The use of fire and human distribution

ABSTRACT

Humans today live in a wide range of environments from the iciest to the hottest, thanks to diverse cultural solutions that buffer temperature extremes. The prehistory of this relationship between human distribution, cultural solutions and temperature conditions may help us to understand the evolution of human biological adaptations to cold temperature. Fire has long been seen as an important factor in human evolution and range expansion, particularly into temperate latitudes. Nevertheless, the earliest evidence for hominin presence in Eurasia, and middle latitudes in northern Europe, substantially predates convincing evidence for fire use in these regions. This review outlines the current state of knowledge of the chronology of hominin dispersal into temperate latitudes, from the earliest occupants to our own species, and the archeological evidence for fire use. Given continuing disagreement about this chronology and limitations to the archeological evidence, new, complementary approaches are worthwhile and would benefit from information from studies of current human temperature regulation.

KEYWORDS:

• fire use
• human evolution
• Lower and Middle Palaeolithic
• middle latitudes
• thermal tolerance

Introduction

“There can be no doubt that fire would be of enormous assistance to hominids in exploiting current temperate regions, even in interglacial times, let alone in glacial periods. Much of the older record, however, lacks clear fire evidence. As the northern record is now a very long one, the hypothesis must at least be considered that early humans became hardy enough to exploit the north before they mastered fire.”¹ (p. 300).

“It would seem… that definite evidence for fire production postdates the time of initial hominin occupation throughout Eurasia, ruling out this technology as a decisive factor but raising for the same reason the question of hominin adaptation to temperate latitudes without it.”² (p. 198).

The temperatures encountered by humans today are mediated by a range of cultural strategies, particularly clothing, shelter, heating and cooling technology, and as a result we are able to function from the iciest to the hottest locations. This is true despite the fact that humans are not particularly well adapted biologically to deal with temperature extremes, particularly cold. Because it has shaped the human thermal environment, the history of the interaction between cultural solutions, human distribution and temperature conditions may help us to understand the evolution of human biological adaptations to temperature. Fire has long been perceived as one of, if not the key, human technological innovations, underlying human evolutionary success, including our widespread distribution, and directing subsequent adaptations. This technology can be used for many purposes, from keeping warm, to cooking food, managing the landscape, making tools and extending daylight hours, many of which are particularly useful in temperate environments. Archaeologists and palaeoanthropologists have devoted considerable effort to identifying evidence for the use of fire in the distant past. Nevertheless, there continues to be widespread disagreement about when fire became a part of the behavioral repertoires of our lineage, and which hominin species first used fire, with suggestions ranging from Homo erectus 1.8 mya to relatively recent Anatomically Modern Humans.^3,4 This is mainly the case because the remains of fire are fragile and preserved only in optimal conditions. As the quotations above indicate, while it seems clear that fire would have been a valuable tool in more temperate, seasonal environments, the archaeological evidence does not unambiguously support this inference.

This article aims to outline our current knowledge concerning the record for fire use, and relate this to the chronology for hominin dispersal into cooler regions. Both fire and dispersal are key topics in human origins, and have been written about at length, and other authors, including those quoted above (see also refs. 5-7), have also considered the relationship, particularly in Europe. Recent debates and critiques of the evidence have stimulated new analyses, making this a rapidly changing field. Rather than focusing entirely on the earliest use of fire and hominin expansion into Eurasia, I will take a longer view, taking into account the likelihood that both the nature of occupation of areas with a temperate climate and fire use changed through time. I will also address both the strengths and limitations to the evidence, and attempt to point to ways forward including information needed from other disciplines.

