Main
The genus Homo has existed for some three million
years4,5. For one third of this stretch of time, human
species were confined to tropical and sub-tropical Africa, which is the
homeland of the genus6,7 and is rich in the warm,
savanna-like environments to which most early hominins were best
adapted8,9. With the emergence of Homo erectussome 2 Ma, our ancestors began to disperse outside of Africa but kept
away from boreal latitudes, possibly because of physiological limits to
cold tolerance1. However, later Homo species
were able to expand their distribution to Northern Europe and Western
Siberia, despite the advent of full glacial cycles which were making
global temperatures colder than ever before during the history of the
genus Homo . Findings in Happisburgh and Pakefield (UK) date the
earliest presence of Homo at the southern edge of the boreal zone
at some 0.7-0.9 Ma10. The occupation of such northern
temperate and boreal zones presents a number of notable challenges. Not
only does the cold itself present a challenge for hominins
physiologically adapted to environments in Africa, but seasonality
imposes extreme annual resource fluctuations, which imply a reliance on
hunted meat for survival7 and limits to population
densities which threaten the maintenance of mating networks.
Technological adaptations facilitating survival in cold environments may
have included the use of fire, shelters or clothing, with vulnerable
infants being particularly susceptible to mortality. Technological
advances such as the use of spears may have facilitated the hunting of
typically large mammals11 whilst social adaptations to
care for higher injury rate may also have been
important2. At the same time cultural changes may have
been necessary to enable knowledge transmission over large regions, and
across generations, with archaeological evidence suggesting the
emergence of larger networks of communication1,2.
Clothing manufacturing leaves very little in the way of fossil
remains12. The first microwear evidence of
hide-scraping (for manufacturing clothes) at Hoxne (UK),
Biâche-Saint-Vaast, Pech de l’Azé and Abri Peyrony (France) and
Shöningen (Germany)13-15 are just some 50ka old at the
most. Only the two most recent human species, H. neanderthalensisand H. sapiens , left incontrovertible evidence that they were
able to produce complex, cold-proof clothing at that time. To make
things more complex, in the particular case of H.
neanderthalensis biological adaptation, besides material culture, was
possibly involved in their ability to withstand the cold. Neanderthals
possessed relatively short limbs, and a large midface and nasal cavity
proposed to be specific cold adaptations, to heat and humidify inspired
air, although the issue is definitely unresolved16,17.
In contrast to any other Homo , H. sapiens is considered
the only species in the genus able to occupy cold regions through a
genuinely cultural process, driven by our technology including the
mastering of fire, ever-improving clothing craftsmanship and
construction of shelters15,18-20. This view setsH. sapiens apart from any other human species in terms of
cognitive skills and implicitly rejects the idea that older Homomay have had sufficiently modern material culture to overcome climatic
harshness21. With such a poor fossil record of
clothes, tools to produce them and great uncertainty about deep-past
local paleoclimates and human dispersal timing and direction, the issue
of when humans first became cognitively and culturally able to extend
their climatic tolerance beyond their physiological limits remains very
difficult to decipher.
Here, we address the more restricted issue of when during the history ofHomo the limits of climatic tolerance expanded, and which species
were involved. We do not specifically address the cultural and social
adaptations that might underlie such tolerance, but rather consider the
implications of our findings for the timing of such adaptations. We
model climatic tolerance limits by associating palaeoclimatic values
with the archaeological record to resolve the detail of the tempo and
mode of past occupation of cold climates. Specifically, we test the
hypothesis that H. sapiens developed greater climatic tolerance
relative to H. heidelbergensis and H. neanderthalensisagainst the alternative that the exploration of climates outside natural
physiological limits had already happened since the earliest of these
species.
To test this, we estimated the rate of change of climatic tolerance
limits across the human phylogenetic tree and searched for possible
shifts in the rate, applying a method which allowed the rates to be
estimated at each branch in the tree. In the present context, shifts in
the rate of evolution of climatic tolerance that accrue to the clade
including the Happisburgh/Pakefield hominins, H. heidelbergensis ,
plus H. neanderthalensis and H. sapiens , would indicate
these hominins first acquired the capacity to develop cold-facing
technological skills and cultural adaptations. Conversely, if either no
rate-shift occurs, or the rate shift coincides with different clades
(e.g. at the root of Homo ) the colonization of Northern habitats
would be not indicative of any sudden increase in the ability to face
environmental harshness.
The human fossil record dataset we used includes 2,597 hominin
occurrences associated with 727 archaeological sites. The time range of
our record spans from the first occurrence of australopiths in East
Africa dated to some 4.2 Ma, to the definitive advent of H.
sapiens in Eurasia almost coincident with the demise of H.
neanderthalensis dated some to 0.040 Ma (see Supplementary Data). Such
a wide range of hominin taxa provides a thorough phylogenetic context
for the analyses.
Deriving spatio-temporally detailed climate data for the past requires
dynamic climate modelling, but the timescales for human evolution exceed
the possibilities of direct model simulation by several orders of
magnitude. To circumvent this limitation, we combine direct simulation
using a computationally efficient, intermediate complexity Earth system
model, PLASM-GENIE, with statistical modelling, to create a PALEO-PGEM
paleoclimate emulator, capable of performing multi-million year
simulations forced by observationally derived proxy timeseries for
ice-sheet state, CO2 concentration and orbital
forcing22. To model climatic niche evolution, we
applied phylogenetic ridge regression
(RRphylo )23. RRphylo allows us to
compute evolutionary rates for each branch of the phylogeny and to
estimate the ancestral phenotypes. Here the ‘phenotype’ comprises
climatic tolerance limits.
By using past seasonal maxima and minima for temperature, precipitation
and annual net primary productivity from PALEO-PGEM, we reconstructed
and projected onto the geographical space the climatic niche limits
corresponding to the ancestral species distributions (the nodes in the
tree) in our fossil database. Using RRphylo , we were then able to
infer climatic niche tolerance limits for each node in the tree and to
assess whether the rate of climatic niche evolution shows any shift
(i.e. acceleration or deceleration) consistent with our starting
hypothesis, while accounting for phylogenetic effects. We further
accounted for phylogenetic uncertainty changing the tree node ages and
the tree topology randomly one hundred times. By incorporating sources
of uncertainty, we defined an overall ‘habitat quality’ metric,
representing the number of iterations (out of 100) a geographic cell was
found habitable (i.e. fell within climatic tolerance limits) for a given
species (or ancestor in the tree).