Thoughts on Human Hibernation

Hibernation is a word many of us will be familiar with, as it is a strategy that some animals living today in Northern latitudes employ to cope with the harsh winter months. However, does there exist potential for humans to hibernate too? And beyond that, did any of our ancient hominin relatives do so in order to deal with the climate? A paper from 2020 in the journal L’Anthropologie comes to the surprising conclusion that ancient humans were hibernating ¹(Bartsiokas & Arsuaga, 2020). Whilst there had been studies that introduced the potential for human hibernation within a laboratory setting, and under certain conditions for a short period of time, it is the first comprehensive investigation into whether hibernation actually occurred in ancient humans¹.

The authors of the study, recognised that many hibernators will suffer from chronic kidney disease as a result of excessive fattening and starvation associated with hibernation, and sought to test this by analyzing the bones of humans from Sima de los Huesos¹. They looked at a number of significant skeletal pathologies (injuries or diseases that affect the skeleton and are evident from the bones) that are supposed to be indicative of chronic kidney disease and hibernation as a whole¹. Some of these, the authors of the paper described as diagnostic¹. However, even the pathologies deemed to be diagnostic may not be such good indicators of hibernation.  Firstly, one of the pathologies the authors of the hibernation hypothesis used as an indicator of hibernation was “Lines of Arrested Growth” (lines which indicate areas of little or no growth in the bones that supposedly represent starvation)  which may have causes other than starvation ^{1, 2}(Papageorgopoulou et al., 2011) . Rather in humans it seems to be caused by normal growth patterns and hormonal change ². Two papers, one from the American Journal of Human Biology and one from the American Journal of Physical Anthropology (now the American Journal of Biological Anthropology) from 2011, discuss the reliability of Lines of Arrested Growth for studying times of stress in populations  ^{2, 3}( Alfonso-Durruty, 2011). The authors of the first paper analyzed 241 tibiae from medieval skeletons in Switzerland, only selecting tibiae free from all other skeletal pathologies for X-ray images ². They compared the number of Lines of Arrested Growth and frequency of Linear Enamel Hypoplasia (a condition that arises from malnutrition and certain diseases which causes the enamel to not properly develop forming horizontal lines on the teeth) per skeleton ². In addition, the authors of that paper also looked at the skeletons to discern whether the bones had pathologies that represented an underactive thyroid condition ². They found no correlation between Linear Enamel Hypoplasia and Lines of Arrested Growth which is to be expected if Lines of Arrested Growth are not caused by starvation ². Whilst they also found that there was a raised level of underactive thyroid condition amongst the skeletons, the number of Lines of Arrested Growth did not reflect the significantly lower life expectancy that is to be expected if it were to represent a skeletal symptom of underactive thyroid condition in a society without the modern understanding of medical care ². Overall the authors of that paper found no correlation between other symptoms of disease or starvation, growth inhibiting diseases or starvation and Lines of Arrested Growth ². The authors of the second paper chose three groups of New Zealand White rabbits, with each group being kept under different conditions ³. The first was a control test where the rabbits were kept in standard laboratory conditions whilst the second was fed a low quality diet and the third were periodically starved ³. New Zealand White rabbits develop in a similar pattern to humans and are very responsive to starvation, meaning they are more likely to develop clearer Lines of Arrested Growth ³. The authors found that overall growth in the skeleton of the rabbits was not hampered by the poor diet and that there was no significant difference in Lines of Arrested Growth between the groups ³. Both of these studies demonstrate that Lines of Arrested Growth do not give an accurate view as to an individual’s health and whether or not they have previously been through a period of starvation or disease. This stands true of bones of deceased individuals far in the past. 

‘Osteocytic osteolysis’ (a type of bone resorption) is another characteristic pathology that the authors of the hibernation hypothesis described as diagnostic of hibernation¹.  However, despite the fact that it can indicate remaining stationary for a long period of time, it has other causes such as lactation by mothers, hyperparathyroidism, Vitamin D deficiency and Calcium deficiency ⁴(Tsourdi et al., 2018). These factors cannot be ruled out and have causes other than hibernation, as they occur in modern day people.    