Hominin dispersal: The archeological and fossil evidence

The earliest hominin fossils come from sub-Saharan Africa, where hot and dry rather than cold temperatures provided the biggest challenges. However by the end of the Early Pleistocene (0.78 mya) stone tools and fossils document hominin presence across much of Eurasia, from the Nihewan Basin in northern China, to Dmanisi in the Georgian Caucasus, to northern Spain^8-11 (Fig. 1). In most of this area the northern limit is 40° North. At these latitudes hominins would have encountered greater seasonality⁸; currently, plant growth stops during a portion of the winter and air temperatures below freezing are not infrequent. At Nihewan, during the current interglacial, January temperatures are as low as −15 to −10°C on average.⁸ Further, between 1.2 and 0.5 mya, cold climate phases associated with northern glaciation became longer and more severe.¹² Perhaps surprisingly, the people who first moved into these regions were small brained¹³ and used simple stone toolkits. Their stature and body size was not exceptional in comparison with other early Homo or current humans.^14-16 In a seasonal environment, the exploitation of meat, evident much earlier,¹⁷ may have been increasingly important. Fire could have been useful for dealing with various challenges presented by temperate latitudes: keeping warm, increasing the caloric yield and edibility of the foods available in winter,¹⁸ light, signaling, and protection from predators.²

Figure 1. Location of key sites mentioned in the text. 1) Happisburgh, 2) Beeches Pit, 3) Pakefield, 4) Schöningen, 5) La Cotte de Saint Brelade, 6) Atapuerca site complex, 7) Caune de l'Arago, 8) Dmanisi, 9) Gesher Benot Ya'aqov, 10) Tabun Cave, 11) Qesem Cave, 12) Ust’-Ishim, 13) Nihewan Basin site complex, 14) Zhoukoudian.

The earliest fossil and archaeological evidence from Europe fits into this general pattern, with the earliest sites located at most 40° North, dating to the end of the Early Pleistocene.¹¹ Quantitative temperature estimates based on herpetological fauna are available for several late Early and early Middle Pleistocene archeological and fossil sites in southern Europe, and are all above freezing.¹⁹ For example, winter temperatures estimated from the hominin fossil bearing level TD6 upper from the Atapuerca site complex in Spain range from 0.5–6.0°C.²⁰ The fossil material from these southern sites is too scanty to indicate whether their body form was warm or cold adapted.^21,22 Evidence for hominin activity that is almost as old has been recovered at 53° North from the east coast of the UK.^23,24 At Happisburgh 3, where the pollen evidence, including the replacement of mainly deciduous with coniferous regional forest, suggests climate cooling at the end of an interglacial, the artifacts are associated mainly with the latter conditions, and are distributed across several different levels and lithostratigraphic units. This suggests that hominins were able to remain in the area and revisit the site several times as the climate cooled.²⁴ Mean winter temperatures are calculated (based on the beetles present) as lying in the range of −3 to 0°C.²⁵ At Pakefield, a small number of stone tools have been discovered that are over half a million years old – this date being based on the ‘vole clock’ (that is, they are associated with bones of a vole species that was replaced by a descendent species across Europe around that time).²³ At this location, hippopotamus were present together with frost-sensitive beetles and plants, suggesting relatively mild winters,²³ which may however have been wetter than now.²⁶ Furthermore, several slightly younger archaeological and fossil sites from the Rhine river system document hominin presence somewhat further east in fully interglacial conditions.^27-29

This suggests that the earliest occupants of Europe were present in locations where temperatures dropped below freezing during the winter. However, whether these artifacts were left behind during a longer term occupation including wintering in the area or a short summer visit is unclear.³⁰ Estimated summer temperatures from Pakefield and Happisburgh 3 were as warm or warmer than present.²⁵ If they spent the winter elsewhere, how far did they travel, and how much difference is this likely to have made to the conditions they encountered? In this period, the maximum distances between the source area of a rock and locations at which artifacts made out of these stones were deposited in archaeological sites tend to be short (tens of kilometres); however multiple factors, including distances traveled by hominins while foraging, but also the availability of raw materials, influence these distances.³¹ Keeping pace with slower children and older adults, foraging on the way, as well as short winter days, are likely to have restricted the distances that could be traveled during a day.^32,33 Sites located in southern Europe, where temperature estimates are all above freezing (as discussed above), are located far from north-western Europe (in the range of 1,000 km as the crow flies) and it seems unlikely that hominins traveled this far. Currently, the east-west gradient determined by proximity to the Atlantic coast is as steep as the north-south gradient in temperature. Even within the UK the coast is warmer than inland by a few degrees, and further west, along the northern French coast, temperatures improve further. Seasonal migration focusing on coasts or rivers could have brought hominins to areas with winter temperatures about 4°C warmer and might have allowed them to avoid freezing temperatures. On the other hand, rainfall also increases with proximity to the Atlantic, and this would have increased the rate at which unclothed hominins lost heat to the environment through evaporation and reduced the effectiveness of fur or clothing.^34,35 Moving along rivers and coasts would also have provided rich, diverse, consistent but exhaustible resources, and hence motivation to keep moving.³⁶ Similarly, hominins in northern China, where conditions are even more severe, are likely to have been occasional summer visitors.⁸ It seems possible that hominins migrated in winter, but the winter conditions that they encountered would still have been challenging.

