The use of fire played a key role in the evolution of the genus Homo1, allowing for warmth, cooking, birch tar production, protection from predators, a venue for social interactions and access to high latitudes and dark caves2. Evidence of hominin fire use is present in the archaeological record beginning around 1.5 million years ago3,4,5, and while it has long been assumed that a variety of hominin species could use fire5,6,7, the degree to which hominins were able to intentionally create and control fire (pyrotechnology) is debated8,9,10,11,12,13,14,15,16,17. Recently, some have argued that this ability was exclusive to modern humans9,12,16, with other hominins, such as Neanderthals, limited to exploiting wildfires. Neanderthals went extinct during the late Pleistocene. The reasons for that extinction remain unclear, and recent genetic data indicate that Neanderthal DNA persists among certain modern populations of Homo sapiens18. Neanderthal extinction has been linked specifically or in combination to fire use, foraging behaviors, a lack of clothing, demography, climate change, and interactions with expanding populations of Upper Palaeolithic Homo sapiens19,20. Evidence for the use of fire among MP hominins includes burning found in archaeological sites, and the construction of hearths. Manganese dioxide blocks, which are useful for fire-starting, have also been excavated at MP sites and are interpreted by some as evidence of Neanderthal fire production21. However, research at MP sites in France claims that fire frequency, measured by thermally altered flint and burnt bone, is positively correlated with warmer periods, when wildfire frequency is assumed to be highest, rather than with colder, glacial periods when fire use would have provided greater benefit; this correlation is derived by associating chronometrically dated stratigraphic layers at these sites with particular phases of Pleistocene global temperature records. These data are collectively interpreted as evidence that Neanderthals had not mastered pyrotechnology, and instead harvested natural fires caused by lightning strikes12,16, though this interpretation is not accepted by all15.

In order to test the hypothesis that MP hominin fire use was correlated with natural fire frequency, we developed a record of polycyclic aromatic hydrocarbons (PAHs) for the MP site of Lusakert Cave 1 (LKT1) in the Armenian Highlands from eighteen sedimentary units associated with MP lithic technology22,23,24. Organic molecular markers of fire, including PAHs, can provide a quantitative record of fire over geological time scales25,26, though lighter PAHs may be more susceptible to the effects of degradation due to their higher solubility27. During biomass combustion PAHs with variable structure are formed typically containing two (e.g. naphthalene) to six (e.g. benzo[g, h, i]perylene) rings (Supplementary Fig. S1). A number of studies have documented the PAH emission from wood burning in hearths and fireplaces28,29,30,31,32,33. While varied PAHs are produced during wood combustion, low molecular weight PAHs (lPAHs) with four or fewer rings concentrate in the gaseous phase, whereas high molecular weight PAHs (hPAHs), with five or more rings tend to concentrate in the particulate phase34,35,36. Hearths in archaeological contexts can reach maximum temperatures of 1000 °C with mean temperatures of 600 °C, which provides sufficient energy for the production of hPAHs37,38.

Unlike wood combustion in hearths, studies document the low production of hPAHs in wild fires of varying intensities, which produce mostly 3- and 4-ring PAHs. For example, soils measured in a recently burned area in southern France did not measure hPAH abundances higher than control samples39. Following forest fires in South Korea, lPAH concentrations were found to be 3 to 28 times higher than hPAH concentrations40. Soil O-horizons in Russia analyzed two, ten and 16 years after a wildfire found that hPAH concentrations were not higher than background soils, despite 9-fold increases in total PAH concentrations41. hPAHs comprised 35% of the total PAH load in unburnt samples from fire prone regions in Spain, but less than 10% in burnt soils after wildfires due to the addition of lPAHs42. Finally, in savannah fires in Australia the frequency of emitted lPAHs was 13 to 30 times greater than hPAHs43. These data attest to the widespread production and dispersal of lPAHs associated with wildfire events.

