Neanderthal DNA in Modern Human Genomes Is Not Silent
From skin color to immunity, human biology is linked to our archaic ancestry.
Sep 1, 2019
Jef Akst
After the 2010 publication of the Neanderthal draft genome sequence, evolutionary biologist Joshua Akey,
then at the University of Washington in Seattle, and his graduate
student Benjamin Vernot began looking into its most provocative
implication: that the ancient hominins had bred with the ancestors of
modern humans. Neanderthals had been living in Eurasia for more than 300
millennia when some human ancestors left Africa some 60,000–70,000
years ago, and according to the 2010 publication, in which researchers
compared the Neanderthal draft genome with modern human sequences, about
2 percent of the DNA in the genomes of modern-day people with
non-African ancestry is Neanderthal in origin.1
To
investigate the archaic ancestry of the living human population, Akey
and Vernot set to work searching for Neanderthal DNA in modern genomes.
They developed a statistical approach to identify genetic signatures
suggestive of Neanderthal ancestry in the genomes of 379 European and
286 East Asian individuals. The endeavor was further powered by the first high-quality Neanderthal genome
sequence, which gave the duo confidence that the sequences they’d
identified were indeed of archaic origin. Still, in the back of Akey’s
mind, he had doubts about the research. “I remember telling Ben [when]
we were working on this, ‘I wake up every day in a cold sweat that this
is all just incomplete lineage sorting’”—a methodological artifact that
would undermine their conclusions about Neanderthal ancestry, meaning
the sequences were the result of the common ancestry the two groups
shared.
Then, as Vernot and Akey were getting ready to submit their work for publication, their department got a visit from Svante Pääbo,
a geneticist at the Max Planck Institute for Evolutionary Anthropology
who had pioneered techniques for extracting and analyzing DNA from ancient specimens
and had led the early Neanderthal genome efforts. They spoke with him
about their ongoing project, and Pääbo noted that his collaborator, David Reich at Harvard Medical School, was pursuing a very similar line of research. So Akey gave Reich a call.
“The
end result [of the conversation] was we agreed to coordinate
publication,” Akey recalls. “We also agreed not to even look at each
other’s papers because we didn’t want to influence the results in any
way.”
See “Simultaneous Release”
Was it just this curious feature of human history that didn’t have an impact, or did it alter the trajectory of human evolution?
—Joshua Akey, Princeton University
Vernot and Akey submitted to Science;2 Reich and his colleagues submitted to Nature.3
The two journals synchronized publication of the papers at the end of
January 2014. The day they went live, Akey anxiously began to read the
paper from the Reich group. “I remember sitting in my office, reading
it, and really sort of just going through the checklist” of the key
results, he says. Quickly, the relief set in. “We essentially said the
exact same thing,” Akey recalls. “Usually when you publish something,
it’s years before you see validation. . . . This was sort of instant
gratification.”
The two groups had used different statistical
approaches to identify Neanderthal DNA in modern human genomes, putting
to bed any skepticism about the history of hominin group interbreeding.
“[It was] the final nail on the coffin that it couldn’t be anything
else,” says Janet Kelso,
a computational biologist at the Max Planck Institute for Evolutionary
Anthropology and a collaborator on Reich’s publication.
With the
issue of Neanderthal/modern human mating settled, scientists could focus
on a new goal, says Akey, now at Princeton University. Namely, what was
the consequence of this interbreeding? “Was it just this curious
feature of human history that didn’t have an impact, or did it alter the
trajectory of human evolution?”
In the past five years, a flurry
of research has sought to answer that question. Genomic analyses have
associated Neanderthal variants with differences in the expression
levels of diverse genes and of phenotypes ranging from skin and hair
color to immune function and neuropsychiatric disease. But researchers
cannot yet say how these archaic sequences affect people today, much
less the humans who acquired them some 50,000–55,000 years ago.
“So
far I have not seen any convincing functional studies where you take
the Neanderthal variant and the human variant and do controlled
experiments” to identify the physiological consequence, says Grayson Camp,
a genomicist at the Institute of Molecular and Clinical Ophthalmology
Basel (IOB) in Switzerland. “No one has actually shown yet in culture
that a human and Neanderthal allele have a different physiological
function. That will be exciting when someone does.”
