evolution of the opposable thumb

[Lisa Rogers <eqwq.lrogers@xxxxxxxxxxx>]

Since somebody asked about thumbs, in relation to labor, tools,
etc....

Opposable thumbs:

Partial source:  Paleoanthropology 1980, by G.E.Kennedy, McGraw-Hill

This book I do not recommend for it's discussion of evolutionary
theory, mechanisms and such, but the documentation of fossil and
living, primate [including hominid] functional anatomy appears to be
excellent.  Unfortunately, it is too old to have the most famous
australopithecine fossil Lucy in it.  Interestingly, it does show
that there was lots of evidence for australopithecine bipedality, in
fossils of lumbar spines and feet, for instance, even before Lucy was
found with her lovely pelvis, hips and thigh bones intact.  But I
must not digress - thumbs only for this post.

Primates first appeared in the Cretaceous period, over 70 million
years ago [mya].  The dominant hypothesis about their form/function
is that they were arboreal, as indeed most modern primates still are.
This is thought to be related to the grasping hands that all primates
share, for use in grasping branches as well as reaching out for and
holding food, as opposed to bringing one's snout to the food.
[Tree-living, as well as predation upon insects, may also be related
to stereoscopic vision and a larger brain area for vision.]

Some non-primates have grasping hands, such as rodents, insectivorous
tree shrews, arboreal marsupials [koalas and opossums], raccoons,
etc.  They cannot stabilize an object by thumb opposability, they use
both hands to hold and manipulate food/objects.  Primates all can do
one-handed feeding.  Aside from a few new-world primates, _all_ have
some rotational function of the thumb, which allows it to face the
other digits and permits a high degree of manipulative ability.

This rotation takes place at the joint at the very base of the thumb,
where it articulates with one of the wrist bones.  [There are 8 small
bones packed together, at the base of the hand.  Feel the long bones
on the back of your hand, tracing them toward the wrist.  You can
feel where they end, as they all meet the group of wrist bones called
carpals.]  New world primates have a simple hinge joint at this
point, at the base of the thumb, so opposability is limited.

True opposability is found in all Old world monkeys, and all apes,
including humans.  The "pulp surface" of the thumb can be touched to
the pulp surface of at least one finger, yielding the "precision
grip".  Also, the fingers can be wrapped around an object and the
thumb folded over the fingers, called the "power grip".  This is made
possible by a saddle-shaped joint, or a ball and socket joint.

It is very interesting that the fully rotational thumb joint evolved
twice, separately, with these two different structures.  The gibbons
have the ball+socket joint, the rest of the anthropoids [monkeys,
apes, humans] all have the saddle-shaped joint.

Another aspect of opposability is a change in the main muscle that
flexes that joint.  It generally reaches from the thumb, between the
first and second joints, down to one of the wrist bones, but in all
Old world anthropoids, this muscle connect to more wrist bones that
are further across the hand, so that it is more effective in rotation
and opposition.  [Touch your thumb to your little finger, and feel
the bulge at the base of the thumb, it's a large part of the 'heel of
the hand' and nearly a quarter of the 'palm' of hand, and it's all
this one muscle.]

True brachiators are gibbons, they can speed through the trees with
arms alone [look Ma, no feet!] and that is their primary locomotion.
Chimps and gorillas are called modified brachiators.  Brach. is not
their primary locomotion, they knuckle walk, i.e. back of the first
two finger joints are on the ground.  Their hands and elbows are
evolved/ built to take the compressive force that this produces.

Gorillas practically never brachiate, they spend a lot of time on the
ground, and eat a lot of foliage.  Chimps are more into fruit.  Both
make temporary sleeping nests in trees at night.  Both will move on
top of branches, but only big branches, because they are heavy.
Orangutans are known, esp. the males, to often come down from one
tree in order to walk to another, because they are just too heavy to
make it from tree to tree on the smaller branches that intertwine.

All of these have grasping feet, because of the divergent big toe,
which is helpful in certain kinds of climbing and hanging around.
Chimps and esp. orangs may well hang by the feet, and in all kinds of
positions in order to reach things, but that is not for traveling,
just feeding and mating.

Thumbs are not used in either brachiation or knuckle walking.

A common feeding posture while in a tree is to stand on one branch,
partially hang from a branch above with one hand, to increase the
range of the free hand, being able to lean and partially hang.  Or to
pull a branch down with one hand to reach the fruit or whatever with
the other hand.  This is seen for other primates as well, including
humans.  Like when I was picking wild elderberries last September.

To sum up the thumb questions, I return to the point about two kinds
of opposability, and quote Kennedy [p.42-3]:

"[W]hile true thumb opposability occurs in no other animal group, it
has arisen within the Hominoidea at least twice in different ways.
Both the saddle-shaped thumb joint of the Old World monkeys, pongids
[apes], and hominids and the ball-and-socket joint of the hylobatids
[gibbons] represent different pathways to full rotation of the thumb.
 It is clear that this action, which results in a fully flexible and
highly manipulative hand, has had important selective advantage
within the higher primates.  Yet, it is equally clear that the fully
opposable thumb, while associated with tool making in the later
hominids, arose long before the emergence of any tool-using practices
in primates."

Hope this answers a few questions.
Lisa



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