Evolution of the Chin
Copyrightã 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.