Introduction

Human use of fire in the Paleolithic period has been widely researched in recent decades due to its major implications for the understanding of human adaptation and evolution (Gowlett & Wrangham 2013; Sandgathe & Berna 2017; Wrangham 2009; Wrangham 2017). There is evidence for the use of fire in caves from ca. 1mya at Wonderwerk Cave (Berna et al. 2012), and somewhat later in Lower and Middle Paleolithic caves in the Levant, Europe, and most probably China (Gao et al. 2017; Goldberg et al. 2001; Ravon, Gaillard & Monnier 2016; Roebroeks & Villa 2011; Shahack-Gross et al. 2014; Shimelmitz et al. 2014; Walker et al. 2016; Zack et al. 2013; Zhang et al. 2014). Still, fire making before the upper Paleolithic period is the subject of ongoing debate, with some arguing that the Neanderthals were able to control fire ignition (Sorensen 2017) and others arguing that they relied on natural fire (Dibble et al. 2018). However, it appears that by the end of the Lower Paleolithic period in the Old World, humans were already starting to use fire on a regular basis (Roebroeks & Villa 2011; Shimelmitz et al. 2014). For example, at Qesem Cave, a superimposed hearth dated to ca. 300kya was identified, and fire was apparently used throughout the 200,000 years of human occupation at the site (Karkanas et al. 2007; Shahack-Gross et al. 2014), most probably for meat roasting (Barkai et al. 2017).

Archaeological research has focused on fire’s advantages: heat, protection, cooking, and light, as major factors in early human adaptation (Gowlett & Wrangham 2013; Kaplan et al. 2000). Hearths were also regarded as essential elements in symbolic or ritual behavior (Jaubert et al. 2016). However, recent studies have also pointed out the possible drawbacks of using fire on a daily basis. Hearth maintenance demanded significant investment of energy for tasks such as wood collection and storage (Goudsblom 1992; Henry 2017; Henry, Büdel & Bazin 2018; Mallol & Henry 2017; Pryor et al. 2016; Twomey 2014). In addition, gathering around the hearth might have contributed to the spread of disease (Cisholm et al. 2016). One major drawback of fire is the emitted smoke. Smoke dispersal in a cave due to the use of a hearth may prevent cave occupation even after several minutes, as shown in an experiment conducted by Gentles & Smithson (1986).

Several ethnographic studies describe and analyze site spatial organization with regard to hearth locations. For example, a study of a humid tropical environment of the Nayaka from the forests of Tamil-Nadu, India, found that the hearth is their main activity area. The Nayaka hearths are about 1 m diameter and are placed outside their houses on the terrace. On cold nights, the Nayaka go outside to the hearth to warm up (Friesem et al. 2017). A study of the South African San group showed that the hearth is located in the center of the camp outside the huts (Leori-Gourhan 1965). Another case from the circumpolar north of the traditional Sàmiturf describes two tents with different structure with a hearth inside the tent. In both cases the hearth is at the center of the tent beneath an open chimney (Beach 2013). Although these studies describe the hearth location at several site types and in different periods and geographical regions, none provide a rationale for hearth location.

Galanidou (2000) analyzes the spatial organization and the location of hearths in 35 cave sites in different countries in the Southern Hemisphere. Galanidou analyzed spatial characteristics such as the site size and location of hearths by dividing the space into activity areas. She claimed that the number of hearths is defined by cultural constraints rather than by the site size or number of occupants, and she found that sleeping areas are usually adjacent to hearths. The research emphasizes that in southern African sites the sleeping areas are placed near the back wall of the cave, while in Australian Aboriginal sites the hearths are located at the center of the cave.

Most research describing spatial organization in a cave or rockshelter focuses on the location of the hearths as the center of activity. For example, in Abric Romani, a Middle Paleolithic rockshelter in Spain, (Vaquero & Pastó 2001), the authors analyzed the distribution of artifacts on the basis of hearth location. They analyzed 10 hearth-related accumulations spread over four levels. Henry (2012) defines several models based on human physical characteristics and social behavior to explain the required distance between hearths. The distance between hearths is used to analyze simultaneous hearth activity at the late Levantine Mousterian rockshelter site at Tor Faraj, Jordan. However, no explanation was proposed for the rationale behind the hearth location.

