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Citation for published item:
O'Higgins, P. and Elton, S. (2007) 'Walking on trees.', Science., 316 (5829). pp. 1292-1294.
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http://dx.doi.org/10.1126/science.1143571
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redistribution. The denitive version was published in Science on 1 June 2007: Vol. 316 no. 5829, pp. 1292-1294 DOI:
10.1126/science.1143571.
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1
Walking on trees
Paul O’Higgins* and Sarah Elton
†
Functional Morphology and Evolution Unit
Hull York Medical School
* University of York, Heslington, York, YO10 5DD, UK
†
University of Hull, Cottingham Road, Hull, HU6 7RX, UK
For decades, standing upright and walking on the ground on two legs have been seen
as defining features of the hominin clade (humans and our closest extinct relatives).
However, there is increasing evidence that some Miocene apes not only had upright
(orthograde) postures (1) but also incorporated bipedalism into their locomotor
repertoires (2,3). Such movement may well have occurred in the trees. This raises
the possibility that preadaptations for hominin bipedalism arose in arboreal settings
rather than in terrestrial environments. On page xxx of this issue, Thorpe and
colleagues present compelling new evidence in support of this. Using observational
data from modern orangutans, they argue that hominin bipedal walking is not novel
but rather a development of locomotor behaviours already established in the
ancestor of great apes. In modern orangutans, hand-assisted bipedalism with
extended lower limbs in the small branches of the forest canopy allows movement
on slender, highly compliant supports, thus enabling them to access resources in the
forest canopy that would otherwise be difficult to procure, or cross between trees
with minimum energy expenditure. These advantages might well have provided
sufficient selective pressure for bipedal adaptations in arboreal habitats.
One important aspect of the orangutan model is that it provides three scenarios for
the emergence of modern great ape and human locomotor strategies from hand
assisted, straight lower limbed, arboreal bipedalism (see Figure). In the first, forest
canopy fragmentation during the Miocene of Africa led to increased vertical climbing.
The authors suggest that this behaviour, which is kinematically similar to knuckle
walking, predisposed gorilla and chimpanzee ancestors to the independent
acquisition of forms of knuckle walking. Meanwhile in South East Asia, orangutan
2
ancestors became even more specialized in traversing, at canopy level, the shrinking
closed-canopy forest. Finally, hominins retained and further adapted pre-existing
arboreal bipedalism to exploit emerging, more open terrain between forested areas.
This scenario is consistent with the long forelimbs that are found in association with
obviously bipedally-adapted hindlimbs in various early hominins. It is necessary in a
model such as this to simplify the nature and tempo of environmental change,
although Thorpe and colleagues do point out the probable fluctuations in forest
coverage that occurred during the Miocene. Inevitably, past environments were
complex, and there was no straightforward transition from forested to more open
habitats. Primate adaptations and radiations were equally complex, and it has been
argued (4) that apes diversified into a variety of environments well before any
significant Miocene forest shrinkage. Nonetheless, locomotion is strongly tied to
habitat, and therefore evolves in response to external pressures, whether they are
caused by environmental change or niche differentiation.
Thorpe et al.’s study reopens the debate about the origins of our own peculiar
commitment to bipedal locomotion. To date, there is no consensus about the
adaptive scenario that could have led to the adoption of terrestrial bipedalism. Many
theories have been proposed, including the postural feeding hypothesis (5); a
behavioural model (6), attributing bipedality to the social, sexual and reproductive
behaviour of early hominins; the thermoregulatory hypothesis (7) which links the
emergence of bipedalism to the need for thermoregulatory efficiency, and the
appeasement model (8) which focuses on bipedal displays that allow for the relatively
peaceful resolution of conflicts. A similar lack of agreement is also evident in
discussions about the locomotor behaviour of the hominin ancestor. One possibility
is that the common ancestor of humans and modern African apes used ground-based
knuckle walking (9) although it has been argued that the exact nature of knuckle
walking differs between African great apes (10). Other proposed locomotor modes
pre-adaptive to bipedalism include arboreal quadrupedalism (11), terrestrial
quadrupedalism (12,13), climbing (14) and a hylobatian model (15) which suggests a
small bodied, arboreally bipedal ancestor of terrestrial bipeds.
3
Central to these debates is whether bipedalism arose in the trees and was taken to
the ground, or whether it arose from an ancestor that was already terrestrial. The
orangutan data presented by Thorpe and colleagues strongly suggest the former, and
could also explain how hominin bipedality arose without needing to go through the
stage of inefficient ‘bent hip bent knee’ bipedalism typical of modern chimpanzees.
Crucially, the orangutan model also illustrates the way in which large-bodied
primates could evolve straight-limbed bipedalism in arboreal contexts.
A number of fossils contemporary with the likely split of the chimpanzee/bonono –
human clades between 4-8 Ma have been claimed to show anatomical evidence of
upright posture and bipedal walking. These include Sahelanthropus tchadensis, Orrorin
tugenensis and two species of Ardipithecus. While there is no general agreement on
the locomotor and taxonomic affinities of these fossils (16), one possibility may well
be that they are evidence of different ways of shifting from the ancestral type of
hand-assisted arboreal bipedality proposed by Thorpe and colleagues. In later
hominins, there is also evidence for locomotor diversity, within and between
lineages. Limb proportions, for example, differ in Australopithecus afarensis and Au.
africanus (17), and there is a range of foot morphologies in hominins from around the
same time period (18). Thus, bipedal walking might have evolved independently in
various early hominins. This could have occurred if multiple lineages originated from
an earlier arboreal ancestor that used hand-assisted bipedalism. If that was the case,
can anatomical evidence for bipedalism really be used as a crucial defining feature of
hominins?
With the orangutan model, Thorpe and colleagues present a plausible and elegant
argument in favour of the emergence of bipedalism in an arboreal rather than
terrestrial context. In doing so, they have reinvigorated the debate over the
emergence of behaviours preadaptive to bipedalism, and have shifted the focus back
into the Miocene. A prediction of their model is that diversity of locomotor
behaviours, including bipedalism and knuckle walking, could have arisen among
decendents of an arboreally bipedal large ape. We must now question whether
morphologies that indicate bipedalism can be used to identify hominins at the base of
4
their radiation. This then raises the issue of whether we can unequivocally identify
any traits that are truly diagnostic of early hominins (19).
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