How a tiny group of human ancestors survived a near-extinction event

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A new study reveals that the human lineage almost went extinct around 900,000 years ago, when a severe population bottleneck reduced the number of breeding individuals to just 1,280. The researchers used a novel technique to reconstruct the ancient population dynamics based on modern genetic data. They also suggest that this bottleneck may have given rise to a new hominin species that is the last common ancestor of Neanderthals, Denisovans and modern humans.

A new method to trace our ancestral history

The study, published in Science, was led by Wangjie Hu and Haipeng Li from the University of Chinese Academy of Sciences in Beijing. They developed a new method to analyse the genetic lineages of 3,154 modern humans from different regions and populations. By constructing a complex family tree of genes, they were able to examine the finer branches of the tree with greater precision, identifying significant evolutionary events.

The researchers focused on the period between 800,000 and one million years ago, which is known as the Early-Middle Pleistocene transition. This was a time of drastic climate change, when glacial cycles became longer and more intense. The fossil record in Africa and Eurasia from this period is patchy and incomplete, making it difficult to infer the population history of our ancestors.

The new method allowed the researchers to fill in some of the gaps and estimate the effective population size (EPS) of our ancestral lineage. EPS is a measure of genetic diversity that reflects the number of individuals who contribute genes to the next generation. The researchers found that the EPS of our ancestors dropped to a minimum of 1,280 around 900,000 years ago, indicating a severe bottleneck that lasted for about 117,000 years.

A close call for human survival

The bottleneck implies that our ancestors faced a high risk of extinction during this period. The researchers estimate that about 98.7% of human ancestors were lost, leaving only a tiny group of survivors. The causes of the bottleneck are unclear, but they could be related to environmental factors, such as harsh climate conditions, habitat loss, disease outbreaks or competition with other hominins.

human ancestors survived a near-extinction event

The bottleneck would have had profound consequences for the genetic diversity and evolution of our ancestors. The researchers suggest that it would have increased inbreeding and reduced genetic variation within the population. It would have also affected the rate and direction of natural selection, leading to changes in traits and adaptations.

Nick Ashton, an archaeologist at the British Museum in London who wrote a related perspective article, says he was intrigued by the small size and long duration of the population. “This would imply that it occupied a very localized area with good social cohesion for it to survive,” he says. “Of greater surprise is the estimated length of time that this small group survived. If this is correct, then one imagines that it would require a stable environment with sufficient resources and few stresses to the system.”

A new species emerges from the bottleneck

The researchers also propose that the bottleneck may have coincided with the emergence of a new hominin species that is ancestral to both Homo heidelbergensis and a species that later split into Neanderthals, Denisovans and modern humans. This species is currently unknown and unnamed, but it may have evolved from an earlier species such as Homo erectus or Homo antecessor.

The timeline of the bottleneck matches some existing genetic estimates that suggest that this new species diverged from H. heidelbergensis around 900,000 years ago. H. heidelbergensis is an extinct hominin that lived in Africa and Eurasia between 700,000 and 300,000 years ago. It is considered to be either a direct ancestor or a close relative of Neanderthals and Denisovans in Eurasia, and possibly also of modern humans in Africa.

The researchers speculate that the new species may have originated in Africa and then migrated to Eurasia, where it gave rise to Neanderthals and Denisovans. Alternatively, it may have originated in Eurasia and then migrated back to Africa, where it gave rise to modern humans. The exact geographic origin and dispersal patterns of this species remain unknown.

Serena Tucci, an anthropologist at Yale University who was not involved in the study, says that the study raises some very interesting questions about human evolution during a poorly understood time period. “We still know very little about the population dynamics of early human ancestors for several reasons, including methodological limitations and difficulties in obtaining ancient DNA data from old Homo specimens,” she says.

She adds that more studies are needed to confirm and refine the findings of this study, especially by comparing them with ancient DNA data from fossils. “Ancient DNA data can provide direct evidence for population size changes over time and can help us identify which hominin groups were present at different times and places,” she says.

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