In this paper, we suggest a method for enhancing our understanding of the location and number of hearths in Paleolithic caves in different seasons. We claim that the location of hearths and their season of use were determined by smoke dispersal in the cave. In order to analyze the smoke dispersal, we propose an air circulation model that describes smoke ventilation patterns. The model is based on data developed by the National Institute of Standards and Technology, USA (NIST) for simulating fire in closed spaces such as buildings and compartments (McGrattan et al. 2017); it takes into account the space volume and structure, fire characteristics, and ventilation characteristics such as the inside and outside temperature gap. The NIST model was verified in an experiment conducted in a quarry simulating Chauvet-Pont d’Arc Cave (Lancanette et al. 2017). We suggest that this model can be used to explain the relationship between smoke dispersal and cave structure, cave opening dimensions, hearth characteristics, and seasonal temperature fluctuations in prehistoric caves. According to our preliminary cave analysis using the air circulation model, we suggest that hearth location was crucial in allowing humans to occupy caves while using fire on a regular basis. We demonstrate the application of this data by presenting several Paleolithic case studies.

Hearth Advantages

Fire offered many potential benefits to early human groups, such as the nutritional advantages of roasted and cooked food due to the improved physical absorption of starch and protein. The starch and protein semi-crystalline granule structures collapse following heating, there by facilitating the digestive process and the absorption of nutrients into the body (Groopman, Carmody & Wrangham 2015). In addition, heat softens tough fibers and neutralizes toxins, thus expanding the variety of food that early humans could consume (Wrangham 2009; Wrangham 2017; Wrangham & Carmody 2010). Fire also provided light, warmth (Clark & Harris 1985), and protection from predators (Brain 1993; Fessler 2006; Malowe 2005), as well as a way to heat adhesive materials for hafting tools (Koller, Baumer & Mania 2001; Mazza et al. 2006). Fire could also be used to clean out dwelling spaces, clear pathways, and exterminate pests (Bentsen 2014; Pyne 1997; Rolland 2004; Sandgathe & Berna 2017). Ethnographic studies have more over revealed that the hearth functions as a social magnet that increases the hours of group activity and contributes to group solidarity (Wiessner 2014).

The Effect of Materials Emitted from a Hearth

One drawback of hearth use within a cave is the emitted smoke. In a few extraordinary cases reported recently, ash particles were extracted from dental human calculus at prehistoric sites, providing direct evidence for the exposure of early humans to smoke. Smoke particle matters from Lower Paleolithic Qesem Cave were found in dental calculus and are assumed to have been inhaled (Hardy et al. 2016). In addition, calculus analysis from Middle Paleolithic El Sidron Cave, Spain, suggests supportive evidence for cooking/smoke inhalation (Hardy et al. 2012).

Smoke may influence human occupation at a cave, as shown by the experiment conducted by Gentles & Smithson (1986). In this experiment, the authors sampled cave temperature when an active hearth was placed at different locations and in different seasons. They state that in the summer, after 20 minutes of interior hearth activity, smoke filled the cave up to about half a meter from the ground level. In winter, the smoke rose higher, enabling cave occupation.

To facilitate an understanding of the effect of smoke dispersal on cave occupation, we describe the by-products of wood burning: the ash that remains in the area of the hearth and the smoke pollution emitted into the air (Aldeias 2017). Wood is composed of two organic polymers: lignin (18–35%) and cellulose, which is a molecule comprised of carbon, oxygen, and hydrogen (65–75%). The lignin-cellulose ratio varies according to the wood type (Pettersen 1984). Since burning produces an intense heat that breaks up the polymers into smaller molecules, the smoke emitted from the combustion contains a number of oxygen-based compounds (Naeher et al. 2007). When wood burning is inefficient, the particles contain inorganic compounds such as ash and soot, as well as compressed inorganic components. The particles can be divided into two groups: particles with a diameter of 10 µm, which, when inhaled, remain in the nose and upper respiratory tract, and particles with a diameter of 2.5 µm, which, when inhaled, can enter the lungs and bloodstream. Smoke from burning wood contains around 200 different chemical materials, mostly of a size that can be inhaled and some that are noxious and carcinogenic (Mannucci et al. 2015; Zelikoff et al. 2002).

