Researchers are often faced with partial remains in the fossil
record, and must create from these an accurate image of what early
human species looked like. However, the principle of bilateral
symmetry often can make fragmentary remains much more informative
than they would at first seem. Humans are roughly symmetrical
across a plane that divides the body into left and right halves.
For example, the right and left arms on an individual are almost
identical in size and shape. Therefore, bones from the left side
of the body can often be used to reconstruct their missing counterparts
on the right side by creating mirror images of the bone. This
principle is often used in reconstructing the anatomy of early
humans from their fossil remains (see also: the Weidenreich
Reconstruction).
In the case of the Australopithecus afarensis reconstruction,
look at the three quarters view of A. afarensis (top).
We can see a large portion of the left parietal bone preserved
(this is the bone forming the side of the skull). However the
right parietal is missing. By using bilateral symmetry, we can
create an accurate reconstruction of the shape of both parietal
bones by assuming that the skull of A. afarensis, like
all other primates, is symmetrical. Additionally, we can use this
principle to reconstruct the mandible (bottom). Draw an imaginary
line down the middle of the jaw. Even though only the incisors
and the third (last) molar are preserved on the right side, you
can see that it is possible reconstruct almost the entire right
side of the lower jaw by using mirror images of the left.
