Scientists have discovered that an unknown group of human ancestors contributed 20% of modern human genes, possibly enhancing brain function and shaping evolution.
The study, published March 18 in Nature Genetics, reveals that early humans split from this mysterious population 1.5 million years ago before merging again 300,000 years ago.
Ancient human populations: A and B
The research, conducted using data from the 1000 Genomes Project and the Human Genome Diversity Project, identified two ancient groups, Population A and Population B. These groups split 1.5 million years ago, with Population A later giving rise to modern humans.
Over time, Population A suffered a sharp decline, losing much of its genetic diversity before recovering. Neanderthals and Denisovans eventually branched off from this lineage.
Genetic exchange and its impact on evolution
DNA analysis suggests that when Population A and Population B mixed again 300,000 years ago, genetic material from Population B played a role in human evolution.
When Asian Homo erectus came back to Africa as Homo sapiens they interbred with the relic African Homo erectus populations.
This chart from the recent study, A structured coalescent model reveals deep ancestral structure shared by all modern humans, is useful for me. I am lazy… pic.twitter.com/V2T39NHHJs
— Bruce R. Fenton (@GeologicalSETI) March 14, 2025
Trevor Cousins, a genetics researcher at the University of Cambridge and co-author of the study, said genes from Population B, particularly those linked to brain function, may have influenced cognitive development.
However, scientists also found that this genetic exchange may have reduced reproductive success in some individuals.
The mystery of ancient human lineages
The study highlights a complex history of human evolution, challenging the idea that species evolve in clear, separate lineages. Instead, interbreeding and genetic exchange were likely common.
Although researchers believe early species such as Homo erectus and Homo heidelbergensis could be candidates for Populations A and B, no direct link has been confirmed.
“The genetic model cannot indicate which fossils should be assigned to Population A or B,” Cousins explained. “We can only speculate.”
Some experts refer to these lost groups as “ghost populations” – ancient human lineages that disappeared but left genetic traces behind.
🧬🔬 Thanks to genetic science, we now know that:
– 10-20% of the ancestors of Eurasian people are neanderthal
– West African populations have up to 19% "Ghost DNA", belonging to an extinct species their ancestors interbred with
– Southeast Asians have DNA of the extinct… pic.twitter.com/0v1sRffHtE
— Keith Woods (@KeithWoodsYT) September 4, 2024
John Hawks, a biological anthropologist at the University of Wisconsin-Madison who was not involved in the study, said the findings suggest a “deep African structure” shared by all humans today.
Limitations of the study
Despite its breakthroughs, the study has limitations. Hawks noted that the research relies on data from the 1000 Genomes Project, which underrepresents African populations. This means the findings may not fully capture the diversity of ancient human groups.
“So I see this as more a proof of principle than a real guide to what ancient humans were doing,” Hawks said.
While the study provides new insights, many questions remain. Scientists continue to refine genetic models, uncovering the complexities of human evolution. What is clear, however, is that modern humans carry the genes of their ancestors that shaped their brain function in ways researchers are only beginning to understand.
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