And in that change from Australopithecus to the first Homo, 2.5 million years ago, something remarkable happened. The brain size doubled! Early Homo habilis, with a brain size of about 700 cc, started to make stone tools and probably had a more socially complex lifestyle. Somewhere along the line, it evolved into Homo erectus, who had a brain size of about 1000 cc, used many tools, had a socially complex life, and, about 500,000 years ago, learned how to use fire to their advantage. These eventually evolved into modern humans, Homo sapiens, who have a brain size of 1350 cc.

What was it that sparked that amazing 50% brain increase? What made us go from the small-brained Australopithecus to the smart new genus Homo? But to help us answer that question, we need another one: why did early humans "want" that big brain? You need extremely good reasons for having a large brain to balance out the disadvantages, such as the fact that brains are metabolically expensive organs ‹ they consume 20% of the total energy modern humans produce. Also, babies are helpless for years after birth because their heads have to be small enough to fit through the birth canal.

One of the reasons that primates are thought to have larger brains than other mammals is that they have color vision, probably for finding colorful fruits. This requires much more "power." But there have been many theories regarding the huge increase in human brains.

In the early 1900's, the common thought was that humans developed a larger brain and then became bipedal, or that these two things happened simultaneously. Later finds, however, proved this wrong. In the 1940's, the "Man the Toolmaker" theory emerged along with finds of stone tools. In the fifties, "Man the Killer Ape" became a widespread theory, depicting early humans as savage killers and hunters who battled vicious tigers and angrily crushed bones left in their cave. In the sixties, "Man the hunter-gatherer" gained acclaim, representing a much more intelligent, humane hominid who was a clever hunter. In the seventies, "Woman the Gatherer" showed females' importance in human evolution and their role in an early human's lifestyle. More recently, "Man the Scavenger" has come strongly forth rendering a kind of human who used cleverness to scavenge kills made by other animals, and who used stone tools to break open bones to get the fatty bone marrow inside.

What's interesting about these theories is that they almost comically reflect the attitudes of people at the time they were believed. For instance, the "Man the Killer Ape" theory was believed when the world was a crisis over World War II, and "Woman the Gatherer" came right along with women's rights movements. But in general, tools and brain increase have gone pretty much hand in hand in the minds of most anthropologists. Indeed, this seems completely logical because the earliest stone tools that have been found come at the same time as that amazing 50% brain increase (about 2.5 million years ago).

But Thomas Wynn, an archaeologist in Scotland, found that all of the spatial concepts needed for making stone tools are found in modern apes! This put into question some of the existing theories. Instead of requiring higher intelligence, did toolmaking only require, say, employing mental abilities comparable to apes, but in a different way? Dick Bryne and Andrew Whiten's 1988 Machiavellian Intelligence hypothesis is based on this and focuses on "Man the social animal," a creature with a socially complex lifestyle.

Everyone knows that bigger group sizes are safer. But as a group gets bigger, it also gets more socially complex. For instance, in a group of five people, a person has to keep track of four direct relationships and six relationships between others in the group. But in a group of 20 people, besides the 19 direct relationships, one has to keep track of 171 between others! That's a lot to keep track of! Psychologist Nicholas Humphrey believes that this theory of social interaction is the main reason for hominid brain evolution.

Going even farther, scientists came up with a reason for why language developed. An ape's form of social interaction is grooming. This can hold groups together up to a certain size, but then you need something else as a means for social interaction. Some anthropologists, such as Humphrey, think that language developed as a sort of "vocal grooming."

There are many advantages of language over grooming. For one, you can do it while working. Also, you can talk to more than one person at a time. For chimps, the maximum group size where everyone knows about everyone is about 50 individuals. For modern humans, the maximum group size is about 150, three times as many. What I find interesting about this is that our brain size is also almost exactly three times as large as that of a chimp!

I think that the social complexity theory is a good one, but it's too extreme. The theory that language developed as a form of social interaction sounds very promising, but it seems likely to me that both tools and social complexity played a role in the evolution of the hominid brain. Social complexity led to larger, better brains, which led to more advanced tool making, which led to better scavenging, which led to more free time in which social complexity could be developed, which led to larger, better brains, which in turn led to more advanced tools, which in turn let to larger, better brains. At this point, it's basically: which came first, the chicken or the egg (or the chicken house)?

Thus we return to the original question: What was it that started that sudden 50% brain increase? Is it, indeed, just a question of which came first? In that case, was it the tools, the social complexity, or increased brain size that came first? If it was the brain size, it was probably by some accidental genetic flaw because there doesn't seem to be a reason for it to just suddenly get bigger. If the increase was because of tools or social complexity, why did it happen then, instead of, say, a million years earlier or later? Often, this kind of alteration is because of a climate change. However indirectly, a climate change could have provided the fuel to spark the amazing brain increase. For instance, the dryer climate could have forced early humans to use tools to survive in conditions with less and less food. But until we find more evidence, the answers to these questions will remain a mystery.