Pre-humans may have lived in a population of only 1,280 people, according to a new technique for analyzing contemporary genetic data.
An unknown species of early human nearly died out around 900,000 years ago, according to genetic analysis. It might have been both the ancestor of Homo heidelbergensis and a species ancestral to our own.Credit: S. Entressangle/E. Daynes/Science Photo Library
According to a study, some 900,000 years ago, human ancestors in Africa were on the verge of extinction. The research, which was published in Science, implies that our predecessors' number was drastically reduced long before our species, Homo sapiens, developed. After being reduced to just 1,280 breeding individuals, the population did not grow again for another 117,000 years.
"About 98.7% of human ancestors were lost," says Haipeng Li, a population geneticist at the University of Chinese Academy of Sciences in Beijing and one of the study's co-authors. According to him, there is a spotty fossil record in Africa and Eurasia from 950,000 to 650,000 years ago, and "the discovery of this bottleneck may explain the chronological gap."
A related perspective was written by archaeologist Nick Ashton of the British Museum in London. He says he was fascinated by the small 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.”
Modern DNA reveals clues
The scientists had to develop brand-new tools in order to make their finding. The researchers created a mechanism that allowed them to fill in the blanks concerning earlier human relatives. Genome sequencing advancements have increased scientists' understanding of population sizes for the time after modern humans arose. Such research was urgently required, according to Serena Tucci, an anthropologist at Yale University in New Haven, Connecticut. “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,”
Through the use of their approach, the researchers were able to reconstruct ancient population dynamics using genetic information from living individuals. The team was able to more precisely investigate the finer branches of the tree and pinpoint major evolutionary events by building a complicated family tree of genes.
The method "put the spotlight on the period 800,000 to one million years ago — for which there is much unknown — in a way that hasn’t been done before," says Stanley Ambrose, an anthropologist at the University of Illinois at Urbana-Champaign.
This time period was a component of the Early-Middle Pleistocene Transition, a period of abrupt climate change during which glacial cycles lengthened and intensified. This resulted in protracted droughts across Africa. According to Li, the climatic changes may have killed off the ancestors of modern humans and pushed the emergence of new human species. These might have eventually developed into the last common ancestor of contemporary humans and our extinct ancestors, the Denisovans and Neanderthals.
The number of pre-humans started to increase once more at 813,000 years ago. Ziqian Hao, a population geneticist at the Shandong First Medical University and Shandong Academy of Medical Sciences in Jinan and a co-author of the paper, says it is still unclear how our ancestors managed to survive and what enabled them to flourish once more. He claims, however, that the bottleneck is likely to have had a significant impact on human genetic variety, driving several significant aspects of modern humans, like brain size. He says that the genetic diversity may have been lost by up to two-thirds. “It represents a key period of time during the evolution of humans. So there are many important questions to be answered.”
Ashton desires more archeological and fossil evidence to support the researchers' conclusions. The authors “suggest that the bottleneck was a global crash in population”, he says, “but the number of archaeological sites outside Africa suggests that this is not the case. A regional bottleneck might be more likely.”
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