Genetic Mapping Unmasks Two Extinct Human Lineages Hidden in Modern DNA
University of California, Berkeley researchers have identified two previously unknown 'ghost' ancestors that interbred with ancient humans. The study reveals that modern genomes contain genetic signatures from lineages dating back nearly 2 million years.
Key takeaways
- UC Berkeley researchers identified two previously unknown 'ghost' ancestors in the modern human genome using a new genealogical tool called TRACE.
- One ghost lineage diverged 800,000 years ago and contributed 1% of the DNA found in all modern humans, regardless of geographic origin.
- A second 'super-archaic' lineage dating back 1.8 million years interbred with Denisovans, eventually passing DNA into the modern human gene pool.
- The study suggests human evolution is an interconnected web of frequent interbreeding rather than a simple branching tree of isolated species.
- The findings align these unknown groups with Middle Pleistocene Homo and Homo erectus populations based on the timing of their evolutionary splits.
The Genomic Web of Human Ancestry
For years, the narrative of human evolution focused on the well-documented contributions of Neanderthals and Denisovans. However, new research published in the journal Science on July 30 indicates that the human family tree is far more entangled. Utilizing a sophisticated analytical tool dubbed TRACE (TRacking Archaic Contributions via ARG Estimation), researchers at UC Berkeley identified genetic material from two previously unidentified hominin groups within the genomes of living people.
This method allows scientists to bypass the need for physical fossils. By examining ancestral recombination graphs (ARGs), the team reconstructed genealogical connections across hundreds of modern genomes. This process identified segments where ancestry stretches significantly further back than typical human lineages, signaling the presence of extinct 'ghost' populations.
A Universal Legacy from Africa
The first 'ghost' ancestor identified by the team contributed approximately 1% of the modern human genome—a proportion comparable to Neanderthal inheritance. This group diverged from the ancestors of modern humans roughly 800,000 years ago, a timeline that aligns with the emergence of Middle Pleistocene Homo groups in Africa. According to Yulin Zhang, a Berkeley graduate student and co-first author, this genetic signature is not limited to specific regions.
"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," Zhang noted. The interbreeding event likely occurred more than 50,000 years ago, prior to the major migration of Homo sapiens into Europe and Asia.
Deep History and Super-Archaic Connections
The second discovery involves an even older lineage, categorized as a 'super-archaic' ancestor. This population appears to have branched off nearly 1.8 million years ago, a date reflecting the era of Homo erectus in Eurasia. While these ancestors did not breed with modern humans directly, they interbred with Denisovans more than 200,000 years ago. When Denisovans later mixed with Homo sapiens, they passed this ancient genetic material down to us.
Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and co-first author, described the finding as a revelation of genetic contributions from a lineage that lived over a million years ago, despite the total absence of sequenced DNA from that specific population. This discovery suggests that early humans shared the planet with various related groups for over a million years, maintaining enough genetic similarity to produce offspring.
Redefining Evolutionary Theory
The findings challenge the traditional 'branching tree' model of evolution. Associate professor Priya Moorjani, who led the research, suggests that human history is better described as an interconnected web of populations defined by persistent migration and mixing. By identifying regions with exceptionally deep ancestry, TRACE has provided a window into the Middle Pleistocene and earlier, periods previously obscured by a lack of fossilized DNA. This research confirms that genetic exchange was a pervasive element of the human experience across vast timescales.
Source: ScienceDaily
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