From our current perspective, the temperatures encountered during winter in north-west Europe by the earliest hominin occupants seem very mild. However, the human body initiates regulatory changes in metabolic heat production to maintain its core body temperature at the much warmer ambient temperature of 29.2°C.³⁷ Humans begin to feel uncomfortable at even milder temperatures, although we begin to experience comfort at cooler temperatures as a result of regular exposure to mild cold.³⁷ As discussed above, high humidity further increases the rate at which the body loses heat to the environment, as does high wind speed. In similar conditions today humans use clothes to insulate the body and shelters to control ambient temperatures, stay dry and reduce airflow, while large mammals are effectively insulated by fur. However similar strategies were not necessarily used by the earliest occupants of middle latitudes. Evidence for fur, clothing and shelter is lacking in this period.^34,38 The majority of estimates suggest that hominins had already lost body hair before moving into middle latitudes in Europe. Dates based on genetic studies of humans and their lice for loss of hair are divergent, ranging from 3.0–1.3 mya.^34,39 Natural shelters such as caves were only available in some areas of Europe, and claimed Lower and Middle Palaeolithic dwelling structures are contested.⁴⁰ The fire evidence is discussed below. Without additional insulation or heat sources, hominins would have risked developing uncontrolled hypothermia. Based on mammalian and human relationships, it is estimated that a hominin with the body proportions of Homo erectus would have had difficulty surviving at temperatures below 11.6°C, which they are likely to have encountered in winter whether at Pakefield or Atapuerca (even without taking into account additional heat losses likely to have occurred as a result of significant airflow over the body⁴¹). This raises an important unanswered question: what behavioral strategies and biological adaptive or acclimatising mechanisms did the earliest occupants of middle latitudes in Eurasia use to stay warm in cool conditions? Even if hominins were able to stay warm enough, cool temperatures present other problems, including seasonal shortages of food, particularly vegetable food. While animal foods and aquatic resources when available may have been present year round, they may be more difficult or risky to eat without further processing.³

After about 500 kya, we start to see evidence for hominin activity in cooler conditions. This includes sites in Central Europe like Schöningen and Bilzingsleben, where, without the ameliorating effect of the Atlantic ocean, the climate in some interglacials would have featured colder winters and warmer, drier summers.⁴² From this time forth we find evidence for hominin presence at sites associated with steppic fauna and geologically attributed to the climatic deterioration both at the beginning and end of interglacial climate phases, including both cooler air temperatures and stronger winds.^29,43,44 The robust proportions of the c. 500 kya old tibia from Boxgrove in the UK may reflect cold adapted shorter extremities or greater overall robustness, either of which could be related to climate.⁴⁵ The much larger fossil sample from Sima de los Huesos in northern Spain is constrained to an age in the early Middle Pleistocene,⁴⁶ providing much more detailed information about hominins in this period. This population had wide bodies and tall stature,⁴⁷ were very muscular and had large cranial capacity.⁴⁸ A relatively broad frame has some advantages in cold conditions,⁴⁹ and high activity levels would have generated metabolic heat while consuming lots of energy. These fossils show a large number of morphological traits characteristic of Neandertals, as well as primitive traits.⁵⁰ The crania suggest a large brain size, more comparable to that of humans than earlier human ancestors, like other fossils of a similar age.^22,51 Which of multiple possible environmental and social factors play a role in brain expansion in this time period is not clear,⁵² but it is likely that it supported greater intelligence in a range of domains.⁵³ The hominins at Schöningen were effective hunters, and the absence of caudal vertebrae and distribution of cut-marks on bones suggest that hominins carefully removed the skins including tails from horses.⁵⁴

