Why did our brains stop expanding?
In the forest the human brain was expanding and expanding at a phenomenal rate. Sometime at around 200,000 to 150,000 years ago, this process came to an end. The brain stopped expanding and started to shrink. This key point in our evolutionary journey has been noted but rarely addressed, and its significance comprehensively ignored.
Christopher Ruff, of John Hopkins University, and his colleagues thoroughly analyzed the fossil record to determine the evolving body mass and brain size of the various Homo species leading up to us. The results show that the assumption of a straight progression from a pea-brained ancestor to the ultrabrainy modern Homo sapiens is decidedly shaky. Hominid brains appear to have remained fairly constant in size for a long period from some 1.8 million years ago until about 600,000 years ago. But then, from 600,000 to 150,000 years before the present, fossils show that the cranial capacity of our ancestors skyrocketed. Brain mass peaked at about 1,440 grams (3.17 pounds). Since then brain mass has declined to the 1,300 grams (2.87 pounds) that is typical today (Ruff 1997).
Of course, brain size alone does not tell the whole story. Brain size also correlates with body size, and the peak of brain size roughly corresponds to the peak in archaic Homo sapiens’ body size (the Neanderthals). The decline in size of the body in Homo sapiens sapiens (modern humans get two “wises” in our name, but do we really deserve it?) over the past fifty thousand years has raised our ratio of brain-to-body size to just above Neanderthal levels. Yet we have done this by shrinking our bodies to a greater extent than our brains have shrunk. There is some evidence that our brains are still shrinking and that they may have done so over the last ten thousand years by as much as 5 percent.