A simple but alternative hypothesis for skeletal pathologies associated with Chronic Kidney Disease or any other aspect of hibernation, is that the humans went through seasonal starvation and fattening whilst remaining active, albeit less so. An animal, or in this case humans, could employ this strategy to deal with the winter months. This can be seen in the Svalbard reindeer, whose Spring body mass is lower than their Autumn body mass by as much as 25 kg. Additionally, Muskox lose 40% of their body weight in Winter and then subsequently gain it back in Summer and Autumn in preparation for the next freeze ^5,6(Norsk PolarInstitutt, n.d.: Adamczewski, 1992). These periods of fattening and starvation could theoretically still elicit the same response from the bones if the changes in body mass were drastic enough. A way to test this would be to study the bones of species that utilize this method and compare them to similar species that hibernate, or contrast individuals of different sexes within the same species where one hibernates and the other does not. For example, the skeletal pathologies of female polar bears could be compared to those of male polar bears, as male polar bears do not hibernate whilst female polar bears do⁷(Robbins, 2012) However the authors of the hibernation hypothesis had an answer for such a proposal¹. They state that “starvation, (without lack of sunlight),  often leads to vitamin deficiencies such as beriberi, pellagra and scurvy but cannot lead to Vitamin D deficiency”¹. The expectations that Vitamin D deficiency is not present where there are adequate light levels and that starvation cannot contribute to Vitamin D deficiency do not align with two fairly recent papers ^8,9(Pereira-Santos, et al  2019: Kueper, et al, 2015). The authors of the first paper found that the percentage of the population affected by “vitamin D deficiency and insufficiency in this country was similar to that of nations with reduced solar availability”, suggesting diet and nutrition played a role in vitamin D deficiency⁸. The other paper found that vitamin D levels were depleted in more extreme cases of anorexia, and that supplementation could help alleviate products of anorexia, such as osteopenia and osteoporosis, indicating a strong dietary component of vitamin D deficiency⁹.  What this all indicates is that even where light levels seem adequate, they may not have been, and diet is and likely has been a significant part of sourcing vitamin D, leaving potential for the hominins to have undergone famine whilst remaining active. This is especially the case in a climate that was colder than today where light levels would likely be lower and famine would have been more common ^10,11(Hosfield, 2020: Candy, Schreve, & White, 2015).

The idea of deep snow drifts and billowing blizzards should not be pictured when thinking of a classic winter endured by the hominins at Sima de los Huesos, as the winters during that period of time were typically dry¹⁰. While colder on average than the region today, most winters at Atapuerca would not be all that different from Winters at present in the area¹⁰. Life under such conditions would be hard without modern comforts but by no means an agonizing slog that could only be endured through hibernating. However, if we are to accept the older dates of the occupation of the site by Homo heidelbergensis and line them up with the younger dates of occupation in a chronological sequence, and then assume consistent occupation for most of the time, then there is the implication that these humans lived through roughly 20,000 years of an extreme cold period known as Marine Isotope Stage (MIS) 13 to MIS 12 ^10,11,12,13,(Berger et al., 2008: Arsuaga et al., 2014).  Temperature estimates for sites in winter a little way North in the UK sit between −10 and −36°C¹¹. It is unclear how this would have affected the temperature at Atapuerca and Northern Spain as a whole, but it is likely that there would be months where the temperature was sub zero^10,11. It is worth mentioning that what we know about the climate of the area from the time is largely available to us from other sites and a number of Atlantic core samples¹¹. 

