Seventy-five thousand years ago, modern humans traveled to Eurasia, where
they met and interbred with Neanderthals. It turns out that Neanderthals
were already carrying human DNA from a far earlier interaction with modern
humans, according to a recent study published in the journal
Current Biology.
The research team led by Penn, with participation from the Universities of
Addis Ababa, Botswana, Fudan, Hubert Kairuki Memorial, and Yaoundé,
demonstrated that an extinct lineage of modern humans migrated to Eurasia
more than 250,000 years ago, where they interbred with Neanderthals. These
people eventually went extinct, leaving a population that was primarily
descended from Neanderthals.
Alexander Platt is a senior research scientist at the Perelman School of
Medicine and one of the study's initial authors. "We found this reflection
of ancient interbreeding where genes flowed from ancient modern humans into
Neanderthals," he adds. "These people departed from Africa somewhere between
250,000 and 270,000 years ago. They were far more like us than Neanderthals,
and they were related to all modern humans."
By contrasting a variety of genomes from contemporary indigenous groups in
sub-Saharan Africa with a Neanderthal genome, the researchers were able to
draw this result.
Senior author of the study and a professor at Penn Integrates Knowledge
University, Sarah Tishkoff, states, "This study highlights the importance of
including ethnically and geographically diverse populations in human
genetics and genomic studies."
Neanderthal ancestry is expected to be limited in sub-Saharan Africa
because the majority of Neanderthal-human interbreeding is thought to have
occurred in Eurasia rather than Africa. However, a recent study made the
startling discovery that several sub-Saharan populations contain segments of
DNA that resemble Neanderthal DNA. The study could not find out how this
Neanderthal-like DNA got into these populations, if it came from modern
humans who had once migrated from Africa, interbred with Neanderthals in
Eurasia, and then moved back to Africa, or if it came from a previous
human-Neanderthal encounter.
It was also unclear if Neanderthal-like DNA is common among sub-Saharan
people because the study relied on a small number of genomes from the 1,000
Genomes Project, all of which share a relatively recent common ancestor in
Central and Western Africa.
Tishkoff's team used a genetically varied sample of 180 individual genomes
from 12 distinct communities in Cameroon, Botswana, Tanzania, and Ethiopia
to better understand how ubiquitous these Neanderthal-like DNA regions are
across sub-Saharan Africa and to unravel their origins. The scientists
located Neanderthal-like DNA areas and searched for Neanderthal ancestry in
each genome.
Next, they made a comparison between the genomes of current humans and a
Neanderthal that lived about 120,000 years ago. The researchers used a
cutting-edge statistical technique to compare the two groups and ascertain
where the Neanderthal-like DNA originated in these contemporary sub-Saharan
populations—from modern humans who inherited the region from Neanderthals,
or from modern humans who inherited it from Neanderthals and brought it back
to Africa.
They discovered that Neanderthal-like DNA was present in every sub-Saharan
community, suggesting that this phenomena is common. Most of the time, the
old lineage of modern humans that gave rise to Neanderthals when they moved
from Africa to Eurasia around 250,000 years ago is the source of this
Neanderthal-like DNA. Approximately 6% of the Neanderthal genome was
inherited from modern humans as a result of this interbreeding between
modern people and Neanderthals.
The researchers also discovered evidence of Neanderthal heritage in some
sub-Saharan communities, which was brought to these populations when people
with Neanderthal DNA came back into Africa. The Amhara from Ethiopia and the
Fulani from Cameroon had the greatest percentages of Neanderthal ancestry
among these sub-Saharan groups, which varied from 0 to 1.5%.
The researchers also looked at the locations of these pieces of
contemporary human DNA to try to determine if carrying it was beneficial or
detrimental when inserted into the Neanderthal genome. The researchers also
looked at the locations of these contemporary human DNA segments inside the
Neanderthal genome in an effort to determine if harboring modern human DNA
was advantageous or detrimental. The majority of the modern human DNA was
discovered to be in noncoding regions of the Neanderthal genome, suggesting
that the presence of modern human genes in a Neanderthal background is
detrimental to fitness. This finding also suggests that modern human gene
variants were being preferentially lost from coding sections of the
genome.
This is comparable to what has been observed in contemporary humans, when
Neanderthal genes have been gradually eliminated from populations due to
natural selection. "A Neanderthal allele may therefore function perfectly in
Neanderthals, but when it is inserted into the current human genome, it
causes issues. Neanderthals and modern humans both gradually eliminated the
other group's alleles," according to Platt.
"In the almost 500,000 years between the ancestors of Neanderthals
splitting off from the ancestors of modern humans and these other modern
humans being reintroduced to Neanderthal populations, we had become such
different organisms that, although we were still able to interbreed quite
readily, the hybrids didn't work so well, which means we were very far along
the path to becoming distinct species."
By locating a genetic reference of a population that is on a branch of the
human family tree that was previously absent from the genomic and fossil
record, this research provides new opportunities for studying the evolution
of humans.
The second first author of the paper, Daniel Harris, is a postdoctoral
research researcher at the Perelman School of Medicine. "Discovering this
ancient lineage of modern humans is really exciting for future research
because it gives us a different lens to look at human evolution," Harris
says. "Because we don't have DNA sequences from modern human fossils from
that long ago, identifying these sequences will shed light on very early
modern human evolution in Africa."
Provided by
University of Pennsylvania