A Mixed HistorySome 350,000 or more years ago, the group of hominins that would evolve to become Neanderthals and Denisovans left Africa for Eurasia.A few hundred millennia later, about 60,000 to 70,000 years ago, the ancestors of modern non-Africans followed a similar path out of Africa and began interbreeding with these other hominin groups. Researchers estimate that much of the Neanderthal DNA in modern human genomes came from interbreeding events that took place around 50,000 to 55,000 years ago in the Middle East. Thousands of years later, humans moving into East Asia interbred with Denisovans.
the scientist staff
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Neanderthal in our skin
Most
Neanderthal variants exist in only around 2 percent of modern people of
non-African descent. But some archaic DNA is much more common, an
indication that it was beneficial to ancient humans as they moved from
Africa into Eurasia, which Neanderthals had called home for more than
300,000 years. In their 2014 study, Vernot and Akey found several
sequences of Neanderthal origin that were present in more than half of
the genomes from living humans they studied. The regions that contained
high frequencies of Neanderthal sequences included genes that could
yield clues to their functional effect. Base-pair differences between
Neanderthal and human variants rarely fall in protein-coding sequences,
but rather in regulatory ones, suggesting the archaic sequences affect
gene expression. (See “Denisovans in the Mix” below.)
A number of
segments harbor genes that relate to skin biology, such as a
transcription factor that regulates the development of epidermal cells
called keratinocytes. These variants may underlie traits that were
adaptive in the different climatic conditions and lower levels of
ultraviolet light exposure at more northern latitudes. Reich’s group
similarly found genes involved in skin biology enriched in Neanderthal
ancestry—that is, more than just a few percent of people carried
Neanderthal DNA in these parts of the genome.
No one has actually shown yet in culture that a human and Neanderthal allele have a different physiological function. That will be exciting when someone does.
—Grayson Camp,
Institute of Molecular and Clinical Ophthalmology Basel
It
was unclear, however, what specific effect the Neanderthal variants had
on phenotype. For that, researchers needed phenotypic data on many
different kinds of traits, paired with genetic information, for
thousands of people. Vanderbilt University evolutionary geneticist Tony Capra has access to such a resource: the Electronic Medical Records and Genomics (eMERGE)
Network. Right around the time the scientific community was beginning
to map Neanderthal DNA in the genomes of living people, eMERGE
organizers were compiling electronic health records and associated
genetic data for tens of thousands of patients from nine health-care
centers across the US. “We felt like we had a chance to evaluate some of
those hypotheses [about functionality] on a larger scale in a real
human population where we had rich phenotype data,” says Capra.
In
collaboration with Akey and Vernot, who helped identify Neanderthal
variants in the genetic data included in the database, Capra’s group
looked for links between the archaic DNA and more than 1,000 phenotypes
across some 28,000 people of European ancestry. They reported in 2016
that Neanderthal DNA at various sites in the genome influences a range
of immune and autoimmune traits, and there was some association with
obesity and malnutrition, pointing to potential metabolic effects. The
researchers also saw an association between Neanderthal ancestry and two
types of noncancerous skin growths associated with dysfunctional
keratinocyte biology—supporting the idea that the Neanderthal DNA was at
one point selected for its effects on skin.4
“This
was crazy to me,” says Capra. “What these other groups had predicted
based on just the pattern of occurrence—the presence and absence of
Neanderthal ancestry around certain types of genes—we were actually
seeing in a real human population, that having Neanderthal ancestry
influenced traits related to those types of skin cells.” What remains
unclear, however, is what the benefits of the Neanderthal sequences were
for those early humans.
At the same time, Kelso and her postdoc Michael Dannemann
were taking a similar approach with a relatively new database called
the UK Biobank (UKB), which includes data from around half a million
British volunteers who filled out questionnaires about themselves,
underwent medical exams, and gave blood samples for genotyping. Formally
launched in 2006, the UKB published
its 500,000-person-strong resource in 2015, and Kelso and Dannemann
decided to see what information they could extract. Conveniently, the
genotyping data specifically includes SNPs that can identify variants of
Neanderthal origin, thanks to Reich’s group, which provided UKB
architects with a list of 6,000 Neanderthal variants.
Among the many links Kelso and Dannemann identified
as they dug into data from more than 112,000 individuals in the UKB
was, once again, an association between certain Neander-thal variants
and aspects of skin biology.5 Specifically, the archaic sequences spanning the BNC2 gene—a
stretch of the genome that Vernot and Akey had identified as having
Neanderthal origin in some 70 percent of non-Africans—were very clearly
associated with skin color. People who carried Neanderthal DNA there
tended to have pale skin that burned instead of tanned, Kelso says. And
the stretch that included BNC2 was just one of many, she adds:
around 50 percent of Neanderthal variants linked with phenotype in her
study have something to do with skin or hair color.