In sedimentary records from fire-prone regions, hPAHs are typically lower in abundance than lPAHs44,45. Different explanations have been given for the lack of hPAHs relative to lPAHs, including changes in temperature or burn intensity44, as higher molecular weight PAHs require higher activation energies for synthesis. Another potential explanation is the proximity to the source of the fire45, as hPAHs are less likely to travel from the source of the fire. Therefore, in the context of a spatially confined archaeological site such as LKT1, we expect that the primary mode of deposition of lPAHs will be through long-range dispersals of wildfire, whereas the primary mode of deposition of hPAHs will be local fire use within the cave by hominins. Given the low abundance of hPAHs produced during wildfires relative to lPAHs, and their concentration in the particulate emissions from wood burned in hearths, it is the most parsimonious explanation that accumulation of hPAHs in the sediments of archaeological sites like Lusakert Cave will be due to increased particulate deposition of residues from wood combustion. These particulate emissions of hearths will concentrate locally, on the scale of 10 s of meters.

In addition to PAH data for local or regional fire, we also analyzed the hydrogen (δD wax ) and carbon (δ13C wax ) isotope composition of long-chain n-alkanes, the molecular remains of epicuticular waxes of terrestrial plants, to constrain regional climate and hydrology through the period of hominin occupation (Fig. 1A,B). Like hPAHs and lPAHs, n-alkanes are preserved over geological time scales. Long-chain n-alkanes at LKT1 have high odd-over-even predominance (OEP), demonstrating that they did not undergo significant microbial46 or thermal47 alteration (Supplementary Table S2). The δ13C values in plant material is primarily a reflection of the photosynthetic pathway of the plant (C 3 , C 4 or CAM). In C 3 plants, δ13C values are influenced by physiological changes as plants balance water loss and CO 2 uptake through the regulation of stomata, altering the partial pressures of intracellular (C i ) relative to extracellular (C a ) CO 2 48,49, causing a positive shift in δ13C values in C 3 plants experiencing water stress50. Fractionation during lipid biosynthesis causes n-alkane δ13C values to be lower relative to bulk plant tissues51,52. Therefore, sedimentary δ13C wax values of n-alkanes predominantly reflect plant photosynthetic type and factors that affect isotopic discrimination during carbon fixation53. δD wax record the isotope values of ambient water during the period of growth54, reflecting mostly precipitation isotope values in terrestrial systems. δD values of precipitation are influenced by temperature, amount of precipitation, and cloud transport history55,56. Most of the variability in modern precipitation δD values in the Armenian Highlands is explained by changes in temperature, and there is no significant trend associated with amount of precipitation57. This is also documented in Global Network of Isotopes in Precipitation stations in Georgia and Turkey (Supplementary Fig. S3). Hydrogen undergoes further fractionation during lipid biosynthesis, which is influenced by the timing of wax formation, plant physiology and functional type58,59,60.

Figure 1 (A) δD wax , (B) δ13C wax , (C) concentrations of hPAHs (red squares), and lPAHs (orange squares) from each sedimentary Unit at LKT1. δD wax (4‰) and δ13C wax (0.5‰) values are plotted with 1σ error bars. LKT1 stratigraphic layers are oriented oldest (right) to youngest (left). Full size image

LKT1 was excavated by an Armenian-American-British team between 2008 and 2011 and is one of the few sites in the region that preserves stratified assemblages of lithic and faunal material (Supplementary Discussion S4, Supplementary Fig. S5). In addition, there is extensive evidence for fire in the form of charcoal and visible combustion structures, as well as thermally altered bone micro-fragments in micromorphological samples (Supplementary Figs S6, S7, S8). The lithic assemblage is made entirely of obsidian from a variety of local (<25 km) and exotic (>25 km) sources, it is based on the Levallois method, and it can be classified as Middle Palaeolithic, a toolkit traditionally associated with Neanderthals in the Caucasus and neighboring regions22,23. Preliminary luminescence and AMS radiocarbon dates, as well as tephrochronological correlations constrain the stratigraphy of the site to 60–40 ka.