The Cautious Climber Hypothesis

Those of you familiar with the literature on hominid evolution will doubtless have read at least something about the evolution of hominid bipedality. In the most popular model, bipedal hominids originated from terrestrial, chimp-like quadrupeds (which were still capable of climbing but not highly specialised for it), sometime within the last 7 or so million years. However, committed adaptation to full-time bipedality did not occur until more recently, and at least some of the hominids included within the ‘australopithecine’ grade of our lineage (and it obviously is a grade, even Australopithecus itself – as currently conceived – being paraphyletic) were seemingly not far from chimps and bonobos in climbing ability.

Hominids - represented here by a gorilla, orangutan and chimpanzee (the human needed to complete the scene is missing) - are different from other anthropoid primates in many important aspects. What particular adaptational history caused them … us… to be so different? This mural is on show at Edinburgh Zoo, Scotland, and is by Russell Dempster. Image: Darren Naish.

Hominids - represented here by a gorilla, orangutan and chimpanzee (the human needed to complete the scene is missing) - are different from other anthropoid primates in many important aspects. What particular adaptational history caused them … us… to be so different? This mural is on show at Edinburgh Zoo, Scotland, and is by Russell Dempster. Image: Darren Naish.

This shift likely occurred in open habitats, and quite why bipedal adaptation evolved has been the subject of copious speculation. Maybe it was to do with being able to see further, to free up the hands for carrying things, to improve social and/or sexual communication, to assist with thermoregulation, to increase foraging reach, to improve wading abilities or… insert favoured model for origin of bipedality here.

Primate evolution: another of those subjects that gets written about  a lot . Here are some of the (mostly hominid-themed) primate books in the TetZoo collection, but far from all of them. Pet and fringe theories abound in the popular and semi-popular literature on hominid evolution. Image: Darren Naish.

Primate evolution: another of those subjects that gets written about a lot. Here are some of the (mostly hominid-themed) primate books in the TetZoo collection, but far from all of them. Pet and fringe theories abound in the popular and semi-popular literature on hominid evolution. Image: Darren Naish.

What I’ve just described might be regarded as ‘the textbook view’, however, since there are indications that things might not have happened as described. Anatomical details suggest that proficient bipedal abilities might not have originated in open, terrestrial environments, but in wooded habitats, earlier in history, and among taxa that spent more time in trees than on the ground.

There’s a lot more that could be said about that particular area, but here I want to concentrate on an even earlier phase of evolution. Namely, that part relevant to those hominoids in existence prior to the split between pongines (orangutans and their relatives) and hominines (African great apes). Such animals can be termed stem-hominids, pre-hominids or early hominoids, depending on your preference, and they’d be closer in position to hominids than are gibbons (aka hylobatids). I’m going to call them ‘pre-hominids’ since I find this to be the least ambiguous term. Which behavioural, locomotory and ecological specialisations led to the evolution of the hominid body form, and what were pre-hominids like?

The red box shows the section of the family tree we’re especially interested in here. The animals concerned are hominoids, but not part of the hominid crown (that is, they’re not part of the hominid group delimited by living hominid lineages). They’re stem-hominids, or ‘pre-hominids’. Image: Darren Naish.

The red box shows the section of the family tree we’re especially interested in here. The animals concerned are hominoids, but not part of the hominid crown (that is, they’re not part of the hominid group delimited by living hominid lineages). They’re stem-hominids, or ‘pre-hominids’. Image: Darren Naish.

Some authors have proposed that pre-hominids were gibbon-like brachiators (perhaps pre-adapted for bipedality). This is the so-called brachiation, brachionationist or hylobatian model (e.g., Morton 1926, Keith 1934, Tuttle 1981). Others have argued that pre-hominids were more terrestrial, chimp-like knuckle-walkers (e.g., Keith 1923, Washburn 1963), a model similar to the ‘trogolodytian’ one actually proposed as a phase within the hylobatian model. Neither of these models, however, appears consistent with the long list of features that hominids share with mammals that are neither brachiators nor terrestrial quadrupeds, but vertical climbers that use what’s termed an antipronograde posture (where the body’s long axis is oriented more than 45° from the horizontal). The proportionally long arms and short legs of hominids (we’re referring here to the assumed ancestral condition, not that possessed by unusual forms like humans), the short thorax, reduced lumbar region where vertebrae are incorporated into the sacrum, wide shoulders and hips, and the anatomy of the shoulders, wrists and feet all parallel features seen in lorises and kin, and in climbing South American monkeys like spider monkeys (Sarmiento 1995). In short, these features – and there’s other evidence too, relating to the way muscles function and so on – suggest that pre-hominids were perhaps specialised for vertical climbing.

Which form of locomotion was typical of those hominoids ancestral to hominids? Were they brachiators, arboreal climbers, or digitigrade or knuckle-walking terrestrial forms? Image: Richmond  et al . (2001).

