So, in this case it's not like the intelligence itself is detrimental, but species with oversized brains should never evolve? I would support starvation, but not "easy life". It is theorized that prehistoric hominins had some kind of "easy life", so evolution said "why not?" and gave them unnecessary large brains.

@Alexander What nutjob had that theory? Early hominins were prey to meat eaters; we have the skeletons to prove it. Neanderthals have more bone breaks than rodeo riders (the most in modern society). Evolution does not say "why not". No major new, complex thing springs into existence fully formed; everything is incremental, and if it is expensive and doesn't pay off more than it costs, negative evolutionary pressure eliminates it by reverting to the norm. Intelligence does not evolve unless it is more useful in every incremental manifestation; whatever the increments may be.

There is simply no good explanation as to why early humans would had huge brains. Another theory is like "Yes, you need a brain that big to be able chip stone", but this is also a theory without hard proof.

@Alexander I disagree, there are plenty of reasons for intelligence, that is why we see it in dolphins, octopi, elephants, chimps, gorillas, and corvids; all capable of planning, rudimentary invention, tool use, lying, and other hallmarks of self-awareness and a theory of mind (knowing that others of their species are thinking and aware and able to make choices). Early humans did not have huge brains; they had chimp sized brains. We stumbled upon the secret formula at a time when it proved important to our survival as a species. At a different and safer time, it would likely have been lost.

In the era stated in the question (2 million years ago), human ancestors already had bigger brains. In the next million - million and a half years, brain size would double. The question is do we really need that much of increase to provide for tech advances of Homo Erectus? Moving from Homo Erectus to Modern humans, brain sizes stabilized, but this is where we have 99.9% of our technological advances.

@Alexander My point is, how do you know how much of an increase was needed? The truth is that like many evolutionary outcomes it was probably a feedback resonance that made our brain larger. Perhaps due to some small but crucial change that made us smart enough to outwit nearly all predators, so that in a few dozen generations humans became the biggest threat to humans, causing an unstoppable arms race of us getting smarter and smarter until we reached the limit of our particular neural biology: stabilized, not because smarter wouldn't be better, just a dead end on improvements.

@Alexander You're looking at things in reverse. The fact that we have invented (and are continuing to invent) so many useful things with our current brain size is the reason why there isn't much selection pressure to increase brain size further. Presumably that wasn't the case earlier in our evolutionary history, so back then, increases in brain size provided an evolutionary advantage. (continued...)

(...continued) Also note that this doesn't mean our current brain size is optimal in some universal sense; another factor is that inter-group communication has advanced to the point where even if a subpopulation mutated up some larger brains and developed corresponding technological advances, the resulting advances would be shared with the population as a whole, and would thus grant little relative advantage to the subpopulation that developed them.

@Amadeus - "humans became the biggest threat to humans" - that is my definition of "easy life" - an apex predator that does not have to compete with other species.

@Ray - that may be true, but 1-2 million years ago out ancestors had to give up a lot (compared to great apes) just to maintain big brains. And, at the moment, payback did not look big. Ignore the last 1 million years for a moment, consider earlier era. Why would homo erectus needed a much big brain?

@Alexander More advanced stone tools, and fire were a pretty decent return on investment.

Yes, but is there really a need for such a big brain for those advancements? If yes, homo ancestors had to suffer for 100,000s of years before getting any benefit of fire. If no, why they had to continue brain development once the fire was harnessed? We should have seen a continuous and marked improvement of tools coincided with brain development - but we don't see it.

@Alexander Early Homo Erectus had much need for a bigger brain. No defensive fur, no claws, weaker than many other predators, but definitely have been shown to be a species of hunters and gatherers. The ones that were born with larger brains were less likely to get killed by superior predators while out in a hunting party or gathering. Evolutionary pressure developed intelligence since being unintelligent got you killed. Each small increment of the improvement in quality of life took another step towards a higher average intelligence.

All other things being equal, hominins were inferior to their cousins big apes. The bigger brain, the bigger disadvantage. Once we stepped on that evolutionary path, bigger brain should have been a bigger advantage. But how? Just by outsmarting the predators?

@Alexander: Out-smarting isn't necessary if you out-coordinate. And selection need not be at the level of the individual; dumb people may have survived because their tribal unit was (on average) smarter and better able to coordinate; remembering where food could be found, developing strategies to evade/discourage predators, etc. Early humans appear to have occupied a grassland niche, where other great apes occupied a jungle niche; if you can only function in an environment that demands greater social coordination/intelligence, it doesn't matter if you wouldn't need it in the jungle.

@Amadeus That is incorrect, or at least misleading. Evolution isn't "only the strong survive", or "traits persist only if they're more beneficial than not to have them." Evolution is based on happenstance and the Just Good Enough Principle. All that is necessary is that the trait not impede reproductive success so much that the trait dies out. It can be disadvantageous and still persist as long as the group remains just fit enough to survive and reproduce across generations. The variety of human genetic disorders is one demonstration of this.

@zibadawatimmy I don't think it is misleading. Intelligence and big brains are very expensive, and evolution is typically ruthless on saving energy. It is why salamanders lose their eyesight after generations in the dark. The rule is, use it or lose it. Anything that takes a lot of calories DOES impede reproductive success, including that impediment called starving to death.

@Amadeus Not all cave salamanders have lost their eyes. Don't think any of them have. Some aren't even blind. They can still have eyes, and even the blind ones can remain photoreceptive for much or all of their lives. Some albino species even darken when exposed to enough light because they can still produce melanin if needed. There are lots of factors at play here, such as environmental niches, resources available, and competition for resources with other species. Some situations give more wiggle room than others to be "just good enough", but rarely is it so simple as "be better or die out".

@Amadeus there are examples of counter-productive evolution. As long as the individual is able to reproduce, the trait will exists, even if it doesn't help the individual. Some evolutionary trait might not have been better when it developed but was just random. We don't know completely how evolution work but it's not "survival of the fittest" stricto sensus, Bad evolutionary trait can appear.

@MauganRa IRL, yes! However, social Intel is stage 2 or 3 and must come after stage 1; e.g. A crow's level of intelligence, problem solving, strategic thinking (which is high). The whole point of this post is how to make intelligence NOT useful; under what conditions would we not ever evolve past Stage 1, or past Stage 2 (chimpanzee, dolphin, elephant levels of intelligence). Maybe elephants & gorillas have not due to their size.**Maybe** Dolphins due to sharks being easy to outwit, so enough is enough. Maybe Chimps due to easy escape by high climbing. I'm answering the Q.

@Amadeus you are right that the discussion about evolution and benefits of intelligence is quite off topic.

Sex as a medium of exchange isn't necessarily a sign of absence of intelligence. Las Vegas makes a fortune off of that very concept. Not a bad idea, now that I think about it...

@Amadeus I was really on the fence with all the answers towards the end, especially since the everyone rallied around your response the most. So I wanted to give you an honorable mention for what it's worth. Your answer really got this discussion moving, and I still like your answer a quite a lot. It's just that Eliot's answer was a better fit for me personally -- a very difficult decision. Thank you nonetheless!

@Amadeus And here's a study by William Jeffery that studies a fish species with both cave and surface (non-cave) varieties. It pretty thoroughly debunks that eye degeneration is a result of energy conservation here, for quite a lot of reasons. It is argued that it is rather pleiotropy that's to blame: selection favors altering gene expression to the advantage of one feature and the detriment of the eyes. The eyes can even be restored with a lens transplant early in development. Not used and not lost.