Humans (Homo sapiens), like all other organisms, are continuously evolving, and the forces that shape human evolution have also changed. Before the development of agriculture, ecological pressures were the primary influence on human evolution. Human populations were scattered and low. Each individual had to work and think harder then because survival in primitive environments required greater endurance and intelligence. Pleistocene Homo sapiens had to do it all. On average Cro-magnon man, also known as European early modern humans, had larger jaws and teeth, a bigger lung capacity, and a larger cranial capacity than present day man. The average cranial capacity of a human being living 20,000 years ago was 1500 cubic centimeters compared to 1350 cubic centimeters for an average human today. This larger brain capacity does mean they were more intelligent, but of course lacked our present day knowledge. During the agricultural revolution social competition replaced ecological processes as the primary influence on human evolution. This led to a division of labor, reducing the need for such a large cranial capacity. Human intelligence has declined all over the world since.
Modern humans have a smaller brain capacity than their recent ancestors. Incidentally, this illustration is misleading. Homo sapiens did not evolve from Neanderthals. Instead, Homo sapiens and Homo neandethalis shared a common ancestor–Homo heidelbergensis. Homo Neanderthals were also a distinct species, not a subspecies.
Domesticated cattle, sheep, goats, and dogs also have smaller cranial capacities than their wild ancestors. Humans culled the larger, wilder individuals; creating tamer breeds. Humans culled people who didn’t conform to society’s rules as well. In this way humans may have eliminated many people with larger brains that just couldn’t fit in. One study of a primitive society in New Guinea showed that as much as 10% of the males were executed because they couldn’t follow society’s rules. The death penalty selects for tamer humans by eliminating aggressive individuals from the gene pool. The death penalty was also common in western societies until the 20th century.
Aggressive intelligent humans had a big advantage over tame humans when the world was a wild place. But some of the characteristics that favored survival in a wilderness are counterproductive in a civilization dependent upon the rule of law to prevent chaos.
Human cranial capacity is on the increase again. Over the past 200 years there has been a major improvement in the average human’s diet. Better nutrition is leading to larger cranial capacities. The brain is a large fatty organ that requires a great deal of energy. With a better food supply humans with large brains are more likely to survive. However, I believe this recent trend is temporary. I predict future generations will suffer from greater incidences of malnutrition as the world population outstrips the ability of farmers to produce enough food for everybody.
References:
Bailey, Drew; and David Geary
“Hominid Brain Evolution: Testing Climatic, Ecological, and Social Competition Models”
Human Nature 2009
McAuliffe, Kathleen
“If Modern Humans are so Smart, why are our Brains Shrinking?”
Discover Magazine 2010
GeorgiaBeforePeople 
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