Pollical distal phalanges: Comparison between Homo sapiens and Orrorin tugenensis

Abstract : The hand and more precisely the thumb have a key role in hand manipulation and precision grasping. The pollical distal phalanx (POP) of the thumb is the focus of this study.  Through  the morphology of this distinctive phalanx, a  biological identification of taxa and an understanding  of a species'  way of living are possible. The aim is not to analyse  Orrorin's tool-making capability but to discuss the POP evolutionary history and the type of locomotion of this late Miocene hominin. lndeed, manipulation and locomotion are the two main selective pressures of hand morphology [2].

The POP is composed of a set of tuberosities and fossae, reflecting the presence of tendons and

muscles. Among them, is the flexor pollicis fongus, which is responsible for the flexion  and  the stabilisation of the POP. This muscle is specific to the thumb and creates a depression on the proximo-palmar part of the phalanx [4]. The general morphology of the POP was also studied, implementing  a set of morphometric measurements, on fossil specimens or on high-resolution surface laser scans. This set was made to allow a comparison between BAR 1901'01, attributed to Orrorin and representing the earliest hominin POP recovered so far, and Homo sapiens.

Even if Orrorin does not have the entire set of extant human POP features correlated to precision grasping, BAR 1901'01 morphology has enough characteristics to conclude that this hominin displays a capacity for precision grasping quite similar to extant humans. A hypothesis is that this human hand proportion is an exaptation, co-opted in early bipedal hominins, but first appearing in arboreal Miocene apes, whose locomotion  required  a powerful grasp that was further strengthened by the involvement of the thumb [1]. Orrorin would therefore be a hominin with a particular way of arboreal locomotion, different from extant apes but capable of bipedalism (as its humerus morphology shows) [3].  Now we are at the very beginning of the comprehension of the POP evolutionary history which can progress with the

discovery of new specimens.