Friday, August 16, 2013

It's all in the wrist

There's a famous scene early in the movie 2001: A space odyssey  in which a tribe of human ancestors learns (from a mysterious monolith ) how to use a club. They leverage this knowledge to whack their rivals--  and the rest, as they say, is history.

As it happens, the ability to grab a club and pound someone involves a complicated wrist motion that might be unique to the human lineage. Careful studies of the wrist bones of Australopithecus afarensis  (the famous Lucy skeleton) have concluded that they likely could not execute a squeeze grip.  However, their close relative Australopithecus sedibus likely did have the wrist geometry necessary for a power grip. So somewhere in between these two, the first ancestor picked up a club and whacked the first rival.
The "squeeze" grip.
Credit:Marzke et al.,
 Am. Journal of Physical Anthropology
These differences in wrist geometry are important--   not just because of the movie, but because the same wrist adaptations provide manual dexterity needed for the manufacture of precision tools, which in turn allows the construction of weapons like spears, the bringing down of larger prey, and probably a survival advantage.  So the motion of the wrist is right up there with brain size and upright posture as a defining adaptation of the human line. 
Human wristbones. The trapeoid, saphoid and capitate (discussed below) are marked with F, A, and G respectively. Credit: Wikipedia

Because of the importance of wristbones in dictating the manual abilities of primates, I was very interested to come across studies of the wristbones of the "hobbit" fossils found on the island of Flores in the Indonesian archipelago. These finds caused a sensation when they were reported in late 2004, because they suggested that very primitive hominins lived on Flores until very, very recently. The skeleton described in the 2004 report belonged to an adult only one meter tall, and with a brain size comparable to that of a human infant. The wristbones of that first specimen were also very primitive, falling somewhere between the shapes of a chimpanzee and of a modern human.  So this was not the wrist of an anatomically modern human, and probably the hobbits could not swing a club.
Specific wrist bones belonging to a chimpanzee (left), hobbit (center) and human (right), viewed in the case of each bone from the same angle for all three species.  Blue, yellow,  and green colors show surfaces where the bones form joints with adjacent bones, and pink indicates surfaces that are not part of joints. Note especially the rotated orientation of the human trapezoid (top row), relative to the hobbit and the chimpanzee This bone is highlighted in the image below. Credit for both: Matt Tocheri, Human Origins Program of the Smithsonian Institution

Excavations at Flores have continued, and there are now additional wristbones reported, which supports the original suggestion that the remains at this dig represent a population of distinct hominins, and not a pathological or damaged modern human. However, the story is not yet over-- the investigators at Flores did find tools at the site, of the sort that probably require a power grip to manufacture. It will take some time, and probably wristbone fossils from other sites and species, to clarify what these hobbits were capable of.
Wristbone arrangements of African apes (left) and modern humans (right). Credit: Matt Tochieri.

1 comment:

Julia Mossbridge said...

I love the way you write about science...and wrists are not only the start of everything...but their continued use allows us to be flexible in motion and thought!