Biology Reference
In-Depth Information
fossilized, as happened with Taung. One can also make an artificial
endocast by coating the interior of a braincase with a thin layer of liquid
latex, then curing it until the liquid dries. In either case, the result is a
cast that reproduces the general shape of the brain as well as any details
of the brain's exterior (the cerebral cortex) that were stamped on the
insides of the braincase during the animal's life.
That brains leave impressions within the skull may seem odd, but
they do to a greater or lesser degree, depending on a specimen's age,
species, and extent of preservation. Although the skeletons we see in
laboratories are dry, brittle, and inert, those of living animals build up
deposits of bone in response to specific pressures and strains. (Living
bone also becomes thinner if stresses are removed.) This is why the bro-
ken bones of an arm, for example, must be properly aligned and secured,
so that the limb can mend in a relatively straight rather than a crooked
manner. It is also why visible ridges of bone appear on the outsides of
skulls where the attachments of chewing muscles exert pressures.
During life, the interior of the braincase responds dynamically to
pressure from the brain, which results in alterations of its bony walls that
conform (more or less) to the brain's shape. Thus, with luck, an endocast
can reveal information about brain size, blood vessels and sinuses, cra-
nial nerves, grooves and bumps of the cerebral cortex (the all-important
sulci and gyri, respectively), and even the sutures where the bones of
the skull are knitted together. What an endocast cannot reveal, however,
are details about the structures and connections between the cells (neu-
rons) of the cerebral cortex or information about the important regions
beneath the brain's surface. What one gets from an endocast is simply a
reproduction of some of the details of the brain's surface.
This is not so bad, however, because the outside of the brain is the
most important part for studying the evolution of human intelligence
and cognition, including aspects of awareness, knowing, thinking, judg-
ing, and learning. Remarkably, although to the untrained eye the con-
volutions of the cerebral cortex may appear like a mishmash of squiggly
bumps, those swellings of gray matter are where we do our conscious
