The Effects of Brain Growth on the Long Bones of Hominids

The Effects of Brain Growth on the Long Bones of Hominids. A Possible Response to "Born to Run." (Endurance running and the evolution of Homo; Nature 432, 345 - 352 (18 November 2004); doi:10.1038/nature03052)

Long bone growth may be determined by the speed of growth. If the brain is competing for growth with the long bones, then Homo sapiens will have longer bones because of the longer time to puberty. This may give the "appearance" of being born to run because all aspects of our growth are affected by the same competition. That is, we have long bones and associated structures because of our brains, not because of environmental effects. The bodies, so-equipped, are better at endurance running.

For example, the long bones of "Down's syndrome" are shorter than normal (Am J Obstet Gynecol. 1989 Nov;161(5):1174-7). If the brain is not competing efficiently, then long bone growth is shortened by the more rapid growth of these bones. The bones grow faster and finish development faster. (I understand there are variations to this hypothesis. If the brain and long bones are competing for the same thing and that "thing" is reduced then both will not develop well, etc.)

I suggest the long time necessary for our brain growth and development may produce the bodies that have been interpreted as "born to run." Once that growth and development of our brains nears finalization, then the competition shifts towards that of the body, re: long bone growth and development, and the bones suddenly develop rapidly and also finish growth, after the brain. That is, once the brain reduces competition, the long bone growth accelerates and quickly finishes.

The following findings may support my hypothesis; the citation containing these findings follows just below. Again, basically I submit that brain growth and development competes with limb growth and development (length). During brain growth, limb length may be extended because of longer period of growth. That is, the limb grows long and thin rather than short and thick. This effect should affect limb growth according to brain growth until puberty when the brain finalizes growth and development. In finding number one, this may be seen, that is, taller children indicates longer brain growth until puberty and shorter indicates a shorter time for brain growth until puberty. The ratios are extended in the taller children.

1. “It was found that the ratios of leg length to height, leg length to arm span, and arm span to height were bigger in taller children in the same age group than the shorter ones in both sexes.”

In number two, the older the child, the longer the period of brain growth. Hence, the ratios are bigger in older children.

2. “All the ratios were bigger in older children in the same percentile than the younger ones in both sexes, showing that the growth rates of leg length and arm span were bigger than that of height in general.”

In number three, I suggest shorter children reach puberty earlier so leg length is shorter until puberty. Taller children, reaching puberty later exhibit longer length because of extended brain growth.

3. “However, growth of leg length is faster in shorter children than in taller children until the onset of puberty, after which growth of leg length in taller children is faster than in shorter children.”

In number four, the rapid growth of leg length from birth to 2 years represents the extended growth in length because this is a period of rapid growth and development of the brain. The growth spurt during the pubertal period represents the switch over in competition towards that of limb growth as the brain finalizes growth. This generates a rapid growth and finishes the bone growth.

4. “The first and most rapid growth of leg length is seen from birth to 2 years, the second growth spurt is seen during the pubertal period.”

The taller children exhibit the rapid leg growth as puberty occurs in number five.

5. “An exceptional increment in leg length between ages from 10 to 15 is also noted in taller children.”

Number six provides information that leg and arm length are also competitive. In the tallest children, the reduced impact of puberty allows a slightly increased time of the rapid growth in the arm as the leg has already grown at this period, then increasing the competition for growth by the arm. Short children, because of the increased impact of puberty have a shortened period of rapid growth which also affects and reduces arm growth.

6. “After puberty, arm span grows faster than height until 17 years of age in the tallest male child, and taller children have longer arm span than height, while arm span in the shortest children never exceeds height.”

I have been told that arm length is longer in some apes. Brain growth and development in apes is reduced compared to humans so the period of increased competition for growth by some of the apes will be extended, therefore, increasing arm length to leg length. That is, ape brain growth competes with the hind limbs in some apes and, when finishing, allows increased arm growth until the growth and development periods are complete.

Ann Hum Biol. 1995 Sep-Oct;22(5):443-58.

Correlations among height, leg length and arm span in growing Korean children.

Yun DJ, Yun DK, Chang YY, Lim SW, Lee MK, Kim SY.

Yonsei University College of Medicine, Seoul, Korea.

A cross-sectional study was performed to find out if any specific correlations exist among height, leg length and arm span in growing Korean children. Height, leg length and arm span were measured in 10,322 healthy children (4740 males and 5582 females). Computed ratios of leg length to height, leg length to arm span and arm span to height for the 3rd, 25th, 50th, 75th and 97th percentiles were made. It was found that the ratios of leg length to height, leg length to arm span, and arm span to height were bigger in taller children in the same age group than the shorter ones in both sexes. All the ratios were bigger in older children in the same percentile than the younger ones in both sexes, showing that the growth rates of leg length and arm span were bigger than that of height in general. However, growth of leg length is faster in shorter children than in taller children until the onset of puberty, after which growth of leg length in taller children is faster than in shorter children. The first and most rapid growth of leg length is seen from birth to 2 years, the second growth spurt is seen during the pubertal period. An exceptional increment in leg length between ages from 10 to 15 is also noted in taller children. After puberty, arm span grows faster than height until 17 years of age in the tallest male child, and taller children have longer arm span than height, while arm span in the shortest children never exceeds height.