Ignoring Ardipithecus in an origins scenario for bipedality is ... lame.

White, Tim D. ; Lovejoy, C. Owen ; Suwa, Gen 等

Living primates have obvious importance for understanding Diodiversity and organismal biology. However, as Thorpe, McClymont and Crompton correctly appreciate (as did Charles Darwin in 1871), "[t]his does not indicate that extinct species should bear any striking similarity to extant taxa" (Thorpe et al. above).

Living orangutans are separated from living people by more than 30 Myr (millions of years) of cumulative biological evolution since we separated from our common ancestor with them. Today each genus is highly specialised in its own anatomical, behavioural and physiological ways.

Even without a fossil record, Darwin (1871: 213) added, "[u]nless we wilfully close our eyes we may, with our present knowledge, approximately recognise our parentage ...". Ours was an ape ancestry, but there was no way for Darwin and his contemporaries to sketch out the specifics of the common ancestors we hominids once shared with living great apes (here, Hominidae encompasses all species on the human side of our phylogenetic split with the chimpanzee lineage).

Accordingly, as with others of his era, Darwin hoped that fossil evidence would help to better reveal human and ape ancestry. It has. However, even with today's emergent Middle and Late Miocene fossil records, the still-elusive common ancestors of great apes and humans have yet to be recovered.

Ironically, Thorpe, McClymont and Crompton's 'short debate piece' actually shortchanges the most powerful evidence available to date, which is the 4.4 Myr hominid Ardipithecus ramidus and its slightly earlier African relatives. This evidence comes from the depths of the Pliocene, closer than ever to the base of the hominid clade. It bolsters their conclusion that knuckle-walking was not an ancestral hominid locomotor mode.

Ardipithecus is situated temporally and cladistically between ancestral apes and later hominids (Australopithecus and early Homo). Ardipithecus primitively retained a widely divergent big toe, along with structures of the lower pelvis and thigh that enabled competent arboreal climbing and clambering. At the same time, this hominid also shared with Australopithecus key evolved features of the upper pelvis and lateral foot that allowed it to engage in terrestrial bipedality with extended hips and knees (White et al. 2009, forthcoming).

Moreover, in limb proportions and in a suite of functionally relevant hand and foot structures, Ar. ramidus shows stronger similarities to Miocene fossil apes that were considerably less specialised in their locomotor skeletons than are extant apes. In particular, Ardipithecus lacks anatomical specialisations related to below-branch suspension (one- or two-armed hanging) and/or knuckle-walking of living great apes.

Rather, when arboreal, Ar. ramidus was probably a relatively slow climber and clamberer compared to more acrobatic extant chimpanzees. This would have involved reliance on both 'pronograde' and 'orthograde' trunk alignments when reaching, bridging and clambering among tree branches. These postures and motions were made possible by its laterally positioned shoulder and other anatomies that enhanced forelimb mobility. Limited forelimb suspension and arboreal bipedality were probably also within its positional repertoire.

The actual emergence(s) of such a versatile arboreal body plan in the Miocene ape ancestors of Ardipithecus, a 'bauplan' different and more advanced than that of quadrupedal 'pronogrady' seen in monkeys and Proconsul (but not yet suspensory-specialised), is obscured by a dearth of sufficiently informative Middle and Late Miocene fossil great apes. However, the recently expanded fossil record increasingly suggests several forms of such parallel derivations across Asia, Europe and Africa. These Miocene apes all lacked the suite of enhanced specialisations for suspensory locomotion exhibited by the more specialised extant great apes (although the insular Oreopithecus of the latest Miocene of southern Europe perhaps approximated it).

Thorpe, McClymont and Crompton's assertion that 'orthograde' (and by their inference), suspensory-inclined apes (their Figure 2) were widely represented in the Miocene after c. 20 Myr is therefore not supported. Indeed, reviewing the plethora of currently known Miocene apes, we (and others, e.g. Nakatsukasa & Kunimatsu 2009; Alba 2012; Almecija et al. 2013) are struck by the conspicuous lack of evidence for suspensory specialisations that characterise all living great apes.

As summarised above, a more generalised ancestral ape is independently suggested by the Pliocene descendant, Ar. ramidus, a primate that differs dramatically from living orangutans. Perhaps this is why Thorpe, McClymont and Crompton do not mention it.

Solicitation of a uniquely specialised suspensory extant ape such as the living orangutan as either a proxy or model for the far more generalised climbing/clambering Miocene apes from which terrestrially bipedal hominids must have emerged represents a dubious undertaking in light of the currently available Pliocene and Miocene fossil records.

It is also evident that observing the locomotor behaviors of extant relict survivors of the richly divergent Miocene ape radiation will never be as revealing as finding the still-missing remains of the actual last common ancestors we once shared with them during the Miocene. As Darwin also appreciated, such revelations can only come from palaeontology. That is why, even more than a century later, living primates are informative, but more fossils are still urgently needed to clarify the origins and subsequent evolution of Hominidae.

Reference

ALBA, D.M. 2012 Fossil apes from the Valles-Penedes basin. Evolutionary Anthropology 21: 254-69. http://dx.doi.org/10.1002/evan.21312

Almecija, S., M. Tallman, D.M. Alba, M. Pina, S. MOYA-SOLA & W.L. JUNGERS. 2013. The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins. Nature Communications 4: 2888. http://dx.doi.org/ 10.1038/ncomms3888

DARWIN, C.R. 1871. The descent of man, and selection in relation to sex. Volume 1. London: John Murray.

NAKATSUKASA, M. & Y. KUNIMATSU. 2009. Nacholapithecus and its importance for understanding hominoid evolution. Evolutionary Anthropology 18:103-19. http://dx.doi.org/ 10.1002/evan.20208

WHITE, T.D., B. ASFAW, Y. BEYENE, Y. HAILE-SELASSIE, C.O. LOVEJOY, G. SUWA & G. WOLDEGABRIEL. 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science 326: 64, 75-86. http://dx.doi.org/10.1126/science.1175802

WHITE, T.D., C.O. LOVEJOY, B. ASFAW, J. CARLSON & G. SUWA. Forthcoming. Neither chimpanzee nor human, Ardipithecus reveals the surprising ancestry of both. Proceedings of the National Academy of Sciences of the USA.

Tim D. White (1), C. Owen Lovejoy (2) & Gen Suwa (3)

(1) Human Evolution Research Center and Department of Integrative Biology, HERC/Museum of Vertebrate Zoology, 3101 Valley Life Sciences Building, University of California, Berkeley, CA 94720, USA

(2) Department of Anthropology, School of Biomedical Sciences, Kent State University, Kent, OH 44240-0001, USA

(3) The University Museum, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan