Fig. 3. Estimated temperature and precipitation ranges at several nodes in the human phylogenetic tree. The individual rows represent the density distribution of minimum and maximum temperature and precipitation, respectively, collapsed together. HnHs = common ancestor to H. neanderthalensis and H. sapiens , MHS = Common ancestor to H. heidelbergensis, H. neanderthalensis andH. sapiens HereHerg = common ancestor to H. erectus andH. ergaster , Homo = common ancestor to Homo species,Paranthropus = common ancestor to all Paranthropusspecies, Australopithecus = common ancestor to allAustralopithecus species, Hominins = common ancestor to hominins.
Although there is consistent evidence that Homo species may have exchanged genes with positive fitness consequences in cold environments by means of introgression, this evidence is limited to the last 40 kya and invariably pertains to local Homo sapienspopulations38,39. Studies on living humans indicate the range of climatic extremes, particularly winter temperatures, experienced by MHS invoke technological and cultural rather than biological adaptation to life in the highly seasonal, cold northern environments. Fire is the most obvious innovation40, and whereas occasional use of fire has deep roots in human history41,42, its habitual use is much more recent, tracing back to no more than the Middle Stone age31. Other less obvious adaptations may have been equally, if not more, important such as fitted clothing43, thrown spears44 or adhesives45. Cultural transmission across regions and generations may also have been significant to occupying more northerly latitudes7, as well as social changes such as healthcare practices2. Whether any of these adaptations played the primary role in enabling expansion remains unclear. Furthermore, the relative significance of biological, technological, social or cultural adaptations will almost certainly be a matter for debate for some period. However overcoming the challenges imposed in the consistent occupation of northern latitudes since the appearance of H. heidelbergensis suggest behavioral modernity is not limited to H. sapiens , and that our own genus became able to master inhospitable habitats well before the appearance of our species, possibly as far back in time as one million years ago.