Human ancestors may have almost died out after ancient population crash | Natural History Museum

The exact paths of human evolution are still uncertain, but the new paper offers new routes to be explored. Image © The Trustees of the Natural History Museum, London.

NewsHuman ancestors may have almost died out after ancient population crash<br>By James Ashworth<br>First published 31 August 2023

Humans may have been driven to the brink of extinction within the past million years, new research suggests.

While the claims of a population bottleneck could explain longstanding mysteries in understanding our evolution, questions remain over the paper’s conclusions.

Almost 99% of all human ancestors may have been wiped out around 930,000 years ago, a new paper has claimed.

The new research, published in the journal Science, used DNA from living people to suggest that humans went through a bottleneck, an event where populations shrink drastically. The paper estimates that as few as 1,300 humans were left for a period of around 120,000 years.

While the exact causes aren't certain, the near-extinction has been blamed on Africa’s climate getting much colder and drier.

Dr Yi-Hsuan Pan, who co-authored the research, says, ‘The novel finding opens a new field in human evolution by raising many new questions.’

‘Where did these individuals live? How did they overcome catastrophic climate change? And did natural selection during the bottleneck affect their evolution? All this remains to be answered.’

While other scientists have welcomed the research, they have questions over just how severe this bottleneck was.

Professor Chris Stringer, an expert in human evolution from the Natural History Museum who reviewed the paper, says, ‘During the period of the bottleneck, there are a number of sites that record evidence of human occupation. This could mean that there wasn’t a bottleneck, but could equally mean it only had a localised impact on part of the human population.’

‘While there are a lot of uncertainties around this proposed bottleneck, it raises some interesting ideas and demonstrates the need for more fossils from this point in human evolution to answer fundamental questions about our origins.’

The bottleneck may be related to ancient climate change in Africa when the continent became colder and drier. Image © Maciej Czekajewski/Shutterstock.

How was the human bottleneck discovered?

Evolution is full of bottlenecks, and the history of humanity is no different. Disease, natural disasters and climate change can all drive species to the edge of extinction, forcing them to adapt or die.

Not all bottlenecks are as violent, however, and perhaps the most notable bottleneck in human history is an example of this. When our species, Homo sapiens, left Africa, only a small group of the entire population went on this migration.

At a genetic level, this means that only a fraction of the Homo sapiens gene pool was brought along for the journey. When examining the DNA of living people, the signature of this bottleneck can be seen in the genetics of people with non-African heritage.

The bottleneck discovered in the current research happened hundreds of thousands of years before this 'Out of Africa' migration. It was identified using a methodology developed by the researchers known as FitCoal, which analysed the genomes of almost 3,200 people from populations inside and outside Africa.

While the mathematics involved in the process are quite complicated, FitCoal is based on the simple idea that all of our DNA and the genes it contains have been inherited from our ancestors over hundreds of thousands of years.

As these genes are traced back through time, they will merge back into older versions that they evolved out of and eventually coalesce into a common ancestor. This process is affected by the size of populations, allowing researchers to assess how this has changed over time.

‘Large populations can contain more genetic variation than small ones, so you can use the coalescence of genes to assess how diverse a population was at a certain point in time,’ Chris says.

The scientists estimate that there would have only been an effective population size of around 1,280 individuals between 930,000 and 813,000 years ago, suggesting the presence of a bottleneck. While that doesn’t mean this was the entire population, only this number successfully bred and passed on their genes to the next generation.

Such a low population size would have posed problems for humans, making it more likely that they would become extinct.

Lead author Dr Wangjie Hu says, ’We first found hints of something really interesting happening in human population size at this time about five years ago, but the real “wow” moment came after we multi-checked our findings using FitCoal.’

‘We realized we had discovered something big about human history and that there's a lot more to uncover.’

Fossils of Homo antecessor are found from the time of the bottleneck,...