Two giant species of bear – once thought to be close relatives – actually evolved independently on separate continents during the Pleistocene epoch, according to a new study.
The huge, short-faced bears, Arctodus simus from North America and Arctotherium angustidens from South America, are two of the largest meat-eating mammals ever to have walked the Earth, weighing upwards of 1,000 kilograms and were as tall as a human when on all fours.
Although both were extinct by the end of the Pleistocene, 12,000 years ago, how they evolved has remained a subject of debate. The modern-day spectacled bear, which belongs to the same family, was thought to be a significant link between the two.
To shed some light on the topic, a team of researchers led by Kieran Mitchell at the University of Adelaide studied an ancient femur bone once belonging to Arctotherium, which was discovered in a cave in Chile.
The researchers analysed the DNA of the South American giant bear and compared it to the genes of its North American counterpart, as well as today’s spectacled bear.
The findings revealed close links between the spectacled bear and its South American ancestor, but also suggest the North American and South American giant bears evolved separately on their respective continents.
The three species, the paper estimates, diverged from each other more than four million years ago, during the Pliocene epoch.
The reason for the simultaneous emergence of two giant bears and the similarities between the two species, despite their divergence, are debated in the paper, published in Biology Letters.
Before the emergence of these giant species, the North American landscape was dominated by smaller-bodied bears, closer in size to today’s species, as well as carnivores such as lions, wolves and sabre-toothed cats.
The researchers suspect the two animals may have each adapted to competition over the carcasses of megafauna. The opportunity to thrive from megafauna would have opened up once some of the other scavenging carnivores had died off – including a species of bone-crushing dog called Borophagus – given that the bears were not built well for predation.
This theory is supported by the fact that evidence of the North American giant bear hasn’t been found dating back to before the Pleistocene period, unlike the fossils of smaller-bodied bears.
Amy Middleton is a Melbourne-based journalist.
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