Emerging evidence from DNA analyses is revealing insights into human ancestors that have eluded researchers due to the absence of fossil records, highlighting possible new ancient lineages within modern genomes, reports BritPanorama.
Decades of research have confirmed that Homo sapiens interbred with now-extinct relatives, such as Neanderthals and Denisovans. However, certain lineages have remained elusive, often categorized as genetic “ghosts” due to the lack of recoverable DNA from their remains. Prior studies indicated remnants of such lineages but were unable to pinpoint the timing of interbreeding or trace their divergence from the ancestors of modern humans.
Recent advancements allow scientists to analyze the genomes of contemporary humans to reconstruct segments of the human family tree previously thought lost. A new study, published on July 30 in the journal Science, reports uncovering two additional ancestor lineages. One lineage directly interbred with Homo sapiens in Africa over 50,000 years ago, while the second, an even more ancient group, contributed to modern humans indirectly through Denisovans.
The study employs a novel method to create genealogies that illustrate the ancestry of human DNA. “These genealogies really allow us to look into the past in a way that we have not been able to do before,” noted Priya Moorjani, a co-author and associate professor at the University of California, Berkeley. This approach not only uncovers previously hidden lineages but also facilitates explorations into why certain hominin groups thrived while others became extinct.
Hunting for ghost lineages
The migration of modern humans from Africa approximately 50,000 years ago marks a pivotal moment of interbreeding with Neanderthals and Denisovans. However, the newly identified lineages add complexity to this narrative. One lineage likely interbred with early humans in Africa prior to this migration, contributing an estimated 0.5% to 1% of the DNA in today’s populations, a figure comparable to the Neanderthal DNA found in modern humans.
The second lineage, termed “super-archaic,” traces back to a branch that diverged around 1.8 million years ago. This hominin likely interbred with Denisovans in Eurasia over 200,000 years ago, with some genetic material eventually passing to modern humans through Denisovan ancestry.
Utilizing a computer-based method named TRACE, researchers sifted through over 500 genomes from a wide range of populations across Africa, Europe, and Asia. This analysis revealed signs of recent ghost ancestry in all populations studied, indicating a common lineage that likely contributed DNA to modern humans before the major exodus from Africa.
“The fact that we harbor DNA from different hominin groups reflects widespread gene flow across populations,” said Moorjani. Despite the challenge of extracting DNA from ancient populations, the modern genome offers a window to these lost ancestors.
Identifying mysterious ancestors
While the specific lineage details remain uncertain, researchers have narrowed down potential candidates based on divergence times within the hominin family tree. One lineage’s divergence aligns with populations like Homo heidelbergensis, which existed in Africa and Europe from around 700,000 to 200,000 years ago. The older lineage might relate to Homo erectus, a group from Eurasia, though no DNA has yet been recovered from either.
“The super-archaic finding is particularly exciting, as it unveils genetic contributions from a human lineage that lived over a million years ago,” said Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and co-first author of the study.
John Hawks, a paleoanthropologist not involved in the research, emphasized the significance of deep ancestral lineages that contribute to modern genetic diversity. “Today we tend to focus on superficial differences, but these reflections show our interconnectedness and shared ancestry, a reality of growing importance,” he added.
The discovery of this younger ghost lineage signals that interbreeding occurred before the interaction with Neanderthals, impacting all contemporary populations. Researchers express hope that new methods like TRACE will reveal further hidden lineages across different species. However, they acknowledge the need for direct DNA from ancient fossils to enhance these findings.
Currently, the evolutionary insights provided by modern DNA are vital to connecting with our long-lost ancestors. “Randomly, two people from the world might inherit the same piece of DNA from an ancestor who we didn’t have an idea of before, but now we actually know,” said Yulin Zhang, a doctoral student at UC Berkeley and co-first author of the study. “From this mysterious ancestor, we have something in common from another angle.”