By the end of the Middle Pleistocene and Late Pleistocene, Neandertals in north-west Europe were living in a wide range of conditions from fully interglacial to much colder, and either abandoned the region or went locally extinct in fully glacial conditions.⁵⁵ They had extended east into the even more continental conditions of the Russian plain,⁵⁶ and recent genetic studies indicate that they ranged yet further into Central Asia and southern Siberia.⁵⁷ Their body proportions were similar to those of recent human populations living in the arctic.⁵⁸ Between approximately 45 and 35,000 y ago, Neandertals were replaced in Europe by Anatomically Modern Humans.⁵⁹ While humans now live in almost all corners of the globe and environmental conditions, these early human occupants did not differ greatly from Neandertals in their ability (or lack of) to hold on during the most severe glacial conditions in the north-west European plain.⁶⁰ However, it is striking that the first modern human fossil in Eurasia, from Ust-Ishim, is from 57° North,⁶¹ further north than any Neandertal fossils discovered thus far.

Evidence for fire use

Resolving the chronology for fire use is more difficult than it might initially seem. For most of human prehistory, hunter-gatherers were relatively mobile and put little effort into building structured hearths. Fires produced by hunter-gatherers for a wide variety of purposes may burn for as little as 15 minutes or as much as several months.⁶² The main archaeological proxies of fire use by hunter-gatherers include the reddened sediments on which a fire was built, heated stone artifacts, charred bone fragments, and pieces of charcoal.⁶ Because natural fires can have the same effects, the occurrence of any of these indicators does not necessarily demonstrate human fire use.⁶³ In addition, such traces are fragile and easily moved by natural processes, particularly in the open air.⁶⁴ Evidence for fire becomes more convincing when several of these different types of archeological indicator are present, when multiple scientific methods are used to investigate these indicators, and when studies of site formation indicate that they are likely to have been deposited on the spot or very nearby and not moved far. Micromorphological studies combined with a wide range of other microarchaeological methods have improved interpretation of fire remains, particularly by making it possible to evaluate site formation processes in caves and rockshelters in which evidence for anthropogenic fires is likely to be best preserved.^65-68 The interpretation of fire residues has been enhanced by experimentation, in terms of fuel type and anthropogenic modification.^69,70 Recent hunter-gatherers used either wood on wood or stone on stone friction to start a fire (the latter generally involving a ‘strike-a-light’ made of flint or another silicious rock, and marcasite or pyrite).⁷¹ These were often combined with a form of ‘tinder’ that catches a spark or coal and can be quickly fanned to flame.⁷¹ The tools for fire making are hard to recognize in this period.⁷¹

Based on palaeoanthropological evidence for anatomic changes, Wrangham has argued that fire was first used by Homo erectus 1.8 mya.³ Claims for the existence of archeological evidence for fire use by 1.6 mya in Africa include red patches of sediments from Chesowanja, Kenya, and from Koobi Fora FxJj 20.^72-74 These sites do not demonstrate that hominins were involved in the production or the use of such fires; the burned materials could have been produced by natural fires and the association with evidence for hominin activities be a matter of chance.^6,63 A large number of heated bones from the cave sites of Swartkrans are somewhat younger (1 mya) and similarly provide evidence of burning.⁷⁵ At Wonderwerk Cave, in strata dating to about 1 mya, micromorphological studies identified well-preserved plant ash remains and bone fragments, which probably burned inside the cave.⁷⁶ These claims are younger than the oldest sites in Eurasia,⁶ where evidence of fire is lacking. The site of Zhoukoudien Locality 1, China, was interpreted as providing early evidence for fire use in Eastern Asia. Studies by Weiner and colleagues showed that ash was absent⁷⁷ in the oldest archeological Layer 10, and demonstrated the extensive role of water in depositing the sediments making it unlikely that the charred bones represented in situ burning.⁷⁸ Subsequent studies have identified ash (based on the presence of siliceous aggregates and elemental carbon) in Layers 6 and 4, as well as magnetic enhancement in areas of Layer 4.^79,80 Another study documents high levels of elemental carbon, with considerable variation, inside the cave in Layer 10 only.⁸¹ In the future, analysis of further samples from all the relevant Layers, including micromorphological analysis of the depositional environment, would be valuable. Other early Asian examples are claimed but have not been examined in detail.⁸²