The effect
that Neanderthal DNA might have on skin appearance and function is
“fascinating,” says Akey. “Something that we’re still really interested
in and starting to do some experimental work on is: Can we understand
what these genes do and then maybe what the selective pressure was that
favored the Neanderthal version?”
See “Effects of Neanderthal DNA on Modern Humans”
Denisovans in the mix
BENCE VOILA, MAX PLANCK INSTITUTE FOR EVOLUTIONARY ANTHROPOLOGY
Once researchers reconstructed the entire high-quality Denisovan genome in 2012 (Science,
338:222–26, 2012), it became clear that, like Neanderthals, Denisovans
had interbred with modern humans during the time that they coinhabited
Eurasia, with analyses suggesting that the introgressed DNA likely came
from multiple Denisovan populations within the last 50,000 years,
sometime after mixing occurred between Neanderthals and human ancestors (Cell, 173:P53–61.E9, 2018; Cell,
177:P1010–21.E32, 2019). Denisovan DNA makes up 4–6 percent of the
genomes of people native to the islands of Melanesia, a subregion of
Oceania, and to a lesser extent they left their genetic mark in other
Pacific island populations and some modern East Asians, while it is
largely absent from the genetic code of most other people. As with
Neanderthal introgression, the question that remains to be answered is:
What effect did these variants have on our own lineage—and are we still
experiencing Denisovans’ genetic influence?
As with Neanderthal
DNA, experts have identified regions of modern human genomes that are
significantly depleted of Denisovan DNA, and they saw that these
“deserts” were the same ones that lacked Neanderthal
sequences—indications of selection against deleterious variants (Science,
352:235–39, 2016). “That’s as close as you can get to sort of a
replication in this type of work,” says Princeton University
evolutionary biologist Joshua Akey. In terms of introgressed bits of
Denisovan DNA that might have been beneficial to modern humans,
researchers have found links to toll-like receptors and other
contributors to immune function, similar to links found with Neanderthal
variants.
Denisovan DNA may have also offered some unique
benefits to ancient humans. One scientific team identified Denisovan
variants in the genomes of Greenland Inuits that include genes involved
in the development and distribution of adipose tissue, perhaps pointing
to advantages in cold tolerance and metabolism (Mol Biol Evol,
34:509–24, 2017). And maybe the strongest suggestion of beneficial
Denisovan introgression comes from a 2014 study in which researchers
linked the archaic sequences with high altitude adaptation among
populations that live in the Tibetan highlands (Nature,
512:194–97, 2014). The particular variant they focused on was so highly
selected, notes Kelso, that “almost everyone living on the plateau
carries this piece of Denisovan DNA.”
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Neanderthal-derived immunity
Another
area of human biology tightly linked to Neanderthal variants in the
genome is the immune system. Given that human ancestors were exposed to a
menagerie of different pathogens—some of which came directly from the
Neanderthals—as they migrated through Eurasia, the Neanderthal sequences
introgressed into the human genome may have helped defend against these
threats, to which Neanderthals had long been exposed.
“Viral
challenges, bacterial challenges are among the strongest selective
forces out there,” says Kelso. Unlike changes in other environmental
conditions such as daylight patterns and UV exposure, “pathogens can
kill you in one generation.”
Hints of archaic DNA’s role in immune
function surfaced as early as 2011, as soon as the Neanderthal genome
was available for cross-referencing with sequences from modern humans. A
team led by researchers at Stanford University found that certain human
leukocyte antigen (HLA) alleles, key players in pathogen recognition,
held signs of archaic ancestry—from Neanderthals, but also from another
hominin cousin, the Denisovans.6 “It’s a cool paper and one that contributed to a lot of people thinking about the effects of introgression,” says Capra.
Several
other studies since then have strengthened the link between archaic DNA
and immune function, branching out from the HLA system to include
numerous other pathways.7
For example, multiple labs have tied Neanderthal variants to altered
expression levels of genes encoding toll-like receptors (TLRs), key
players in innate immune responses. In 2016, Kelso, Dannemann, and a
colleague found that pathogen response and susceptibility to develop
allergies were associated with Neanderthal sequences that affect TLR
production.8
Viruses, in particular, appear to be potent drivers of adaptation. Last year, University of Arizona population geneticist David Enard
and colleagues found that one-third of Neanderthal variants under
positive selection were linked to genes encoding proteins that interact
with viruses.9
Viral challenges, bacterial challenges are among the strongest selective forces out there. Pathogens can kill you in one generation.
