Ramidus Returns

Ardipithecus ramidus (Ardi) is back. In 2009, the skeleton discovered by Tim White’s team in the mid-90s was published in full. Dated to approximately 4.4 million years old based on volcanic stratigraphy, ramidus was found in the Middle Awash river valley in Ethiopia. The most complete individual, over 40% of a female skeleton, had the brain approximately the size of a chimp and exhibited a similar level of facial prognathism. Hand and foot morphology pointed to an arboreal creature, most notably a divergent big toe that would make many primates proud.

However, ramidus exhibits a bunch of other traits which seemed derived in the direction of Homo, which is what the authors of the study are pushing. These traits include reduced canines, and aspects of the foot and pelvis which could indicate a level of at least facultative bipedalism.  Critics argue that it is possible that these traits were present in many apes of the time period around the human-chimp split, and that chimps rather than humans moved away from these adaptations.

Ardipithecus ramidus from White et al. 2009
Ardipithecus ramidus from White et al. 2009

The new study published in PNAS furthers the original argument made by the discoverers that Ardi is in fact a hominin.  In particular, the reconstructed basicranium was observed in the study, led by William Kimbel of Arizona State University. Kimbel and his team demonstrate that like Homo and Australopithecus, Ardi had a short basicranium, and a relatively anteriorly placed foramen magnum. The placement of the foramen magnum, in particular, is particularly important in determining whether or not a species was bipedal. Along with the broadening of the cranial base, came notable modifications/shifting of tympanic elements and other foramena.  In short, the authors put forward the argument that because of the age of ramidus, it is possible to say that these types of changes in the cranial base were some of the earliest derivations towards Homo.

Paleoanthropologists are in a tough place. Inferring descent through morphology alone is tough, but it is all fossils really give us right now. This was just demonstrated in an even more recent case where H. heidelbergensis was demonstrated to be more closely related to Denisovans on its matriline than Neandertals, with whom they shared many morphological affinities. Going back millions of years doesn’t make it any easier, and we’re not going to be recovering 4.4 million year old DNA any time soon to help give answers. Till then here’s to the many personalities of paleoanthropology, who direct our understanding of human evolution with biology, anthropology, and sometimes a sprinkling of ego.

Matthew Magnani

5 thoughts on “Ramidus Returns

  1. “Ardi is in fact a hominin”… This is deplorable pre-Darwinian anthropocentrism typical of many paleo-anthropologists: they all want “their” fossils to be “human ancestors”. But what the authors call humanlike (non-honing canines, short basi-cranium, “characters related to facultative bipedality”) are simply primitive hominoid traits found in many Mio-Pliocene apes including australopiths, see my guest post in Greg
    Laden’s blog last year (google: Laden Verhaegen).
    Most perhaps all early hominoids (e.g. clearly in Moroto- & Oreopithecus) had vertical spines (orthogrady), apparently not for humanlike bipedality, but for hanging vertically from branches as still seen in gibbons, or for climbing vertically in the branches above the swamp or wading on 2 legs like lowland gorillas in forest swamps (google: aquarboreal). This early-hominoid orthogrady was not only preadaptive to gibbon brachiation, orang suspensory locomotion & chimp & gorilla knuckle-walking, but also to human bipedalism (google: econiche Homo).

  2. Ardipithecus was clearly hominin and probably the last common ancestor of Homo and Australopithecus. Austrlopithecus is far to derived in it’s dentition for it to be ancestral to HOMO, IMO.

    Marcel

  3. Bipedalism appeared early”? What is “early”? How do we define “bipedalism”? Gibbons & indris are often bipedal.
    Orthogrady (vertical spine) appeared very early (e.g. in Morotopithecus c 20 Ma & Oreopithecus c 8 Ma), but was not necessarily for bipedalism (e.g. wading), e.g. gibbons are orthograde (hanging & swinging from branches), gorillas often wade vertically on 2 legs & float vertically in forest bais, feeding on floating vegetation, all apes climb vertically (arms overhead) in the branches & hang from branches, and australopiths were found in wetlands, had curved hand phalanges for below-branch locomotion, and might have waded bipedally in forest swamps, not unlike lowland gorillas today.
    The idea that H.erectus were endurance runners on open plains, however, is just-so thinking, which is easily disproved (google “econiche Homo” table 4). Instead, archaic Homo apparently followed coasts & rivers, they had numerous convergences with littoral mammals (cranial & postcranial pachyostosis, platymeria, platycephaly, platypelloidy, plantigrady, protruding nasals, brain explansion, stone tool use, intercontinental diaspora, island colonisation etc.), so they were obviously parttime divers (slow & shallow diving for sessile foods), but how often they waded bipedally is uncertain (e.g. seasonally following anadromous fish such as salmon?).
    In early H.sapiens, the reduced diving & the bipedal wading adaptations are obvious (see discussion in my recent paper Hum.Evol.28:237-266, 2013): longer & more vertical processus spinosi mid-thoracally, less valgus knees, longer legs (esp. tibias), loss of cranial pachyostosis, basi-cranial flexion etc. Apparently they collected a lot of food in very shallow water & at the waterside, but fully modern bipedalism (including running as in a few E.African populations) is very recent (google “Laden Verhaegen”).

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