When considering the skeletal pathologies of Homo heidelbergensis it is important to recall that the scarcity of resources during the winter months would limit access to a varied diet containing plant foods. It is possible that H. heidelbergensis was largely dependent on red meat as a food source in the coldest seasons. Undoubtedly there was red meat available, with Aurochs (giant wild ox), at least one species of bison, a species of giant deer, the list goes on^14-18(Domingo et al., 2017: Rodríguez-Gómez et al., 2017: García & Arsuaga, 2011: Rodríguez et al., 2011: Van der Made, 2013). This is significant because diabetes, one of the leading causes of chronic kidney disease today, can be caused by an overconsumption of red meat ^19,20,21,22(Webster, et al., 2017: Kovesdy, 2022: Evans & Taal, 2011: Feskens, Sluik, & van Woudenbergh, 2013). Diabetes makes up between 30-50% of all cases of chronic kidney disease¹⁹. It is estimated that for every 100g of red meat that is consumed daily, the percentage of risk of developing diabetes goes up by 13%²². If we are to take the upper amount of calories that the humans at Sima de los Huesos would have to consume it would sit in the region of between 3000 kilocalories (kcal) and 5500 kcal based off of estimates for their descendents (the Neanderthals) ²³(Sorensen & Leonard, 2001). From there we can work with a mid range estimate of about 4250 kcal. There are approximately 1450 kcal in a kilogram of Bison meat although it should be worth mentioning that this is an average extrapolated from a whole bison and it undoubtedly could be higher or lower based on the specific part of the body ²⁴(Henrikson, 2004). It is also worth mentioning that the hominins would likely be eating other animals as well and that Bison meat is only used to estimate the risk of developing chronic kidney disease. However once we accept this is only an estimate we can divide the mid range estimate for daily calorie requirements by the kilocalories per kilogram of Bison meat then we get the kilograms of Bison meat required each day which is 2.93kg. Then we convert it to grams and divide it by the 100g mentioned earlier. This gives us 29.3 which we then multiply by the 13% of risk of developing diabetes per 100g of red meat consumed daily²³. It should be discussed that the risk of developing diabetes may be different for the Sima de los Huesos hominins than for modern humans so this does rely on the assumption that it is the same. This gives us 381% which is the percentage at which we can expect diabetes to be higher in the Sima de los Huesos population. 381% is then multiplied by 0.4 as the percentage of diabetes cases that develop into chronic kidney disease is 40%^19,20. The result is 152% which is the percentage at which  chronic kidney disease can be expected to reach if such humans were eating a largely meat based diet. Now we convert this to a decimal and multiply it by the percentage of the population today that have chronic kidney disease. This is 11% so 11 multiplied by 1.52 is 16.72%¹⁷. So therefore we can expect chronic kidney disease cases to represent about 1:5 of the hominins at Atapuerca, which is a significant proportion, and when taken together with evidence against the techniques used by the authors of the hibernation hypothesis, this certainly makes for an interesting alternative proposal. However, eating that much red meat would have other consequences such as protein poisoning and overdose of Vitamin A which would undoubtedly affect the bones causing osteoporosis ²⁵(Hockett, 2012). This suggests it could only be maintained if the humans subsisting off of this diet did so for short periods of time. There is also the potential that the 40% figure for diabetes resulting in chronic kidney disease is an overestimation as a paper suggested that at least some of the cases of chronic kidney disease in diabetics may not have a causal relationship²⁰. In addition, the extreme cold period MIS 13 to 12 only lasted for 20,000 years from 470,000 years ago and does not encompass all of the human activity at Sima de los Huesos, so there would not necessarily be the need for the hominins to consume excessive amounts of meat all of the time¹¹.  

The authors of the hibernation hypothesis also use the representation of adolescent skeletons at Sima de los Huesos¹.  On fairly recent archaeological sites where adolescents are significantly underrepresented in the site’s archaeological record, there is an important  connection between preservation and assignment of age to the skeletons ²⁶(Lewis, 2016). To quote Mary Lewis from her 2016 paper²⁶: 

“Late fusing epiphyses (i.e. the end of the clavicle nearest the sternum, or first vertebra of the sacrum) used to identify an individual as a ‘young adult’ are less likely to survive than the features of mature age, such as osteoarthritis, meaning that it is likely many more young adults are consigned to the ‘adult’ age category than any other age group.”

It is to be inferred that such diagnostic features of adolescence would be not only easier to spot, but also present to a much greater degree on sites with far better preservation. From that it can then further be inferred that a more substantial percentage of skeletons would be adolescent in an environment with better preservation such as Sima de los Huesos. However this on its own may not be enough to account for the high adolescent death rate as the adolescent mortality consists of roughly 1:2 of the hominin assemblage when looking at the craniums alone^1,13. Perhaps a better suggestion is that there is high adolescent mortality in societies without modern medicine, particularly in cases of low genetic diversity. A famous case of this would be of Charles Darwin’s family where, despite being a wealthy family, three of his children died during childhood including one dying during adolescence ²⁷(Bogin, 2012). This is even more present when one looks at the percentage of mortality that children under 15 made up ²⁸(Roser, 2019). This figure was almost 50% just a century ago and includes juveniles and early adolescents²⁸. In addition, one archaeological site known as “Indian Knoll” in Kentucky, USA, suggests that the death rate before adulthood was even higher in some prehistoric hunter gatherer populations²⁸. The highest proportion of mortality for any society before or at the beginning of the 20th century was by far that of infant mortality with 27% newborns dying in their first year of life²⁸. So hypothetically we should expect to see high numbers of fossil remains of newborns and young children amongst the Sima de los Huesos collection if it represented the demographics that died in the cave. However newborns often do not fossilize well, which could equate to a misleading death assemblage demographic ²⁹(Halcrow et al. 2020). This is backed up by the underrepresentation of newborns and infants in the fossil record where at the most 300 of the 6000 hominin individuals found so far are newborns or infants ^29,30(Smithsonian Institution, 2022). This is equivalent to 5% nearly a sixth of what the true representation should be.

Another point worth mentioning is anatomically modern hunter gatherer societies and tribes often collaborate to take care of the ‘juveniles’ and adolescents, and with anatomically modern humans’ extended ‘juvenile stage’, while these members of the group are semi-independent, they are still unable to meet their daily caloric intake and so members of a group are required to aid them in obtaining enough food²⁷. However this may not have been the case for highly mobile lower paleolithic hunter gatherers like the Sima de los Huesos hominins. The argument can be made that the adolescent proportion of the population of hominins at Sima de los Huesos may have represented a larger percentage of the overall living population and so played a more significant role in obtaining the group’s daily calorie requirements, resulting in them disproportionally often facing down the prey they were hunting, in addition to coming into conflict with other groups of humans and rival predators. This competition between rivaling groups of humans and other predators may have also resulted in a deficiency in the hunting ecology of Atapuerca as a whole. This could likely cause many of the humans found in Sima de los Huesos to be unable to meet their daily caloric demands, explaining both the pathologies on the hominins and the distribution of mortality being biased towards adolescent hominins. There is some evidence of this from the overall stability of the surrounding ecosystem, where total caloric demands for the predators would have been greater than that of the caloric availability¹⁵. What this means is that it would have been a daily struggle for the hominins of Sima de los Huesos, contending with either starvation or being forced to compete with voracious predators, some of which could have easily eaten them¹⁵.

Another possibility is that the opposite is true and many of the adolescents and juveniles at Sima de los Huesos died when the group encamped at the entrance of the cave, and adult hominins are rather the anomaly in the sense that there is less of them because many of them would be hunting and so likely dying away from the cave. A worthwhile approach would be to compare Sima de los Huesos to other hominin bearing cave sites similar to it, both at Atapuerca and in other parts of Spain and Europe from the same time period, in order to better rule out a bias in the fossil record from a circumstance such as type of camp placement and age related duties. This is a particularly important argument as, when we look at Neanderthal remains from later on sites, it appears that on the whole there is a bias in the fossil record towards adolescent individuals³¹(Trinkaus, 1995). However there are only a few sites where adolescent mortality is abnormally high, and at these sites it is so raised that it skews the overall data for Europe³¹. When sites are weighted against each other this abnormal adolescent mortality rate drops significantly across Neanderthal sites³¹. Despite this, young adult mortality remains high even after the sites are weighted³¹. Such a high proportion of mortality in the young adult population that is not mitigated by the weighting of sites suggests a more accurate representation of death rates and life expectancy³¹. What this all means is that a greater understanding of overwintering behaviors, such as hibernation, could be better grasped by using a larger sample size and properly weighting sites (particularly those that are population dense), and that one site with abnormal adolescent death rate is not enough to extrapolate from. 

So the question now stands: how did the Sima de los Huesos hominins end up in the cave?

The remarkably low amount of carnivore activity that is present on the bones suggests that they were not transported to the cave by a predator or scavenger ³²(Sala et al, 2023). If anything the fossils appear to have been scavenged within the cave, likely by bears when the bodies had largely decayed³². The presence and high representation of hand and foot bones indicates the bodies of the hominins were transported to the cave intact as these are elements that are among the first to disarticulate³². The fact that the preservation of the hand and foot bones is better than that of other ancient sites, even those involving burial of hominin remains suggests the bodies were buried in the cave and were only accessed later by the bears³².  

In conclusion, did the Sima de los Huesos hominins hibernate all those many millennia ago? Honestly, it is hard to tell, but as the case I have made here clearly shows, there is currently not enough evidence to point to hibernation as being an overwintering behavior that these hominins practiced. Often in these cases it is the simplest scenario that has the most credibility so here I apply Ockham’s razor :  the case for seasonal starvation whilst remaining active is yet to be rebutted, therefore this is the explanation I have to go with.  

Sources:

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