—Janet Kelso, Max Planck Institute for Evolutionary Anthropology
Researchers
have also identified several less-easily explainable phenotypic
associations with Neanderthal introgression. In their 2017 analysis, for
example, Kelso and Dannemann found that Neanderthal variants were
associated with chronotype—whether people identify as early birds or
night owls—as well as links with susceptibility to feelings of
loneliness or isolation and low enthusiasm or interest. The associations
with mood-related phenotypes jibe with what Capra’s group had found the
year before in its dataset of medical information, which linked
Neanderthal variants to risks for depression and addiction. “These were
associations that were quite strong,” says Capra. “And when we looked at
genes where these regions of Neanderthal ancestry fell, in many cases
they made sense given what we already know about those genes.”
Why
these associations exist is still a mystery. Kelso suspects that light
might be a unifying factor, with both changes in day-length patterns and
UV exposure reductions as they moved to more-northern latitudes. But
that’s just a hunch, she emphasizes.
“It’s fun speculating about
how [Neanderthal introgression] could have been advantageous, or how
variants that make us depressed in the modern environment could have
been beneficial,” says Capra. “I don’t really even know what depression
meant 40,000 years ago. That’s both the challenge and the fun,
provocative part about all this.”
A question of functionality
Even
with more straightforward associations, such as with skin traits or
immune responses, conclusions thus far are drawn from correlations
between genotypes and phenotypes. While such genetic and statistical
approaches can conceptually link Neander-thal introgression with
phenotypes and hint at why such sequences may have been selected for in
humans’ early history, researchers have not yet published in vitro
validation studies.
“Studying Neanderthal DNA more closely on a
molecular level in the lab is pretty tricky,” says Dannemann.
Neanderthal variants tend to come in packages, and the linkage between
the variants makes it difficult to identify the function of each one, he
explains.
That challenge hasn’t stopped researchers from trying.
As a postdoc in Pääbo’s lab in Germany, Camp, along with Vernot, Kelso,
and Dannemann, established a handful of brain organoids
from induced pluripotent stem cell lines of modern Europeans who vary
in their Neanderthal-derived genetics, and tracked single-cell
transcriptomes as the cultured cells matured. The early data suggest
that the Neanderthal variants affect gene expression in the same way as
documented by previous work, validating the model.
See “Minibrains May Soon Include Neanderthal DNA”
But
such research is still in the proof-of-principle stage, says Camp, who
is continuing this work in his own lab in Switzerland. “Now you just
need to increase throughput. You need to do this for 100 or 200
individuals.” Even then, he adds, the conclusions researchers will be
able to draw will be limited. “I am a bit cautious and maybe pessimistic
[about whether] you can really identify . . . impacts [of Neanderthal
variants] on some physiological outcomes.”
There are other
fundamental questions that are proving difficult to answer about
Neanderthal introgression, says Akey, from the number of hybridization
events to the timescale over which those events took place, and whether
there was sex bias in patterns of gene flow. “There are all these
important things that are really hard to estimate,” he says. “I think
the field is kind of stuck right now.” But he’s hopeful that as more
genomes from various archaic hominin groups and from modern humans come
online, researchers’ power to model how all of these groups interbred
will strengthen. A second high-quality Neanderthal genome was published
in 2017 (Science, 358:655–58), and researchers now have the genome of a 40,000-year-old human who had a Neanderthal ancestor just a few generations back. Last year, researchers published the sequence of a first-generation hybrid of Denisovans and Neanderthals.
See “Girl Had a Denisovan Dad and Neanderthal Mom”
Those
data will likely yield some surprises. Capra has found evidence, for
example, that some of the Neanderthal segments that correlated with
modern phenotypes may not affect those pheno-types directly. His work
has uncovered cases in which the correlation was driven by sequences
close enough in the genome to Neanderthal variants that the two always
appear together. These sequences were carried by the common ancestor of
Neanderthals and modern humans but were missing from the group of humans
who founded the modern Eurasian population. These variants, which had
been retained by Neanderthals, were then reintroduced to the ancestors
of modern non-Africans during periods of interbreeding.10
“These genetic variants existed in modern [Eurasians only] in the
Neanderthal context, but these were not of Neanderthal ancestry,” Capra
says.