Which form of locomotion was typical of those hominoids ancestral to hominids? Were they brachiators, arboreal climbers, or digitigrade or knuckle-walking terrestrial forms? Image: Richmond et al. (2001).

We can go further, since hominids don’t merely possess general features associated with vertical climbing: they have additional features specific to what are known as cautious climbers. These are those vertical climbers that rely on the grasping of (sometimes discontinuously sized) supports and do not leap. Cautious climbers among mammals include lorises, some colobine monkeys, tree sloths and the extinct palaeopropithecid lemurs (Sarmiento 1995) (though some of these taxa – sloths in particular – have specialised for suspensory climbing too). The features we’re talking about here include dorsal migration of the scapula relative to its position on the ribcage in other primates, increased breadth of the manubrium (the big, anterior-most section of the sternum), a reduced number of tracheal rings, a flat-topped diaphragm with a central tendon, a tendon that connects the protective membrane around the heart to the diaphragm (termed the pericardiophrenic tendon), and reduction or loss of the tail.

Did hominids start their history as ‘cautious climbers’, convergently similar to such arboreal mammals as lorises, sloths, various leaf-eating Old and New World monkeys, and some extinct lemurs? The pre-hominid at far right is a hypothetical animal - a ‘concestor’ - that matches this prediction. A slow loris and three-toed sloth are shown at left. Image: Darren Naish.

Did hominids start their history as ‘cautious climbers’, convergently similar to such arboreal mammals as lorises, sloths, various leaf-eating Old and New World monkeys, and some extinct lemurs? The pre-hominid at far right is a hypothetical animal - a ‘concestor’ - that matches this prediction. A slow loris and three-toed sloth are shown at left. Image: Darren Naish.

Because living cautious climbers share a set of physiological, anatomical and behavioural features, we can infer that they were likely present in cautious climbing pre-hominids too. Cautious climbers use slow, deliberate movements, often hold the foot in a hooked pose, use the foot from the toes to the heel when grasping, and are often bipedal when on the ground. Their slow, deliberate and often powerful movements are in keeping with a high proportion of slow twitch or red muscle fibres (Sarmiento 1995).

Cautious climbers also tend to be – but are not always – folivores (leaf eaters), and their reliance on leaves means that they’ve evolved large guts and have slow metabolisms. They’re therefore mostly big (c 10-40 kg), bulky animals with long gestation periods and a reliance on tropical habitats with a guaranteed supply of canopy foliage. Their slow metabolisms also predict that they’re relatively good at dealing with toxins, and it’s been argued that their slow-moving, low-energy strategy makes them inclined to evolve laryngeal specialisations and an ability to broadcast loud sounds over distance.

Cautious climbers generally don’t leap, or drop from height onto other structures when climbing. Instead, they mostly climb slowly and deliberately, with actions like these (here depicted in a potto) being used to move from one branch to another. Image:   Napier & Napier (1985)   (and based on an original by Charles-Dominique).

Cautious climbers generally don’t leap, or drop from height onto other structures when climbing. Instead, they mostly climb slowly and deliberately, with actions like these (here depicted in a potto) being used to move from one branch to another. Image: Napier & Napier (1985) (and based on an original by Charles-Dominique).

The idea that pre-hominids were cautious climbers of this sort, and that the biological and ecological correlates of this adaptive regime were present in these animals, was explored in depth by Esteban Sarmiento in a few papers from the 1990s (Sarmiento 1995, 1998). The significance of the cautious climber model is not just that it better allows us to imagine what pre-hominids might have been like; it’s also interesting in that imaging hominids as animals that went through this phase helps explains why they were evolutionary shaped in the ways that they were.

Hominid skeletons - this is that of an orangutan, photographed at Musée d'histoire Naturelle, Tournai (Belgium) - possess numerous features indicative of arboreal adaptation. Image: Michel Wal, CC BY-SA 3.0, wikipedia (original  here ).

Hominid skeletons - this is that of an orangutan, photographed at Musée d'histoire Naturelle, Tournai (Belgium) - possess numerous features indicative of arboreal adaptation. Image: Michel Wal, CC BY-SA 3.0, wikipedia (original here).

Hominids took to frugivory on later occasions within their history, for example, but a folivorous initial phase might explain why they have the teeth that they do (the incisors are relatively small, the canines are shortened, there are crushing surfaces on the premolars, enamel wrinkling is present on the molars, and so on), why the front of the face is reduced, and why the lower jaw has such a large and vertical ascending process, a big section adjacent to the molars and a broad but short condyle (Sarmiento 1995). A folivorous heritage could also help explain why hominids are relatively slow growing, resistant to many poisons toxic to other mammals, and equipped with an ability to make loud, complex calls. It would appear, however, that pre-hominids were not as specialised for folivory as are cautious climbers like sloths, since hominids never evolved a complex stomach and always maintained good terrestrial abilities.

