Every person alive today carries DNA from an extinct human lineage that has never been found in a single fossil. That’s the finding of a study published in the journal Science on July 30 by researchers at the University of California, Berkeley, who identified genetic traces of two previously unknown ancestors hidden within modern human genomes.
Homo sapiens once shared the planet with several other human forms, and earlier research had already confirmed interbreeding with Neanderthals and Denisovans.
To detect lineages without sequenced DNA at all, the Berkeley researchers, headed by computational biologist Yulin Zhang and geneticist Arjun Biddanda of Johns Hopkins University, came up with a novel approach, known as TRACE, capable of detecting the ancient ancestry of people by their contemporary genomic structure.
The first lineage, known as “the ghost ancestor”, branched off from the lineage of the present-day humans more than 500,000 years ago, at the same time when the ancestors of Neanderthals and Denisovans emerged.
This lineage was mixing with the lineage of the present-day humans in Africa more than 50,000 years ago prior to the mass migration of people from Africa and currently comprises 0.5% to 1% of each person’s genome.
“We have now been able to identify and map the relevant DNA segments and show that this ghost lineage is found in all people living today, not just in Africans,” stated Zhang.
The second group, known as the “super-archaic” group, diverged some 1.8 million years ago, about the time when Homo erectus migrated into Eurasia.
This lineage was interbreeding with the Denisovans over 200,000 years ago, influencing modern humans indirectly via their genes in the Denisovans that are currently in Oceanians.
“The evidence for this super-archaic ancestor is particularly exciting because it reveals genetic contributions from a lineage that lived more than a million years ago, even though no DNA from this population has yet been decoded,” said Biddanda.
According to Priya Moorjani, a professor at the University of California-Berkeley, this new discovery supports the changing view about human origin that is now viewed not in the form of an evolutionary tree but rather in the form of a web in which different populations met, mixed, separated, and continued on their way.