Akey has come upon another interesting twist: Africans do
have Neanderthal ancestry. Unpublished work from his group points to the
possibility that some of the ancient modern humans that bred with
Neanderthals migrated back to Africa, where they mixed with the modern
humans there, sharing bits of Neanderthal DNA. If true, that would mean
that Africa is not devoid of Neanderthals’ genetic influence, Akey
notes. “There’s Neanderthal basically all over the world.”
All About Regulation
Am J Hum Genet, doi:10.1016/j.ajhg.2019.04.016, 2019; the scientist staff
In
their seminal 2014 studies, the groups of David Reich of Harvard
Medical School and Joshua Akey, then at the University of Washington,
noted that the Neanderthal variants that correlated with human
phenotypes did not appear in coding regions. Two years later, a
genome-wide analysis published by investigators in France found that
Neanderthal ancestry was enriched in areas tied to gene regulation (Cell,
167:643–56.e17, 2016). The implication was that sequences that
originated in Neanderthals tend to have “less impact through protein and
more impact through gene expression,” says coauthor Maxime Rotival, a
geneticist at the Pasteur Institute in Paris.
To ask this question
more directly, Akey turned to the Genotype-Tissue Expression (GTEx)
Project, which has cataloged gene expression data from roughly 50
tissues for each of 10,000 individuals. “It’s this really fine-scale
record of gene expression,” says Akey. His then-postdoc Rajiv McCoy, now
an assistant professor at Johns Hopkins University, developed a method
to assess messenger RNA levels based on which allele was being
expressed—the one from an individual’s father or mother—and the
researchers applied this approach to people in the GTEx database who
were heterozygous for a particular Neanderthal variant. Comparing
expression levels based on which allele was being expressed, the
researchers found that a quarter of the stretches of Neanderthal DNA in
human genomes affect the regulation of the genes in or near those
stretches (Cell, 168:P916–27.E12, 2017).
“We’ve known for
a long time that gene expression variation is an important source of
phenotypic variation within populations and phenotypic divergence
between species,” says Akey. “We were interested in asking whether
Neanderthal sequences make any contribution to gene expression
variability.” The answer was a resounding yes.
Earlier this year,
Rotival and two colleagues calculated ratios of Neanderthal to
non-Neanderthal variants across the genome and compared those ratios for
protein-coding
regions and various regulatory sequences, specifically enhancers, promoters, and microRNA-binding sites. Consistent with previous results, they found a strong depletion of Neanderthal variants in coding portions of genes, and a slight enrichment of the archaic sequences in regulatory regions (Am J Hum Genet, doi:10.1016/j.ajhg.2019.04.016, 2019). “What we see is that in coding regions, the ratio of archaic to non-archaic variants is much smaller than the ratio outside of coding regions,” says Rotival. |
References
- R.E. Green et al., “A draft sequence of the Neandertal genome,” Science, 328:710–22, 2010.
- B. Vernot, J. Akey, “Resurrecting surviving Neandertal lineages from modern human genomes,” Science, 343:1017–21, 2014.
- S. Sankararaman et al., “The genomic landscape of Neanderthal ancestry in present-day humans,” Nature, 507:354–57, 2014.
- C.N. Simonti et al., “The phenotypic legacy of admixture between modern humans and Neandertals,” Science, 351:737–41, 2016.
- M. Dannemann, J. Kelso, “The contribution of Neanderthals to phenotypic variation in modern humans,” Am J Hum Genet, 101:P578–89, 2017.
- L. Abi-Rached et al., “The shaping of modern human immune systems by multiregional admixture with archaic humans,” Science, 334:89–94, 2011.
- H. Quach et al., “Genetic adaptation and Neandertal admixture shaped the immune system of human populations,” Cell, 167:643–56.e17, 2016.
- M. Dannemann et al., “Introgression of Neandertal- and Denisovan-like haplotypes contributes to adaptive variation in human toll-like receptors,” Am J Hum Genet, 98:P22–33, 2016.
- D. Enard and D.A. Petrov, “Evidence that RNA viruses drove adaptive introgression between Neanderthals and modern humans,” Cell, 175:P360–71.E13, 2018.
- D.C. Rinker et al., “Neanderthal introgression reintroduced functional alleles lost in the human out of Africa bottleneck,” bioRxiv, doi:10.1101/533257, 2019.
Jef Akst is the managing editor of The Scientist. Email her at jakst@the-scientist.com.
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