Depending on who you ask, we are either approaching or in the midst of the sixth mass extinction – and it's being triggered by human activity. But our species has a long history of annihilating others, as a study published in Scientific Reports this week goes to show.
Modern humans' arrival in Europe just so happened to correspond with the decline (and eventual extinction) of the cave bear (Ursus spelaeus), beginning approximately 40,000 years ago.
Cave bears were one of the more than 150 genera of megafauna to roam the planet during the Late Pleistocene. The group also included now-extinct species like the woolly mammoth, the woolly rhinoceros, and the saber-toothed cat.
But while early humans' hunting dexterity has been linked to several extinctions, the precise cause of death of the cave bear is contested. How responsible were our ancestors for their demise? And what role did environmental changes play in the run of events?
To find out, researchers analyzed the mitochondrial DNA of 59 cave bear fossils collected across 14 sites in Europe, from France in the west to Serbia in the east. Mitochondrial DNA (or mtDNA) is inherited only from the mother and can determine the relationships between animals across distances, thereby revealing just how diverse different populations of animals are, genetically-speaking.
These were compared to 64 already-published cave bear mtDNA. The results revealed five distinct mtDNA lineages, showing cave bear populations 50,000 years ago were far more diverse than previously thought. What's more, they remained pretty stable until humans arrived on the scene. This is despite two cold snaps and fluctuations in temperature.
It suggests the species was reasonably resilient to environmental changes. Instead, the results show their decline started approximately 40,000 years ago, around the time early humans established a base in Europe and before the last Ice Age.
As early humans settled in and started hunting cave bears (possibly for food or because they were competition for food), bear populations became fractured and isolated. Those that held out were driven to remote corners of the continent, such as the Italian Alps. Here, they may have struggled to find enough food, while their genetics would have weakened over time as a direct result of this isolation – a factor that could put them at increased risk of disease.
"It is the clearest evidence we have so far that humans might have played a big role in the extinction of the cave bear," lead author Verena Schuenemann of the University of Zurich told the BBC.