In Europe, a rich, well-studied record from multiple depositional contexts is available, providing one of the best opportunities to examine the chronology of archeological evidence for fire use.⁶ In a recent review, published sites with fire evidence were evaluated.⁶ The earliest example is Beeches Pit in the UK, dated to the warm interglacial Marine Isotope Stage (MIS) 11 (c. 400 kya). At this site, several large features consisting of darkened sediment, are associated with heated lithics as well as charred microvertebrate bones, and are interpreted as the remains of hearths,^83,84 although heating of the sediments has not yet been demonstrated using micromorphological and other analytical techniques.^84,85 At Schöningen, where archeological evidence is attributed to the subsequent interglacial,⁸⁶ detailed analysis of possible hearths, possible burned sediments and possible burned wooden implements indicated that these had not been exposed to heat.⁸⁵ This suggests that the earliest regular evidence for fire use is younger than the earliest evidence that hominins were active in Europe by more than half a million years.⁶ This argument is more convincing because of several well-preserved open air and cave sites with long sequences in which traces of fire are very scarce indeed.⁶ For example, traces of fire are found in younger layers at Caune de l'Arago, dating to 350 kya, while older layers contain no evidence of fire.^6,87 Evidence for an episode of human burning in the form of heated chert and charred and calcined bone heated to quite high temperatures has recently been reported from a deep level at the cave site of Cueva Negra del Estrecho del Río Quípar.⁸⁸ This level is dated by palaeomagnetism and biostratigraphy to 0.8 mya.^88,89 However, given the caves' complex sedimentary history, the palaeomagnetic evidence may be relevant only to older sediments preserved at the back of the cave, and an early to mid Middle Pleistocene age can be supported by the fauna present.⁹⁰ This would be more consistent with the pattern elsewhere in Europe. Given the contrast in dates, it is clearly necessary to resolve the dating of this site.

The record from the Near East shows a similar pattern. In the long sequence at Tabun Cave in Israel, frequencies of heated flints are minimal at first, and begin to form about 10% of the total by 357–324 kya, becoming increasingly important through the Middle Palaeolithic.⁹¹ Other Levantine cave sites show a similar chronological pattern. For example, at Qesem Cave fire was habitually used between 380 and 200 kya;⁶⁵ a large, repeatedly used central hearth marks an increase in fire evidence in the sequence after 300 kya.⁹² However, at the open air site of Gesher Benot Ya'aqov, clusters of heated stone microartifacts and small plant materials, present in multiple levels, are interpreted as indicating regular use of fire considerably earlier, c. 780 kya.^93-95 Currently this remains a strikingly isolated case, contrasting with a change in the nature of fire use in several regions after about 450–350 kya.⁶

