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.