Exposure to smoke causes burning eyes and respiratory-related issues, such as coughing. In addition, use of a hearth raises the nitrogen level in the air and can cause methemoglobinemia, a condition in which red blood cells are unable to release oxygen efficiently to the body’s tissues. This produces dizziness, fainting, weakness, dyspnea, confusion, and at higher levels of nitrate concentration leads to loss of consciousness, coma, blood vessel injury, and edema (Hasselblad, Eddy & Kotchmar 1992; Naeher et al. 2007). Although the use of fire also has many direct advantages, we may assume that the continuous use of a hearth in caves required careful selection of its location in order to contend with the possible negative effects. In the following we describe the cave air-circulation model, which we then employ to highlight the significance of hearth characteristics and location in caves with respect to seasonal changes in air pressure inside and outside the caves.

Method

In this section, we use a basic air circulation model for caves with an active hearth to estimate the smoke levels that early humans might have been exposed to in different areas of the cave (Cigna 1968; De Freitas et al. 1982; Faimon & Lang 2013; McGrattan et al. 2017). This model can be used for cave structures as it was verified in a quarry test simulating Chauvet-Pont-d’Arc Cave (Lancanette et al. 2017).

The cave’s internal temperature is almost constant throughout the year and is equal to the external annual average temperature. In accordance with the second law of thermodynamics, the cave’s internal temperature changes to be equal to the outside temperature, creating air circulation. Warmer air fills a larger volume, resulting in lower air pressure. Thus, cold air flows near the floor while warm air flows near the ceiling (Petrucci et al. 2011). Figure 1a presents an example of airflow in the winter, where the outside temperature is lower than the internal temperature (Cigna 1968; De Freitas et al. 1982).

The difference between the internal and external temperature affects the airflow rate, which is defined as the air mass flow through the cave opening, termed cave ventilation rate. The air mass flow per second (amps) is based on Bernoulli’s equation, which depends on the size of the cave opening (t, w) and the temperature difference (ΔP) (Peacock et al. 2017):

where, t is the opening height, C is the orifice coefficient taken to be 0.7 (Steckler, Baum & Quintiere 1985), ρ is the gas density, w is the cave opening width, and ∆P(z) is the air pressure difference at height z.

When the hearth is in the interior of the cave, the emitted smoke is warmer than the surrounding air and thus rises to the cave’s ceiling, exiting through the cave opening due to the lower air pressure inside the cave compared the external air pressure. The smoke line in a cave is defined as the point where the outflowing smoke meets the cold air entering the cave. The smoke line is influenced by the cave’s ventilation rate (amps) (Figure 1b). The cave entrance constitutes a tunnel for the two-way airflow; thus, the larger the opening, the larger the amps and the higher the smoke line. A larger temperature difference, due to the interior hearth activity, increases the amps through the cave entrance as defined above. In both cases, the smoke line is farther from the floor due to the larger amps. Thus, if two caves have identical diameter hearths, the cave with the larger opening will exhibit larger amps and thus a higher smoke line than the cave with the smaller opening and smaller amps. Moreover, if two caves have the same structure but different diameter hearths, the cave with the larger diameter hearth will exhibit more smoke with similar amps, producing a smoke line lower than that of the smaller diameter hearth.

The caves might also have a chimney or a number of openings. In this case, the warm air exits the highest opening in winter and cold air enters from the lowest opening, whereas in the summer the airflow directions are reversed: warm air enters the cave from the highest opening and cold air exits from the lowest opening (Cigna 1968; Russell & MacLean 2008). In this type of cave, when a hearth is used, the smoke exits through the chimney or upper opening. Thus, the smoke path flows from the hearth location to the chimney and might elevate the smoke line height in the cave. In the case of a cave with two or more openings at the same height, the hearth smoke exits the cave through all the openings, increasing the amps and elevating the smoke line height.

