Fig. 2. Climatic niche evolution in hominins. a, The climatic niche volume occupied by hominin clades through time. b , The hominin tree used in this study. The branch colours are proportional to the multivariate rate of climatic niche evolution for each branch in the tree. At the MHS common ancestor (14) an acceleration in the rate of evolution in climatic tolerance limits occurs (shaded area). The common ancestor to all species within Homo is indicated by node 12.c , The distribution of the rates of niche evolution for the MHS clade (deep blue) compared to the rest of the branches in the tree (light blue). d , The individual rates of niche evolution for the tree branches forming the MHS clade. The average rate for the entire tree is indicated by the vertical blue line. MHS = modern Homospecies, EHS = Homo species exclusive of MHS, Australopiths = species in the genus Paranthropus and Australopithecus .
As expected, we found that the clade identified by H. heidelbergensis, H. neanderthalensis, and H. sapiens and their common ancestor experienced a significant rate shift towards wider climatic tolerance (Fig. 2). The rate shift does not depend on the phylogenetic hypothesis we applied, neither it depends on the selection of species we used. Shuffling the tree node ages (to account for common ancestors dating uncertainty) and species positions in the hominin tree (to account for phylogenetic uncertainty) 100 times the shift reappears for this very clade 95 times (Table 1). Subsampling the most abundant species (randomly selecting no more than 100 fossil occurrences per species) to account for sampling differences between species still leads, 91 times out of a hundred, to a significant rate shift in the MHS clade. We also repeated the phylogenetic reshuffling by randomly removing one species at once. Under this latter design, the MHS shift occurs 86 times out of 100, and 23 additional times the shift involves two, rather than three, MHS species. Individually, H. sapiens andH. heidelbergensis appears in 86 significant rate shifts,H. neanderthalensis in 85, and no shift appears outside the MHS clade, meaning that the rate shift pertains to these species only and is not guided preferentially by any of the three (Table 1).
The estimates of climatic niche limits at nodes in the hominin phylogeny suggest that the rate shift in the climatic niche limits for the MHS clade was not an exclusively biological process. At the root of the hominin tree (node 11, Extended Data Table 1), the predicted range in temperatures spans from 20°C (coldest quarter of the year) to 29.9°C (warmest quarter), and in mean rainfall from 12 mm (driest quarter) to 512 mm (wettest quarter). This is entirely consistent with today’s African savannah environment25. At the node subtending the pair H. ergaster plus H. erectus (which is the first hominin to disperse over Asia and Europe), the corresponding figures are 0.7°C to 31.9°C for temperature range and from 4.8 mm to 1080 mm for precipitation range. These estimates are reasonable considering both the range expansion into temperate regions and the colonization of warm and humid environments (Indonesia) by H. erectus 6,26,27. Yet, at the common ancestor to the three MHS, the estimates for temperature extremes span from minus 21.1°C to plus 31.4°C and precipitation from 0.7 mm to 905 mm. Although the common ancestor to MHS was an African species which probably never experienced these extreme climates28, the estimates agree perfectly with the notion that a sudden widening to climatic niche limits occur with the advent of this ancestor, whose offspring lived after the onset of fully glacial Pleistocene conditions29. The massive increase in the (estimated) range of thermal conditions suitable the MHS clade taxa (marked by a 20°C decrease in minimum temperature as compared to the hominin tree root, Fig. 3) does not depend on the phylogenetic hypothesis we applied. By using 100 different tree topologies and branch lengths to account for phylogenetic uncertainty, we found a significant trend in the temperature of the coldest quarter experienced by hominins 97 times (Fig. 3), whereas no trend is found in the warmest quarter temperatures. This means that hominins, and MHS in particular, were able to expand, rather than track, their climatic envelope, by constructing their own niche culturally, starting with the appearance of the MHS. In fact, the increase in temperature range cannot be explained by physiological adaptation per se 12,30. We found that in African species and ancestors, the average temperature of the coldest quarter of the year was no less than 9.4°C, meaning that the winter chill is unlikely to have been a problem for them (Extended Data Table 5). In contrast, within the range of temperatures experienced byH. heidelbergensis , the coldest quarter of the year was as cold as -12.3°C, suggesting specific cultural equipment to fend off the risk of hypothermia was needed (Extended Data Table 5). Although H. erectus may have occasionally faced similarly extreme temperature minima, it lived under much warmer and more humid climates on average (Extended Data Table 1a, Fig. 3), and the ability to master fire (which might greatly mitigate the effects of cold weather) is not attested before 350 ka31, which is much later than the first appearance of H. erectus at nearly 2 Ma but still fully consistent with its use by any or all MHS.
Our data therefore indicate cold climates (hence Northern latitudes and temperate latitudes during glacial periods) remained unavailable for any species other than the MHS triplet. It is important to remark that a literal interpretation of our modelled estimate of annual mean minimum temperature of - 20°C as the condition experienced by our ancestors is unlikely and not necessary to our conclusions. There is uncertainty in the paleoclimatic estimates22 and our ancestors might have used cryptic refugia offering milder conditions32. In addition, all MHS species used to dwell in cave environments that were climatically less extreme than open air, and the use of fire and clothing shields against extreme temperatures and wind chill12,15,30. Instead, we are suggesting that MHS had sufficient technology to produce clothing and manage fire at will on a daily basis. The emergence of behavioural modernity, including complex social interactions, the habitual use of fire31 and the ability to work hide, wood and ivory11,14 may well place the appearance of such ‘modern’ cultural attributes to the Middle Stone Age at some 350 kya, and so extend to extinct human species3. Brain asymmetry and right-handiness, usually linked with advanced cognitive skills33,34, is common to H. heidelbergensis ,H. neanderthalensis and H. sapiens 35-37. It has been demonstrated that range expansion towards northern, sub-boreal regions similarly pertain to these three species only10, which is important since range expansion to previously unoccupied regions is itself part of behavioral modernity3.
In contrast to MHS, EHS either did not venture outside Africa or went across Eurasia longitudinally. Homo erectus spread across Africa and Eurasia up to Java at some 1.7 Ma, but never settled north of the Mediterranean area or southeast China6. Yet, from the appearance of H. heidelbergensis onward, the far North was no longer completely uninhabitable, despite it was becoming more inhospitable than ever before because of the cooling trend in global climate. The jump in the rates of evolution in climatic niche width (driven by a sudden increase in tolerance to the cold, Fig. 3) is the most important result we found.