When Yulin Zhang sat down to study human DNA, she expected to find the usual traces of Neanderthal and Denisovan ancestry. Instead, she and her colleagues at UC Berkeley discovered something unexpected: two completely unknown groups of ancient human relatives hiding inside all of us.
One of these mysterious ancestors, which scientists call a "ghost ancestor," shared genes with modern humans in Africa more than 50,000 years ago. That encounter happened even before our ancestors left Africa to settle the rest of the world. Today, about 1% of every person alive carries DNA from this unknown relative — roughly the same amount we inherit from Neanderthals.
"We were actually able to find and map genomic locations in modern humans that are from this ghost lineage and show that this ghost ancestry is in all modern humans, not only in Africans," said Zhang, a Berkeley graduate student and co-lead author of the study.
The second discovery is even older. The researchers found genetic material from what they call a "super-archaic ancestor" — a human lineage dating back roughly 1.8 million years. This incredibly ancient group passed DNA to Denisovans, who later mixed with modern humans. In this roundabout way, a tiny piece of genetic material from over a million years ago still survives inside people living today.
"The super-archaic finding is particularly exciting because it reveals genetic contributions from a human lineage that lived over a million years ago, despite the absence of any sequenced DNA from that population," said Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and study co-author.
The team developed a new computer technique called TRACE to find these hidden ancestors. Rather than needing fossils — which are extremely rare — the method reads the genealogical "family tree" written in the DNA of people alive now. By comparing hundreds of modern genomes, the researchers could spot branches pointing to unknown ancestors.
What does this mean for understanding who we are? The findings add to a growing picture of human evolution that looks less like a simple tree and more like a tangled web. Our early ancestors kept bumping into related groups, and whenever they did, they sometimes had children together.
"We often think of human evolution as a branching tree, but new genomic data and analytical methods reveal a much more interconnected history," said Priya Moorjani, a Berkeley associate professor who supervised the project. "More like a complex web of populations connected by repeated episodes of migration and mixing."
Scientists still do not know exactly who these ghost and super-archaic populations were. But the timing suggests they might be connected to other human groups that lived in Africa around 800,000 years ago and to Homo erectus populations that spread across Asia starting 1.8 million years ago.
The research, published in the journal Science on July 30, shows that every person alive today carries genetic echoes of encounters that happened tens of thousands of years before recorded history. We are, it turns out, living libraries of ancient relationships — a reminder that humanity has always been woven together.