Our ancestors were pushed to the brink during ancient African extinction event

Almost two-thirds of all African mammals were wiped out as the world's temperature dropped over 33 million years ago.

This is the finding of a group of international scientists, who discovered that rodents and primates lost 63% of their diversity in only a few million years. Their work challenges previous assumptions that African mammals rode out this extinction event due to the continent's warmer climate. 

Instead, rates of extinction approached the global average, pushing our early ancestors to the brink. 

Professor Erik Seiffert, who co-authored the paper, says, 'In our anthropoid ancestors, diversity bottoms out to almost nothing around 30 million years ago. 

'It's an interesting story about the role of that bottleneck in our own early evolutionary history. We came pretty close to never existing. Luckily, our monkey-like ancestors survived the extinctions that took place 30 million years ago.' 

The research, by British, American and Egyptian scientists, was published in Communications Biology.

The times are a changin' 

The extinctions took place at a period of time known as the Eocene-Oligocene transition around 33.5 million years ago. The Eocene is when most major modern bird and mammal lineages evolved, while it was during the Oligocene that the earliest animals such as giraffes and felid cats appeared. 

At the time of the transition, the planet is known to have undergone a period of intense cooling, though the exact reasons remain unclear.

Meteor strikes like that which wiped out the dinosaurs have been suggested, but other studies have shown no link between strikes and global cooling at this time, suggesting alternatives like extended volcanic activity and a sharp fall in carbon dioxide levels.  

In any event, the effect of the Eocene-Oligocene transition was devastating. While not quite catastrophic enough to qualify as a mass extinction, 67% of all known species at the time are thought to have been killed off. 

While these events were confirmed by the fossil record in Asia and Europe, Africa's fossil record is much sparser. This is likely due to sampling biases across the scientific community, with a 2020 research article suggesting it could take 274 years to assess 90% of the continent's living species, let alone extinct ones. 

This lack of evidence left some to suggest that Africa's climate could have been a refuge for mammals, with its warmer climate providing a buffer against falling global temperatures. 

The researchers wanted to find out more about how the transition affected these mammals, and turned to fossilised teeth to do so. They used teeth from a range of ancestral groups, including Hyaenodonta, a group of extinct hyena-like predators, Hystricognathi, which includes guinea pigs and mole rats, and the simians, which contains monkeys, apes and humans. 

Teeth are a good way of finding out a lot about dead organisms. Patterns of growth and wear can tell scientists what the animals ate and how old they were, while the minerals inside them can give clues about what the wider environment was like. 

In this case, the researchers looked at dental topographic metrics (DTMs). These describe the unique features on teeth that are specific to a species and reflect the different diets an animal may have had. These diets can then be linked to environmental changes. 

Science with teeth 

The researchers found that the teeth of species belonging to many groups were significantly different before and after the Eocene-Oligocene transition. Beginning with 42 lineages before the transition, this dropped to as few as 15 immediately afterwards. 

'It was a real reset button,' said Dr Dorien de Vries, the paper's lead author. 'We see a huge loss in tooth diversity, and then a recovery period with new dental shapes and new adaptations.' 

The disparity between different tooth shapes dropped sharply in rodents, especially in the ancestors of guinea pigs, while those in our early relatives actually increased before falling sharply afterwards.  

This suggests that species were rapidly dying out, with only a few types of teeth shapes being passed on. As a result, this limited the range of diets descendant species could have had, as evolution had less variation to work with. 

While the results suggest that African mammals were just as affected by the Eocene-Oligocene transition as other groups around the world, the research doesn't offer a definitive cause.  

There is conflicting evidence over whether temperatures in the continent dropped or not.

Instead, a number of geological upheavals around the same time could be to blame for these extinctions, while at the same time there were some of the largest super eruptions ever recorded which would have covered large areas in lava. Meanwhile, the Red Sea and the Gulf of Aden were opened up and flooded large areas of land. 

Together, events such as these would have impacted life on the African continent. The scientists suggest that together with changes happening globally, this caused many species to be lost over just a few million years.  

Though these extinctions took place over 30 million years ago, they can help us understand how modern climate change will affect Earth's ecosystems and the species within them. 

'Climate changes through geological time have shaped the evolutionary tree of life,' says co-author Dr Hesham Sallam. 'Collecting evidence from the past is the easiest way to learn about how climate change will affect ecological systems.'