Biology Reference
In-Depth Information
been linked with microcephaly. By comparing the microstructures of
the normal forms of the relevant genes in humans and other primates,
geneticists have identified at least two genes
(ASPM
and
CDK5RAP2)
that appear to have been under natural selection related to the increase
strongly associated with neonatal brain size. Because mutations in them
interfere with growth of the brain, some geneticists believe that their
normal variants were likely to have been targets of positive selection
for increased brain size during primate, including hominin, evolution.
The normal form of these and other genes that are sometimes mutated
and associated with microcephaly could also possibly have influenced
entists are right, then the pathological condition of microcephaly that
disrupts normal brain development may be thought of as providing a
kind of window or pointer into the past. Not that anyone is suggesting
that microcephalics are evolutionary throwbacks. Instead, it seems that
the disruption of certain genes in living people interferes with the nor-
mal expression of traits (such as big brains) that evolved during hominin
evolution. (Similarly, concluding that the much smaller-sized brains
of our early ancestors were anything but normal would be incorrect.)
Thus, mutations of specific genes that contribute to brain development
may result in pathological traits that appear like those that were normal
for our ancestors before they evolved the more advanced variations.
The idea that the physical manifestations associated with certain ge-
netic pathologies might provide glimpses into the distant past is intrigu-
ing. If the small brains of microcephalics point to genes that, in their non-
pathological states, were under intense natural selection during primate
brain-size evolution, might similar reasoning hold for other features of
microcephalic brains, such as their shapes? If so, would the fossil record
contain ancestors who normally had brains that were not only the size but
on brain shape in microcephalics and early hominins suggests that this
is, indeed, the case. The narrow frontal lobes with flattened under-
