If you were born a baby spider, things would have turned out much differently for you. You'd have been mostly brain, for example. Researchers have long suspected that tiny spiders -- the young of which are routinely born deformed yet grow into normally proportioned adults -- are born with very large brains. Now they know it, thanks to what I imagine is research that amounted to dissecting deformed spider babies carried out by arachnid specialists at the Smithsonian Tropical Research Institute, down Panama way.
The researchers posit that spiders are born with the minimum brain mass they need to be able to function, which is still a substantial amount, especially considering the size of some of the tinier spider species. Essentially, nature has found a way to shrink neurons and other brain cells to as small a size as they're going to get right now; spiders need a set amount to function normally and so when they're born, they're packed with so much brain that it deforms them. Eventually, they grow into their huge brains but even adult spiders have a lot more brain than you'd suspect. Adults of some of the smallest spider species, like nymphs, are up to 80% brain,with as much as a quarter of their legs containing brain tissue. That's a lot of brain.
This is somewhat related to another issue I came across recently, the question of why humans aren't smarter than we are. The answer, the researcher came up with is that we have topped out. We humans have achieved maximum smartness and anything additional would likely drive us insane. This position is based on the idea that schizophrenia is the result of lowered latent inhibition, which is our ability to filter out superfluous stimuli -- people with schizophrenia, the hypothesis goes, have a decreased filter and their brains try to make sense of the barrage of information, turning it into delusions and hallucinations.
The upshot of the paper was that the evolutionary reason we're not smarter is the same as why spiderlings are born deformed: brains can only get so small and still work correctly. If we humans were born with larger brains, our head size would have to increase. This would have to correlate with an increase in pelvis size in women or else maternal mortality rates would skyrocket, erasing us as a species. Since we wouldn't be able to move as well with larger pelvises, ipso facto, our brains aren't going to get any bigger, hence we'll just have to be satisfied with being as smart as we are now.
Of course, human evolution could mimic that of spiders and stuff more brains throughout our body cavities.