From about MIS 9, an interglacial dating to about 340 kya, extensive evidence for fire use can be found in both cave and open air settings.⁶ For example, at La Cotte de Saint Brelade, Jersey, high densities of burnt material, particularly bone, are present in every level during the period of occupation from MIS 7 to 4.⁹⁶ Roebroeks and Villa document a steady increase in the number of sites with fire evidence through time.⁶ Based on the scarcity or absence of fire evidence in cold phases at 2 cave sites in France, Pech de l'Azé IV and Roc de Marsal, Sandgathe and colleagues⁴ argue that Neandertals were opportunistic users of fire, dependent on exploiting natural occurrences. However, on a regional scale, evidence for Neandertal fire use does occur in full glacial as well as interglacial conditions.^97,98 Neandertals seem to have been sophisticated fire users, using fire as a tool, for example to haft materials,^99,100 and to cook.¹⁰¹ A number of likely Neandertal ‘strike-a-lights’ have been identified.⁹⁸ It is even possible that they used manganese dioxide powder to modify ignition temperatures and fire properties.¹⁰² Stone-lined or delimited hearths are not common but are documented in some Middle Palaeolithic sites⁶ and are present as early as 230,000 y ago at Bolomor Cave level XIII.¹⁰³ The sophistication of Neandertal fire use seems similar to that of modern humans, with both including subtle technological applications and sometimes constructed hearths. The early record for modern humans in Europe includes the striking constructed clay hearths from Klisoura Cave.¹⁰⁴ However, the majority of fires were not so carefully made. For example, of 89 Aurignacian sites in southern France and northern Spain, only 10 sites have structured hearths while many more contain fire proxies such as heated flints, charred bones or charcoal, and ashes dispersed in the sediments.⁶

Discussion and conclusion

Based on the evidence currently available (Fig. 2), it is possible to sketch a scenario for the role of fire use in changing hominin distribution. Current archaeological evidence does not support a long chronology in which regular fire use emerged 1.8 mya, causing anatomic changes and range expansion. However, at lower latitudes hominins may have exploited new food resources in recently burnt landscapes, and improved access to these new resources by setting light to additional areas.¹⁰⁵ Outside Africa less frequent lightning strikes and natural fires could have meant that this was a less regular part of daily life.¹⁰⁶ The combination of absence of evidence and negative evidence implies that the earliest occupants of temperate latitudes in Eurasia were able to cope with cool conditions and seasonal food shortages without producing and using fire at will. These conditions included air temperatures close to and sometimes below freezing as well as rain. Even if they were short-term visitors, as well as favoring an ‘Atlantic’ coastal and fluvial context within north-west Europe, keeping warm could have been a challenge.

Figure 2. Timeline of the main events mentioned in the text.

Around 350 kya the evidence for fire becomes more frequent and convincing. This may be the result of a change in the frequency of use of fire, perhaps based on an ability to produce fire at will.⁶ Searching for older microwear traces of fire production on stone tools may provide relevant evidence.⁷¹ It is tempting to argue that fire played a role in the expansion into cooler conditions which is indicated both by palaeoenvironmental and fossil evidence c. 500 kya. From this time on hominins faced the increased demands of survival in cooler conditions (including the energetic costs associated with producing additional body heat) while at the same time they apparently had reliable sources of energy to support large brains and needy young. They may have been successful hunters with regular access to cold-weather food combined with access to animal hides for insulation. It seems that hominins in this period faced new challenges and developed a range of strategies, of which fire would have been complementary and perhaps necessary. Important benefits of fire for these hominins could have included providing thermal radiation to heat the body when resting or sleeping outside, and increasing ambient temperatures in a rockshelter or cave by convection and radiation; warming the ground before sleeping; drying wet clothes; increasing the energy obtained from food; processing meat for consumption; and acting as a hunting aid.

Neandertals were clearly dealing with colder conditions, within some fairly extreme limits, using biological and cultural solutions, and there is almost no question that one of these was fire. Use of fire by the earliest Homo sapiens in Europe did not differ materially from that by Neandertals: it seems likely that fire was a well established technology by this point and did not in itself make a major difference in dispersal or competition. Our own species probably evolved cold adaptations in a context in which the thermal environment was shaped by cultural solutions including fire.

