Monday, December 9, 2013

Apes in the trees

Reconstructed Paranthropus boisei skull at the Australian
Museum.  Credit: Stuart Humphreys.  Image source.
Continuing our apparent recent theme of paleoanthropology, a report was published last week on the open-access journal PLOS One detailing a partial Paranthropus boisei skeleton found in Tanzania.  Though this is not the first specimen of the species to be found, the authors of the study claim that this is the first skeleton found associated with teeth that can be definitively assigned to P. boisei.  Upon examining the remains, the authors found that this individual was extremely robust and strong, more so than previously thought, particularly in the upper arms.  They concluded that it was well-built for at least partial life in the trees.

Paranthropus is considered to be closely related to Australopithecus (famously represented by the skeleton named "Lucy"), though some paleoanthropologists maintain that they are the same genus.  Paranthropus (particularly P. boisei) is more heavily built than Australopithecus, leading the original discoverers to propose that they feasted on hard nuts, giving the original P. boisei specimen the nickname "Nutracker Man."  Subsequent study of the teeth, however, revealed that they probably fed on grass.  Australopithecines (consisting of Australopithecus, Paranthropus, and their immediate relatives) are actually not considered to be ancestors of humans but are seen as an extinct offshoot of the evolutionary line from apes to humans.  Nevertheless, the group is plagued by controversy and assumptions.

One key development that distinguished human ancestors from other apes, evolutionists say, was the ability to walk upright (bipedalism).  Bipedalism freed the hands to carry tools, leading to the development of technology and to evolutionary selection of greater intelligence.  It is critical, then, for evolutionists to demonstrate that hominins ("apes" that split from chimpanzees to give rise to humans) showed signs of evolving away from tree-dwelling (arborealism) toward full bipedalism.  It is understandable, then, that scientists are a bit surprised that this new Paranthropus specimen seems so suited to life among the branches.  Curiously, the authors say that it only had "occasional bouts of arboreality" while spending the rest of the time walking on two legs on the ground.  Their analysis of the anatomy did not discuss any form of walking, and according to their own claims, hip and leg bones of other P. boisei specimens are either unknown or of indefinite classification.  Another Paranthropus species, P. robustus, supposedly shows signs of bipedalism, so the authors likely and reasonably assumed that P. robustus and P. boisei had similar locomotive methods.  What, then, is the evidence for P. robustus walking upright?

A survey of human evolutionary history points to several anatomical features that appear indicative of a bipedal P. robustus:
  1. A "strikingly humanlike" hallucal metatarsal.  This is the foot bone that your big toe attaches to.  In humans, this bone is quite thick and specially designed to allow you to spring off of your big toe while walking.  Tree dwellers, on the other hand, have a thinner, curved first metatarsal that lets them more easily grasp branches with their feet.  It appears that only one hallucal metatarsal has been found attributed to Paranthropus, and indeed it does appear humanlike.  On the other hand, there is little to indicate that it is not actually human.  The bone was found in Swartkrans, a site in South Africa that has yielded a variety of archaeological and anthropological remains, including tools made of bone and stone.  Most of the remains are attributed to Paranthropus, but some are from Homo erectus (human).  Statistically speaking, the metatarsal most likely belongs to Paranthropus, assuming accurate identification of the other bones, but this does not rule out a human origin.  Despite the noted similarity to modern humans, it is never compared to the first metatarsal of H. erectus, so its exact identification is not well established, in my opinion.
  2. Hip and pelvic structure similar to Australopithecus.  This argument, of course, only holds weight if Australopithecus was bipedal.  Even then, the hypothesized bipedal gait of australopiths more closely resembles a toddler learning to walk than a normal human gait.  The question comes down to the angle of the hips.  This can get quite deep and technical, so I will refer you to this short video from an old episode of NOVA demonstrating how much interpretation gets in the way of studying the pelvis.

    The anatomist in the video, Dr. Owen Lovejoy, is somewhat well-known for his work on Lucy, the famous Australopithecus specimen.  He thought the angle of Lucy's hip was too chimpanzee-like for Lucy to walk upright with any regularity, so he proposed that the bone was broken at some point before its discovery and needed to be reshaped in order to accurately reflect the true structure in life.  When he finished sculpting a replica, he attempted to fit the pieces back together "like a jigsaw puzzle," leaving a gaping hole in the middle of the (cast) bone.
  3. Hands not suited to arboreal locomotion.  In light of today's article, I question the validity of this claim.  Also, the hand bones that are referenced in support of this claim were found at the same site as the previously-mentioned metatarsal, leaving open the option that they were actually human bones.  In fact, a radius (forearm bone) found in the P. boisei specimen more closely resembles that of modern African apes than of H. erectus fossils from Swartkrans, according to the authors.
All told, the idea that these extinct hominids were bipedal is based on the belief that they have evolutionary connection with humans, rather than on any direct fossil evidence.  Even so, studies like this one continue to push these creatures higher into the trees, widening the gap between humans and apes.  I have no doubt that much more work will be done in an attempt to show that they evolved the capability to walk as we do, so I will try to report it as it comes.

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