Many mammals on Earth have the capacity to hibernate. Yet, until recently, it was believed that evolution had cruelly never even given humans the option to sleep right through the winter months.
Now, thanks to a recent study on fossils found in Spain, researchers believe that early humans did in fact hibernate when conditions were particularly harsh and food was scarce.
Hints of hibernation in early humans
The study details evidence from bones found at Atapuerca, near Burgos in northern Spain, one of the world's most important fossil sites.
A cave at the site is effectively a mass grave, according to researchers who say that they have found countless bones dating back from 400,000 years ago that likely belonged to early Neanderthals or their predecessors.
Scientists argue that lesions and other signs of damage in fossilized bones of early humans are consistent with those left in the bones of animals that hibernate.
This leads them to believe that our hominid predecessors may have dealt with extreme cold hundreds of thousands of years ago by slowing down their metabolisms and sleeping through the winter months.
In their paper, published in the journal L’Anthropologie, study lead Juan-Luis Arsuaga and Antonis Bartsiokas, of Democritus University of Thrace in Greece, detail seasonal variations on bones from the site.
These seasonal variations, they say, suggest that bone growth was disrupted for months at a time at yearly intervals.
This suggests that early humans were periodically "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," the researchers say.
The same pattern of growth disruptions, presented as lesions, is present in bones of hibernating mammals, including cave bears.
What's more, the researchers point out that the remains of a hibernating cave bear (Ursus deningeri) were also found in the Sima pit, proving at the very least that the location was an ideal spot for hibernation.
Debating the likelihood of hibernating humans
The researchers also tackle one of the strongest counter-arguments out there for those who believe humans' predecessors never hibernated: modern Inuit and Sa´mi people live in incredibly cold, harsh conditions and they don't hibernate.
Arsuaga and Bartsiokas argue that the Inuit and Sa´mi diet, rich in fatty foods such as fish and reindeer, provided these populations enough nutrients during the winter to remove the necessity for hibernation.
In contrast, the area surrounding the Sima site in modern Spain half a million years ago would not have provided anywhere near enough food.
"It is a very interesting argument and it will certainly stimulate debate," forensic anthropologist Patrick Randolph-Quinney of Northumbria University told The Guardian.
"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."
The next step will likely be to test the hibernation theory against the genomes of the Sima people, Neanderthals, and other early humans to help establish whether our ancestors could indeed sleep through the winter.