anyway, here is some info I found, havn't verified it however

Evolution of the Chin

Copyright ã 1997 by James Michael Howard.

 
An explanation of the evolution of the human chin has occurred to me,
directly derived from my theory of human evolution. It accounts for the
development of the chin in Homo sapiens, and the lack of a chin in
early hominids.
Basically, my theory of human evolution suggests that increases in
testosterone in hominids initiated human evolution. (Male and female
humans produce more testosterone than male and female chimpanzees,
respectively.) Testosterone along with another, less potent hormone, is
also directly involved in human evolution. That hormone is
dehydroepiandrosterone (DHEA). The explanation for the chin is an
interaction of the effects of testosterone and DHEA on growth and
development of the mandible and teeth.
It is my theory that teeth are exquisitely sensitive to levels of DHEA.
I suggest that the two dentitions of humans result from two peaks of
DHEA during our life spans. There is a large peak of DHEA at birth,
which rapidly declines to reach very low levels around three to four
years (this will vary). DHEA, around four to five, then begins to
increase to reach another peak around eighteen to twenty. (The
beginning of the increase of DHEA of late childhood is called
"adrenarche.") DHEA then begins a steady decline, reaching very low
levels in old age. I attribute the first dentition to activation, by
DHEA, of the genes controlling tooth development when the peak is high
from birth to young childhood. As this level declines, the small teeth,
which developed rapidly due to this first peak, begin to degenerate
(deciduous teeth). As DHEA increases at adrenarche, teeth begin to
develop again. This period of DHEA lasts the remainder of our lives,
until it declines. Hence, these teeth become "permanent." As DHEA
declines in old age, the teeth, being sensitive to DHEA levels, begin
to degenerate again.
My work suggest that all tissues require DHEA for optimal transcription
of genes. Also, all tissues compete for the available DHEA. I think the
brain is able to absorb more DHEA than any other tissue. Hence,
increased DHEA results in increased transcription in the brain,
compared to other tissues. Brain is characterized by high metabolic
activity and exhibits two to three times the transcriptional activity
of other tissues. (J. Neurochem. 1991; 56: 812). This means that the
"dip" in available DHEA between the peak from birth to early childhood
and adrenarche is due to use of DHEA by the developing brain. As the
brain finishes its development, the available (measurable) levels of
DHEA increase. This is the increase in DHEA from adrenarche until about
the teens.
I am specifically saying that brain development causes the loss of the
deciduous teeth. That is, when brain development increases, the brain
absorbs available DHEA at the expense of the early teeth. As brain
development begins to use less and less DHEA, the teeth begin to
develop again, to produce permanent teeth.
Monkeys produce much more DHEA than chimpanzees, with humans producing
just a little less than chimps. This follows the relationship of the
brain to the levels of available DHEA. As the brain increases in size,
measurable levels of DHEA decline. The brain is using the DHEA. This
also follows the size of the denditions. Lots of DHEA means large
teeth. My point in all of this, if you are still with me, is that a
large brain means small teeth. (I am aware of Neanderthal's large
brain, large teeth, and no chin. I will explain this, below.)
Bones are sensitive to testosterone. Increased testosterone means
stronger bones. Black women produce more testosterone than white women,
and black women have fewer problems with osteoporosis than white women.
Also, men have fewer problems with osteoporosis than women. (I have
posted about this in the past.) Now, as humans evolved to produce
increased testosterone, our bones and muscles became stronger and
bigger. It is part of my theory that the increased testosterone
triggered changes in the body before the brain increased. This includes
upright, bipedal locomotion and increases in the mandible, also. So,
early hominids exhibited increased mandible size and teeth size, as the
brain was not increasing much at this time, there, not using much DHEA.
As we evolved larger brains, the ratio of DHEA to testosterone
decreased. That is, measurable levels of DHEA declined. The end result
is an animal that produces less DHEA for growth of teeth and more
testosterone for growth of the mandible. During the "plastic" change in
hominids caused by this change in the ratio of the two hormones, the
teeth and their supporting bone recede while the mandible increases.
Hence, the chin develops when the brain reaches very large proportions.
My work suggests that Neanderthal developed in an area which would
produce animals of high DHEA and high testosterone. If this were the
case, very high levels of DHEA could produce a very large brain, along
with very large teeth. It is part of my theory of human evolution that
migration away from the equator results in increased DHEA. That is,
DHEA stimulates metabolic energy. The probability of existence in a
cold environment would increase as a result of increased DHEA. It is
known that Neanderthal had a very large brain, very large teeth, and
small chin. Even though I think Neanderthal was a high testosterone
type, the ratio of DHEA must have been very high.
So, large brains produce small teeth. Large brains, small teeth, and
high testosterone produce a chin.