A mystery engraved in DNA: traces of a "ghost ancestor" are found
A study by the University of Berkeley has discovered that humans reproduced with two unknown archaic lineages
BarcelonaScientists from the University of Berkeley and Johns Hopkins University have revealed a new secret in the history of the human species: they have found traces from two unknown ancestral lineages in human DNA. These are lineages we did not inherit from Neanderthals or Denisovans, but rather from older species. However, their appearance, where they lived, or which species they belonged to are questions still without an answer.
Human history is much more complex than we believed. The first lineage described in the study, led by Yulin Zhang, Arjun Biddanda, and Priya Moorjani, is the most surprising: it comes from a “ghost ancestor”. Far from being an imaginary creature, this scientific term refers to an ancient population with whom our ancestors coexisted, but from which no sequenced fossil DNA exists, nor fossil remains. Nevertheless, experts deduce that it existed because there are genetic fragments from this ancestor that have left a mark on the DNA of its descendants.
According to the study, this "ghost ancestor" separated from the lineage that would eventually give rise to Homo sapiens more than 800,000 years ago. Although the two branches continued to evolve in parallel, they met again in Africa and, according to this study, reproduced thousands of years later – specifically, more than 50,000 years ago. Despite being a singular interbreeding that was unknown until now, it left an indelible mark on our genetics. The resulting descendants, carrying DNA from the "ghost ancestor", expanded it throughout Europe and Asia when modern humans made the great migration.
The genes resulting from this interbreeding now form part of between 0.5 and 1% of our genome. That is, the same proportion of Neanderthal DNA as is within our genetic makeup. Although previous research had suggested that we might have "ghost ancestors", it was not yet known when the two populations mixed. Furthermore, it was also believed that this mysterious genetic trace was present only in Africans. The novelty of this study, according to Zhang, is that genetic traces of these ancestors have been geolocated around the planet, which has allowed us to conclude that it is found in "all modern humans".
More complex relationships
The other mysterious ancestor discovered by the study has been categorized as "superarchaic". According to experts, it belonged to a lineage of hominins that encountered Denisovans in Europe and Asia, and later interbred with Homo sapiens, also leaving a trace in the DNA of modern humans. In particular, this finding is especially revealing because it demonstrates not only that modern humans shared space and time with various groups of hominins from lineages originating in Africa, Europe, and Asia.
The research also shows that they were genetically compatible enough to interbreed with them. "It is particularly exciting because it reveals genetic contributions from a human lineage that lived more than a million years ago," explains Biddanda. Furthermore, the research also shows that, contrary to what has been thought for many years, the relationships between lineages are more complex than previously believed. As Priya Moorjani explains, different populations are not like "a branching tree", but rather have a much more interconnected history through migratory and reproductive movements.
The study has used a new method, Tracking Archaic Contributions via ARG Estimation (TRACE), which allows the detection of DNA from ancient human populations even when no fossil with sequenced DNA is available. The usual method consists of comparing our genome with that of a known ancestor. In contrast, TRACE allows the analysis of hundreds of genomes from current individuals to reconstruct the genealogical history of small DNA fragments; a kind of complex family tree indicating when different DNA fragments shared a common ancestor.
