The past is present when
it comes to human evolution.
That seems to be the
takeaway from new research that concludes “archaic” humans, somewhere
in Africa during the last 20 to 60 thousand
years, interbred with anatomically modern humans and transferred small amounts
of genetic material to their offspring who are alive today.
University of Arizona geneticist Michael Hammer and a
team of evolutionary biologists, geneticists, and mathematicians report the
finding in Proceedings of the National
Academy of Sciences.
The National Science
Foundation (NSF) in Arlington,
Va., funded the study.
“It appears some
level of interbreeding may have occurred in many parts of the world at
different times in human evolution,” says Hammer, whose work is the first
to definitively suggest interbreeding between separate human forms inside of Africa.
Previous studies primarily
examined modern human interbreeding with Neanderthals in Europe or other
archaic forms in Asia. Analyses of ancient DNA
in those studies suggest a small amount of gene flow occurred from Neanderthals
into non-African offspring sometime after anatomically modern humans left Africa.
discovered evidence that a distinct group of extinct archaic humans who lived
in Southern Siberia, called ‘Denisovans,’
contributed genetic material to the genomes of present-day Melanesians.
As Hammer and colleagues
write in their recent report, “Given recent fossil evidence, however, the
greatest opportunity for introgression was in Africa” where anatomically
modern humans and various archaic forms co-existed for much longer than they
did outside of Africa.
“We estimate that the
archaic DNA fragments that survive in modern African genomes come from a form or
forms that diverged from the common ancestor of anatomically modern humans 700
thousand to 1 million years ago,” says Hammer.
“This archaic genetic
material is more prevalent in west-central African populations, possibly
reflecting a hybridization event or process that took place in central Africa,” he says. “The populations that
interbred were similar enough biologically so that they were able to produce
fertile offspring, thus allowing genes to flow from one population to the
Hammer and team studied
DNA sequence data from three sub-Saharan African populations: Mandenka, Biaka,
and San to test models of African archaic admixture.
The Mandenka are an
agricultural population from West Africa, while the Biaka and the San are
historically isolated hunter-gatherer populations from Central and Southern Africa, respectively.
The team conducted
extensive simulations to test the likelihood of gene flow from an archaic
population to anatomically modern humans. The simulations rejected the default
position, or null hypothesis, that no admixture took place.
The result gave the
scientists confidence to infer that contemporary African populations contain a
small proportion of genetic material—about 2%—that moved from a species of
archaic humans into the gene pool of anatomically modern humans about 35
thousand years ago.
They surmise that this
archaic population split from the ancestors of anatomically modern humans about
700 thousand years ago.
“We do not have
ancient DNA, i.e., from a fossil specimen, to directly compare with DNA from
contemporary populations, so our approach was indirect or inferential,”
says Hammer. But there are several candidates in the African fossil record that
may have added their genetic material to modern humans such as Homo heidelbergensis, an extinct
species of the genus Homo
that lived between 600 and 400 thousand years ago.
represents an approach to answering long-standing questions about the
contributions of archaic, extinct forms of our genus to the gene pool of our
modern human species,” says Carolyn Ehardt, Program Director for
Biological Anthropology at NSF. “Previous to the advent of contemporary
genetic techniques and approaches, the addressing of such pivotal questions in
bioanthropological science was prohibitive.”
Additionally, the research
could provide greater insight into human physiology and assist the
understanding of human diseases.
“It is entirely possible that some of the genes that were picked up
from archaic forms by the ancestors of modern Africans were beneficial and are
now part of the functional physiological machinery of contemporary
populations,” says Hammer. “This could be in the form of disease
resistance alleles or other gene variations that led to novel adaptations in
the modern population.”