However, given the limitations of the fire record, there is always a chance that the lack of older evidence is a result of the lack of preservation and that this will be recovered as methods for recognizing fire traces improve. Further, mobility represents a key means of adjusting to circumstances by hunter-gatherers, and if earlier hominins moved around in a substantially different way, for example, traveling much more frequently and not bringing resources to central places (and using rockshelters and caves in a different way from later hominins) any fire traces would be less likely to be preserved and discovered. Further, the scarcity of sites and particularly both fire and fossil evidence makes it difficult to make a strong argument for a causal link between use of fire and occupation of cooler conditions c. 500 kya. It is striking that for example there is no convincing evidence for fire use at Schöningen where hominins probably faced more continental conditions. However, it is worth noting that the evidence from Schöningen seems to represent brief activities undertaken while moving through the landscape, and it is possible that fire is absent by chance.¹⁰⁷

There is clearly still room for progress in refining the interpretation of fire evidence, as well as developing other innovative proxies for fire use.^108,109 Perhaps equally important is the need to consider fire as a behavioral strategy that was adaptive for hominins, with a suite of existing behavioral and biological characteristics, in a specific environmental context. One interesting avenue for investigation is the physiology of thermoregulation and how this could affect the use of fire. For example, there is a complicated interaction between different areas of skin and internal organs in informing the thermoregulation system and triggering thermal behaviors,¹¹⁰ which may suggest higher energetic benefits of fire use. Another starting point is to identify plausible fire-free strategies for survival, particularly keeping warm, preparing food and warding off predators.¹⁰⁶ An ‘Atlantic’ coastal and fluvial strategy provides some possible answers, which it may be possible to test in the future.³⁶ As Gowlett and Wrangham¹⁰⁶ point out, this is necessary to support the interpretation that regular fire use was not important for the colonization of temperate latitudes. Understanding hominin strategies for coping without regular use of fire may also help us to understand why and when it became important. This can build on existing studies addressing how hominins survived on a day to day basis in the Lower Palaeolithic.³³

Further progress in addressing this question, with a focus on keeping warm, will involve a more detailed consideration of the nature of the environments in which hominins were active. This should acknowledge the range of factors affecting heat exchange with the environment. Alternative scenarios for occupation, including winter residence and seasonal mobility, should be taken into account. Earlier hominins may well have had different behavioral and biological thermoregulatory mechanisms from us. Comparative data sets including human biology, ethnography and natural history may prove stimulating in identifying a wider range of plausible strategies for survival in such conditions. Many large mammals have solved the problem of wintering at temperate latitudes, for example with thick winter fur and fat. Mammals also use social and behavioral mechanisms such as huddling which can save substantial amounts of energy.¹¹¹ Recent and historical hunter-gatherers use a range of simple and highly sophisticated cultural solutions; these include for example smearing their bodies with grease, simple fur cloaks and tailored clothing. While relevant studies are scarce, the small amount of evidence available suggests that in these populations cultural solutions were combined with a range of biological cold defense mechanisms.¹¹² Biological anthropology studies of traditional populations living in yet colder conditions point to seasonal and longer-term adaptations which may also be relevant for hominins lacking key cultural strategies to buffer themselves from the cold.¹¹³ Experimental studies of western industrial populations provide detailed quantitative information about cold acclimation and the mechanisms underlying this.¹¹⁴ Human origins studies can benefit from further developments in these related disciplines, and specific information is needed for this. Medical studies focus on comfort rather than tolerance, and biological anthropology studies do not yet address fitness effects,¹¹⁵ leaving us with limited knowledge of human tolerance of cold.

Once such strategies and their tradeoffs have been identified, it will be possible to assess whether these are plausible for survival in the conditions faced by hominins. The next very important step will be to identify testable hypotheses for a range of aspects of hominin behavior and biological adaptations. These might concern distribution, use of the landscape, diet, cultural insulation, metabolic regulation, among many others. The broader nature of these hypotheses will mean that there are multiple possible ways of testing them, which will be complementary to further studies of fire and distribution.

Abbreviations

kya	=	thousand years ago
MIS	=	Marine Isotope Stage
mya	=	million years ago

Disclosure of potential conflicts of interest

No potential conflicts of interest are disclosed.

Funding

This research was aided by the Royal Netherlands Academy of Arts and Sciences (Academy Professor Prize program).