Was Sarmiento right about cautious climbing and folivory being all that important in primates close to the ancestry of later hominid lineages? This isn’t an area that’s been all that intensively discussed but – when comments on ‘pre-hominid’ evolution have been provided – authors have tended not to state special preference for the idea, at least considering it as likely as suspensory behaviour or generalised climbing (e.g., Pilbeam & Young 2004, Begun 2016). Richmond et al. (2001), however, noted that ideas positing antipronograde climbing postures were “[a]rguably the most popular [of evolutionary models pertaining to pre-hominid lifestyle] … during the last several decades” (p. 81) and argued that the anatomy and biomechanics of extant hominids are consistent with an arboreal climbing ancestry. They didn’t specifically have cautious climbing and/or folivory in mind though.

This article isn’t about SpecBio. But if pre-hominids were vaguely sloth-like in some ways, a different trajectory of hominid evolution might have resulted in a radiation of increasingly sloth-like primates… in which case, maybe things like this could have evolved. Maybe. This is  Giganthropus , a fictional sloth-like hominid featured in   Dougal Dixon’s 1990  Man After Man   . Image: Philip Hood, in   Dixon (1990)  .

This article isn’t about SpecBio. But if pre-hominids were vaguely sloth-like in some ways, a different trajectory of hominid evolution might have resulted in a radiation of increasingly sloth-like primates… in which case, maybe things like this could have evolved. Maybe. This is Giganthropus, a fictional sloth-like hominid featured in Dougal Dixon’s 1990 Man After Man. Image: Philip Hood, in Dixon (1990).

As goes fossils, animals that pertain to the right approximate part of the cladogram do seem to provide support for the model. Morotopithecus from Early Miocene Uganda is large (20-40 kg) (Gebo et al. 1997) and has been inferred to have been a vertical climber (and perhaps a cautious climber), while Pierolapithecus from the middle Miocene of Spain has a set of features which appear inconsistent with suspensory behaviour and more in line with vertical climbing (Moyà-Solà et al. 2004) (and, again, perhaps with cautious climbing too).

All in all, I find the ‘cautious climber’ model of pre-hominid evolution intriguing and worthy of more attention, hence this article. Long-term readers might remember me mentioning this article – as an in-prep piece, waiting completion – for some years now. It is, at last, ticked off the list.

Articles like this are possible because of the support I receive at patreon. Please consider supporting my research and writing if you don’t already, thank you so much.

For previous TetZoo articles on primate diversity and evolution, see…

 Refs - -

Begun, D. R. 2016. The Real Planet of the Apes: a New Story of Human Origins. Princeton University Press, Princeton and Oxford.

Dixon, D. 1990. Man After Man: An Anthropology of the Future. Blandford, London.

Gebo, D. L., MacLatchy, L., Kityo, R., Deino, A., Kingston, J. & Pilbeam, D. 1997. A hominoid genus from the Early Miocene of Uganda. Science 276, 401-404.

Keith, A. 1923. Man’s posture: its evolution and disorders. British Medical Journal 1, 451-454, 499-502, 545-548, 587-590, 624-626, 669-672.

Keith, A. 1934. The Construction of Man’s Family Tree. Watts, London.

Morton, D. J. 1926. Evolution of man’s erect posture: preliminary report. Journal of Morphology and Physiology 43, 147-149.

Moyà-Solà, S., Köhler, M., Alba, D. M., Casanovas-Vilar, I. & Galindo, J. 2004. Pierolapithecus catalaunicus, a new Middle Miocene great ape from Spain. Science 306, 1339-1344.

Napier, J. R. & Napier, P. H. 1985. The Natural History of the Primates. British Museum (Natural History), London.

Pilbeam, D.  & Young, N. 2004. Hominoid evolution: synthesizing disparate data. C. R. Palevol 3, 305-321.

Richmond, B. G., Begun, D. R. & Strait, D. S. 2001. Origin of human bipedalism: the knuckle-walking hypothesis revisited. Yearbook of Physical Anthropology 44, 70-105.

Sarmiento, E. E. 1995. Cautious climbers and folivory: a model of hominoid differentiation. Human Evolution 10, 289-321.

Sarmiento, E. E. 1998. Generalized quadrupeds, committed bipeds, and the shift to open habitats: an evolutionary model of hominid divergence. American Museum Novitates 3250, 1-78.

Tuttle, R. H. 1981. Evolution of hominid bipedalism and prehensile capabilities. Philosophical Transactions of the Royal Society 292, 89-94.

Washburn, S. L. 1963. Behavior and human evolution. In Washburn, S. L. (ed) Classification and Human Evolution. Aldine, Chicago, pp. 190-201.