Sporting activities are essentially modified forms of hunting behavior. Viewed biologically, the modern footballer is in reality a member of a hunting group. His killing weapon has turned into a harmless football and his prey into a goalmouth. If his aim is accurate and he scores a goal, he enjoys the hunter's triumph of killing his prey. To understand how this transformation has taken place we must briefly look back at our forefathers. They spent over a million years evolving (进化) as cooperative hunters. Their very survival depended on success in the hunting field. Under this pressure their whole way of life, even their bodies, became greatly changed. They became chasers, runners, jumpers, aimers, throwers and prey killers. They cooperated as skillful male group attack. Then about ten thousand years ago, after this immensely long period of hunting the food, they became farmers. Their improved intelligence, so vital to their old hunting life, was put to a new use—that of controlling and domesticating (驯养) their prey. The hunt became suddenly out of date. The food was there on the farms, awaiting their needs. The risks and uncertainties of the hunt were no longer essential for survival. The skills and thirst for hunting remained, however, and demanded new outlets. Hunting for sport replaced hunting for necessity. This new activity involved all the original hunting sequences (后果), but the aim of the operation was no longer to avoid starvation. Instead the sportsmen set off to test their skill against prey that was no longer essential to their survival. To be sure, the kill may have been eaten, but there were other purposes, much simpler of obtaining a meaty meal. What is the main idea of the passage? ______.