In The Red Queen, Matt Ridley popularized the theory that human intelligence is the byproduct of an evolutionary arms race of sexual selection. It goes something like this:

• fact: human brains are proportionately much larger than almost any other species, including any other primate
• fact: one proven catalyst for extreme evolutionary change is an arms race, where two sets of genes compete with all other things more or less equal. For example, a predator and its prey each get faster and faster, but relative to each other they stay competitive. This continues until all other things are no longer equal; e.g., the cost to be so fast can no longer be paid. For example, it is argued that humans and other primates pay for all of that metabolically expensive brain tissue with smaller digestive tracts, an tradeoff that clearly has limits.
• fact: sexual selection occurs, in which a trait which is not otherwise critical to fitness becomes attractive for purposes of attracting mates, leading to extreme exaggeration of that trait. Example: peacock tails.
• Theory: at some point displays of intelligence became important for proto-human males to attract females, and conversely female intelligence was required to select good male mates. This turned into an arms race, leading to dramatic increases in brain size, a byproduct of which was modern human intelligence.

This is not the only explanation for human brain size; social cooperation and language are also factors. Here's a more recent study which posits something similar: "... the balance of evidence now clearly favors the suggestion that it was the computational demands of living in large, complex societies that selected for large brains. However, recent analyses suggest that it may have been the particular demands of the more intense forms of pairbonding that was the critical factor."

Today's New York Times has an article about the search for genetic causes of schizophrenia, which is taking longer than expected because none of the big obvious causes pan out. Instead, it seems likely that "the genetic component of the disease is due to a large number of variants, each of which is very rare, rather than to a handful of common variants." What this means is that evolution has done a very good job of eliminating the big causes of (some kinds of) mental illness, leaving only lots of little things that aren't as simply selected for deletion. In other words, there is evolutionary pressure to have good brains. This is surprising to a lead researcher because "I would have thought the brain was a luxury organ when it comes to reproductive success."

I guess he's not current on the Red Queen and human sexual selection for brains. Chalk up a supporting point for Ridley's theory.

Although, an alternate explanation does occur to me. Perhaps we are being bred by brain-eating zombies for taste and flavor.