In winter, the cave temperature is higher than the external temperature. Using a hearth increases this difference. Thus, the air circulation rate (amps) is higher due to the greater difference in air pressure. Lighting a hearth in summer also raises the cave’s internal temperature, decreasing the internal and external temperature difference and thus reducing air circulation. This forces the smoke to disperse inside the cave. In summer, when the cave’s temperature exceeds the outside temperature (because of the hearth), the air circulation exhibits the same process as in winter: some of the smoke exits near the ceiling, but since the circulation rate is lower than in winter, the smoke line height is lower.

Hearth location also has an impact on air circulation within the cave. When the hearth is located at the cave entrance, some of the generated smoke is dispersed within the cave and some of it flows out (Figure 1c). The dispersion rate in the two areas depends on the entrance structure and the exact location of the hearth. In the case of caves with a rather small entrance, the smoke flows towards the ceiling because the smoke density is lower than the air density in the cave, and thus the smoke flows near the ceiling up to the back wall, or the smoke temperature reaches the internal air temperature and the smoke exits near the floor. This type of air circulation renders cave dwelling problematic because of the high concentration of smoke remaining in the cave. On the other hand, in a cave with a high and wide entrance, only a small portion of the smoke remains in the cave (lower smoke concentration) and dwelling there is not likely to be a problem.

A chimney or cracks in the cave ceiling adds another way for the smoke to ventilate, as shown in Figure 1d. The amps is increased due to the chimney. Increasing the chimney size increases the ventilation rate. Thus, a chimney or cracks cause an increase in smoke height.

Examples of Paleolithic Caves with Hearths

In this section we provide a preliminary illustration of how the air circulation model can be used to reconstruct airflow and smoke dispersal within selected Paleolithic caves.

The Late Acheulian-Early Mousterian Lazaret Cave in France shows evidence of human occupation dating to 190–44kya. The cave is 35 m long and 13 m wide, with a 10 m high ceiling and an entrance width of about 3 m. In layer 25, dated to 190–120kya, a hearth was found in the cave’s interior in Area F, which is located about 13 m from the entrance; for an illustration of this hearth, see Figure 4 in Valensi et al. (2013). The researchers suggested that the hearth produced low-temperature smoke intended mostly for meat preservation (Falguères, De Lumley & Bischoff 1992; Valensi et al. 2013). In addition, in layer 26, four small hearths, 20–30 cm in diameter, were found in areas T15, R14, Q13, and P12 located at about 13 m from the entrance; for an illustration of this hearth, see Figure 1 in Azemard et al. (2013). The hearths in both layers are located in the center of the cave. According to the proposed air circulation model, the smoke from the hearth is dispersed partially to the back of the cave and partially to the cave entrance. In the winter, since the temperature outside is lower than the interior temperature, the smoke is dispersed near the cave ceiling, allowing for human occupation. Since the cave ventilation rate is higher in winter than in summer due to the larger difference between the cave’s interior temperature and the external temperature, the amount of smoke in the cave is greater in the summer, making human occupation unpleasant. A detailed analysis of the faunal remains suggests that cave was indeed used during winter (Valensi et al. 2013). Animal dental remains were analyzed and sorted into age groups. By integrating dental structure, erosion, and the expected season of birth, it was possible to determine the animal’s age at time of hunting. The researchers concluded that hunting and meat preservation procedures were usually performed in autumn and winter, between October and December, in order to stock up on food before the cold set in (Valensi et al. 2013). Their study conforms to the air circulation model: the small-diameter hearths in the cave’s depth were used in winter when the airflow rate was higher than in the summer because the cave ventilated faster and more efficiently.

Bolomor cave in the Valencia region bears evidence of human occupation dated to 350–100kya. The cave with the terrace area is about 600 m² and features several archeological levels, about 15 m depth from the dripline and with maximum width of about 30 m (Figure 2). The cave contains 14 hearths (diameter 20–120 cm and thickness 5–10 cm) in Levels 2, 4, 11, and 13. On the side of the site, the two interior hearths in Level 13 (Sublevel 13 c, Blocks F2 and D1), near the side wall, are 0.45 m and 0.51 m in diameter and located 1.12 m from one another. The four hearths in Level 4 (section CBIV-3) were found at the entrance of the cave, which at this stage had become a kind of rockshelter (Figure 2, red circles). Figure 2 shows that the hearths are located near the wall beneath the dripline towards the outside of the rockshelter (Die 2008; Peris et al. 2012; Sañudo, Blassco & Peris 2016). According to the air circulation model, the emitted smoke from hearths near the wall flows to the ceiling and then outside the rockshelter, with some smoke dispersing to the cave’s interior. Since this site is about 30 m wide, the hearths in this site can be used throughout the year. The air circulation model suggests that the site may contain additional hearths near the cave back wall; however, those sections have not yet been excavated.

