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 study, published in December’s issue of L’Anthropologie, suggests that an extinct human species found in Spain may have had the ability to hibernate.

Study authors Antonis Bartsiokas and Juan-Luis Arsuaga examined the skeletal remains of hominins using close-up photography, microscopes and CT scanning. The remains were found in a deep shaft of the Sima de los Huesos cave at UNESCO heritage site Atapuerca in Spain, where fossils hundreds of thousands of years old have been found.

During this examination, Bartsiokas and Arsuaga found that these remains showed signs of vitamin D deficiency, as well as growth spurts in adolescents that appear to be linked to the seasons — all signs that this species may have hibernated in dark enclosures through the winter.

“The hypothesis of hibernation is consistent with the genetic evidence and the fact that the Sima de los Huesos hominins lived during a glacial period,” Bartsiokas and Arsuaga write.

But the study’s authors note that this human ancestor’s hibernation was not necessarily successful or healthy, as hominins were unlikely to be able to build up the necessary fat reserves that other species, such as bears, can. These physiological limitations to hibernation could lead to bone and kidney disease.

Questions remain as to the exact species the researchers studied, though some of the remains have been positively identified as Homo heidelbergensis from the Middle Pleistocene, a geologic age spanning between 770,000 years ago and 126,000 years ago.

The researchers acknowledge that their research is still preliminary and can read like “science fiction.”

“While many questions about their life histories and metabolism are still open, there is no doubt as to the immense consequences that hibernation has for hominin/human physiology and life history,” they write.