Bears do it. Bats do it. Even European hedgehogs do it. And now it turns out that early human beings may also have been at it. They hibernated, according to fossil experts.
Evidence from bones found at one of the world’s most important fossil sites suggests that our hominid predecessors may have dealt with extreme cold hundreds of thousands of years ago by sleeping through the winter.
The scientists argue that lesions and other signs of damage in fossilised bones of early humans are the same as those left in the bones of other animals that hibernate. These suggest that our predecessors coped with the ferocious winters at that time by slowing down their metabolisms and sleeping for months.
The conclusions are based on excavations in a cave called Sima de los Huesos – the pit of bones – at Atapuerca, near Burgos in northern Spain.
Over the past three decades, the fossilised remains of several dozen humans have been scraped from sediments found at the bottom of the vertiginous 50-foot shaft that forms the central part of the pit at Atapuerca. The cave is effectively a mass grave, say researchers who have found thousands of teeth and pieces of bone that appear to have been deliberately dumped there. These fossils date back more than 400,000 years and were probably from early Neanderthals or their predecessors.
The site is one of the planet’s most important palaeontological treasure troves and has provided key insights into the way that human evolution progressed in Europe. But now researchers have produced an unexpected twist to this tale.
In a paper published in the journal L’Anthropologie, Juan-Luis Arsuaga – who led the team that first excavated at the site – and Antonis Bartsiokas, of Democritus University of Thrace in Greece, argue that the fossils found there show seasonal variations that suggest that bone growth was disrupted for several months of each year.
They suggest these early humans found themselves “in metabolic states that helped them to survive for long periods of time in frigid conditions with limited supplies of food and enough stores of body fat”. They hibernated and this is recorded as disruptions in bone development.
The researchers admit the notion “may sound like science fiction” but point out that many mammals including primates such as bushbabies and lemurs do this. “This suggests that the genetic basis and physiology for such a hypometabolism could be preserved in many mammalian species including humans,” state Arsuaga and Bartsiokas.
The pattern of lesions found in the human bones at the Sima cave are consistent with lesions found in bones of hibernating mammals, including cave bears. “A strategy of hibernation would have been the only solution for them to survive having to spend months in a cave due to the frigid conditions,” the authors state.
They also point to the fact that the remains of a hibernating cave bear (Ursus deningeri) have also been found in the Sima pit making it all the more credible to suggest humans were doing the same “to survive the frigid conditions and food scarcity as did the cave bears”.
The authors examine several counter-arguments. Modern Inuit and Sámi people – although living in equally harsh, cold conditions – do not hibernate. So why did the people in the Sima cave?
The answer, say Arsuaga and Bartsiokas, is that fatty fish and reindeer fat provide Inuit and Sami people with food during winter and so preclude the need for them to hibernate. In contrast, the area around the Sima site half a million years ago would not have provided anything like enough food. As they state: “The aridification of Iberia then could not have provided enough fat-rich food for the people of Sima during the harsh winter - making them resort to cave hibernation.”
“It is a very interesting argument and it will certainly stimulate debate,” said forensic anthropologist Patrick Randolph-Quinney of Northumbria University in Newcastle. “However, there are other explanations for the variations seen in the bones found in Sima and these have to be addressed fully before we can come to any realistic conclusions. That has not been done yet, I believe.”
Chris Stringer of the Natural History Museum in London pointed out that large mammals such as bears do not actually hibernate, because their large bodies cannot lower their core temperature enough. Instead they enter a less deep sleep known as torpor. In such a condition, the energy demands of the human-sized brains of the Sima people would have remained very large, creating an additional survival problem for them during torpor.
“Nevertheless, the idea is a fascinating one that could be tested by examining the genomes of the Sima people, Neanderthals and Denisovans for signs of genetic changes linked with the physiology of torpor,” he added.