Manot Cave, located in the western Galilee, Israel, is mostly associated with the Upper Paleolithic period dated to 41–33kya. The cave has an 80 m × 10–25 m main hall and two smaller, lower halls on the main hall’s northern and southern flanks. Area E, which is located next to one of the estimated cave entrance locations, contains two areas with burnt remains, one of which has been identified as a hearth with a 0.6 m diameter (Figure 3) (Barzilai, Heshkovitz & Marder 2016; Marder et al. 2013). According to the excavators, the hearth is located next to the cave’s assumed entrance. A few meters after the entrance there is slope downwards to the main hall. In the case of a small entrance, an interior hearth near the cave’s opening would have produced a great amount of smoke in the habitation level. According to the air circulation model, smoke dispersed from a hearth near the entrance is partially ventilated outside the cave, while the rest of the smoke is dispersed to the main hall. The smoke temperature decreases and the smoke fills the main hall. This would have caused respiratory irritation for the early human inhabitants. Consequently, if the cave entrance was indeed located where the researchers suggest, we may assume that the hearth was either located away from it or that the cave had another opening. This example shows that the air circulation model can help predict or verify the location of different cave openings.

Examples of Paleolithic Rockshelters with Hearths

There are many Paleolithic rockshelter sites with evidence of the use of hearths. A rockshelter has a different structure than a cave: a very large opening that facilitates faster and more efficient smoke ventilation. In this section we briefly describe two rockshelter sites and explain how their hearths differ from those in caves.

The Abric Romani rockshelter in Barcelona, Spain, features 25 levels dated to 70–40kya. Level O, dated to 55kya, has 24 hearths (Vallverdú et al. 2012) and Level N has 19 hearths (Vallverdú et al. 2010) (Figures 4 and 5). In Level O, the hearths are divided to two groups: one located near the rockshelter wall and one located outside of the dripline (Figure 4). The hearths in the first group are of small diameter. According to spatial distribution analysis, most of the artifacts are located near the back wall of the rockshelter. There are no hearths in the middle of the shelter. The analysis of the fauna and lithic concentrations revealed that the hearth area functioned as a sleeping space (Gabucio, Fernández-Laso & Rosell 2017; Vallverdú et al. 2010). According to the air circulation model, the size and location of the hearths would enable the sleeping space to be heated while the smoke concentration remained low, as small hearths emit small amounts of smoke and the proximity to the back wall of the shelter forces the smoke to ascend to the ceiling and flow out. Smoke emitted from the hearth located close to the rockshelter entrance most probably flowed out of the shelter thanks to the wide opening.

In level N (Figure 5), most artifacts are located in the central zone around hearths 9, 10, 11, 14, 15 and 16. Around hearth 13 there are scattered artifacts. In the frontal zone, scattered artifacts were found around hearths 6 and 7 and dense artifacts around hearth 8, which has a 30 cm diameter. Faunal remains were found around hearths 9–12 and 14–16. A low density of artifacts was found around hearths 4 and 5. The researchers suggest that hearths 1–5 were used for sleeping. According to the air circulation model, the location of hearths 1–5 near the back wall and their small size helps the smoke ventilate near the ceiling. The middle cave hearths (9, 11, 14, 17, 19) are all very small in diameter (less than 20 cm²), and thus would emit little smoke. Hearth 13, which is larger (72 cm²), is located near the back wall. The smoke in this location flows up near the back wall and outside near the rockshelter ceiling. Hearths 8 and 16 have an area of 1 m² and are located near the dripline. According to the air circulation model, hearth 8 was probably not activated in parallel to the hearths in the sleeping area since the smoke emitted from it would flow to the sleeping area. Hearth 16 is far from the sleeping area and its smoke would be dispersed mostly outside the rockshelter.

