At this point it will be useful to consider man's arboreal
apprenticeship and how he became a terrestrial journeyman. Professor
Wood Jones has worked out very convincingly the thesis that man had no
direct four-footed ancestry, but that the Primate stock to which he
belongs was from its first divergence arboreal. He maintains that the
leading peculiarities of the immediate precursors of man were wrought
out during a long arboreal apprenticeship. The first great gain of
arboreal life on bipedal erect lines (not after the quadrupedal fashion
of tree-sloths, for instance) was the emancipation of the hand. The
foot became the supporting and branch-gripping member, and the hand was
set free to reach upward, to hang on by, to seize the fruit, to lift it
and hold it to the mouth, and to hug the young one close to the breast.
The hand thus set free has remained plastic--a generalised, not a
specialised member. Much has followed from man's "handiness."
The arboreal life had many other consequences. It led to an increased
freedom of movement of the thigh on the hip joint, to muscular
arrangements for balancing the body on the leg, to making the backbone a
supple yet stable curved pillar, to a strongly developed collar-bone
which is only found well-formed when the fore-limb is used for more than
support, and to a power of "opposing" the thumb and the big toe to the
other digits of the hand and foot--an obvious advantage for
branch-gripping. But the evolution of a free hand made it possible to
dispense with protrusive lips and gripping teeth. Thus began the
recession of the snout region, the associated enlargement of the
brain-box, and the bringing of the eyes to the front. The overcrowding
of the teeth that followed the shortening of the snout was one of the
taxes on progress of which modern man is often reminded in his dental
troubles.
Another acquisition associated with arboreal life was a greatly
increased power of turning the head from side to side--a mobility very
important in locating sounds and in exploring with the eyes.
Furthermore, there came about a flattening of the chest and of the back,
and the movements of the midriff (or diaphragm) came to count for more
in respiration than the movements of the ribs. The sense of touch came
to be of more importance and the sense of smell of less; the part of the
brain receiving tidings from hand and eye and ear came to predominate
over the part for receiving olfactory messages. Finally, the need for
carrying the infant about among the branches must surely have implied an
intensification of family relations, and favoured the evolution of
gentleness.
[Illustration: _Photo: New York Zoological Park._
THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS SKULL AND
DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE ADAPTATION OF ITS LIMBS
TO ARBOREAL LIFE]
[Illustration: _Photo: New York Zoological Park._