago. This implies that some population closely related to early modern humans must already have been
in Eurasia at that time. Surprisingly, this interbreeding resulted in
the replacement of the original Neanderthal Y chromosomes with those
of early modern humans, a pattern similar to what has been seen for
Neanderthal mitochondrial DNA in an earlier study.

Selection for Y chromosomes from early modern humans At first, the
complete replacement of both Y chromosomes and mtDNA of early Neanderthals
was puzzling, as such replacement events are quite unlikely to occur by
chance alone. However, the researchers used computer simulations to show
that the known small size of Neanderthal populations may have led to an accumulation of deleterious mutations in their Y chromosomes which would
reduce their evolutionary fitness. This is quite similar to situations
where extremely small population sizes and inbreeding can sometimes
increase the incidence of some diseases. "We speculate that given the
important role of the Y chromosome in reproduction and fertility, the
lower evolutionary fitness of Neanderthal Y chromosomes might have caused natural selection to favor the Y chromosomes from early modern humans, eventually leading to their replacement" says Martin Petr.

Janet Kelso, the senior author of the study, is optimistic that this replacement hypothesis could be tested in the near future: "If we can
retrieve Y chromosome sequences from Neanderthals that lived prior to
this hypothesized early introgression event, such as the 430,000 year
old Neanderthals from Sima de los Huesos in Spain, we predict that they
would still have the original Neanderthal Y chromosome and will therefore
be more similar to Denisovans than to modern humans."

========================================================================== Story Source: Materials provided by Max-Planck-Gesellschaft. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Martin Petr, Mateja Hajdinjak, Qiaomei Fu, Elena Essel, He'le`ne
Rougier,
Isabelle Crevecoeur, Patrick Semal, Liubov V. Golovanova,
Vladimir B.

Doronichev, Carles Lalueza-Fox, Marco De La Rasilla, Antonio Rosas,
Michael V. Shunkov, Maxim B. Kozlikin, Anatoli P. Derevianko,
Benjamin Vernot, Matthias Meyer, Janet Kelso. The evolutionary
history of Neanderthal and Denisovan Y chromosomes. Science,
2020 DOI: 10.1126/ science.abb6460 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200924141449.htm

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