Tor Faraj rockshelter at Ma’an Plateau, Jordan is dated to 50–70kya and assigned to the Middle Paleolithic Levantine Mousterian. The ceiling of the rockshelter is about 10–13 m high, its depth is 5–6 m, and its width is about 24 m, with a total area of about 136 m² (Henry 2003). The hearths are mostly concentrated near the back and side walls (Figure 6) (Hietala 2003; Henry et al. 2004). The average hearth diameter is 57 cm. Henry analyzed the hearth distribution and found that the average distance between hearths is 1.5 m and many of the hearths in the site are near the back wall. Floor I includes six hearths, three near the back wall (B), one near the side wall close to the dripline (C), and two in the center between the dripline and the back wall. A bedding area is located near the back wall in area B. Floor II includes 13 hearths, five in area B near the back wall, three near the side wall (C), three at the center of the rockshelter, and two in area A, one close to the back wall and one at the center (Henry 2002). The bedding areas are found in areas B and C close to the walls. On both floors, the hearths are located mostly near the rockshelter walls and at the center of the rockshelter. According to the air circulation model, smoke emitted from the hearths near the back wall flows up near the wall and outside near the ceiling while the hearth heats the surroundings. The smoke is ventilated through the wide opening of the rockshelter. A phytolith study by Rosen (2003) shows that the Tor Faraj site was used in the cold season from February to May, supporting the conclusions of the air circulation model.

Discussion and Conclusions

Hearths are used as a pivot element in spatial analysis of caves and rockshelter sites. In this paper, we showed that hearth location and season of use are not randomly determined and can be explained using the air circulation model. The smoke dispersal in the cave depends on the hearth location and cave structure. Smoke height can be estimated according to air circulation parameters in the cave, taking into account hearth location, size and season of use. Thus, we used an air circulation model to determine the most probable season of occupation in Paleolithic cave sites where hearths were in use. The input parameters for the model are cave structure, cave opening dimensions, the difference between the interior and exterior temperature, and hearth characteristics. We showed that the expected smoke levels in the colder season are higher than the smoke level in the warm season. In addition, we showed that the preferred hearth location is near the back wall and not at the cave entrance.

Case studies of Paleolithic caves with active hearths support our hypothesis about the relationship between hearth location and probable effect on season of use. For example, Lazaret Cave and Tor Faraj rockshelter had a few small hearths in the cave’s depth, implying human occupation during the winter season. Faunal and botanical remains from both sites support claims regarding winter habitation. In winter the difference between the cave’s internal and external temperature is greater, and thus smoke ventilation is increased. A study of faunal remains from several Middle and Upper Paleolithic cave sites with hearths in the Ach Valley in Germany suggests that the sites were used during the winter and spring season (Münzel & Conard 2004), as our model indeed suggests.

Our short survey of sites has also revealed that more hearths were used in Paleolithic rockshelters than in caves. In the Abric Romani and Tor Faraj rockshelters, many hearths were scattered around, some near the shelter’s entrance and others near the back wall. The large entrances characteristic of rockshelters afforded the inhabitants greater leeway with regard to hearth location, and the group activity area seems to have been the dominant parameter in this regard. This is in contrast to caves, where smoke emitted from the hearth might have had an immediate effect on the occupants. Paleolithic caves are therefore expected to contain fewer hearths, located towards the back, as suggested by our study. A broader survey should be conducted to confirm these preliminary results.

Our preliminary analysis suggests that cave hearths were better suited for cold season use, when heat from the fire increased the temperature difference between the cave’s interior and the external environment, resulting in faster air circulation and more efficient smoke ventilation. During the hot season, when the internal and external temperature difference is smaller, smoke ventilation is poorer. This would have led to a higher concentration of smoke in the cave and a lower smoke height, making habitation difficult.

Since hearths emit a great deal of smoke, it would seem that early humans must have reasoned about air circulation when positioning a hearth within a cave, in order to benefit from its many advantages. We hope that the preliminary observations presented in this paper will promote a better understanding of human uses of fire within Paleolithic cave and rockshelter sites, and we intend to pursue this line of investigation further by enhancing the model, simulating different smoke circulation scenarios in caves, and analyzing the correlations between the various parameters.