Increase in human brain size a key to increase in body size

Lucy, considered to be the ancestor to all humanity was a very short creature about three and a half feet tall, weighing some 60 to 65 pounds and lived around 3.2 million years ago in Ethiopia. Perhaps the growth period among the australopithecines was much shorter than that of the modern day humans and hence simply by this yardstick, there has to be a lot of difference in body size between them. The longer the growth period the larger the body size and this is what seemed to happen to the humans during evolutionary history.

Recently Mark Grabowski, a researcher at American Museum of Natural History, New York, observed in his research paper that “Bigger brains led to bigger bodies… as over the last four million years, brain size and body size increased substantially in our human ancestors” (Current Anthropology, Vol. 57, 174-196, April 2016). These observations were not new and were clearly understood by the scientific community earlier also. However, numerous hypotheses put forth had emphasized the role of natural selection on different traits independently. But none of them had gone in the direction of a correlated response to natural selection in favour of enlarging the brain size and the body size together. These viewpoints had concluded that increase in brain size and body size were the products of separate natural selection forces. However, Mark Grabowski states that “some genes cause variation in both brain and body size, with the result that selection on either trait can lead to a correlated response in the unselected trait.” This is a new explanation to the problem. It highlights the role of correlated outcomes of the natural selection phenomena occurring to one trait but affecting the other trait even if that is not selected for. It is similar to saying that as the brain size increased from Lucy to Homo erectus so did the body size as if the animal pulled itself up and increased in size proportionately as well to keep pace with the increase in brain size. Brain use has been maximizing in the human ancestors and the brain size was rapidly increasing. Strong selection pressures for increasing the brain size has been happening which may be the primary cause for an increase in the body size as well so as to accommodate a larger brain within a larger body.