Cf. the "Starchild" skull that conspiracy theorists still love to speculate about, long after it was shown to be the skull of a hydrocephalic individual.
It's not impossible that this was a lineage of large-brained hominids, distinct from our own. Evolution favors local optima; it does not follow a teleological pattern. If big brains weren't advantageous to the local circumstances, they wouldn't have survived. But the big-brained, distinct-species theory speculates quite a bit on very little evidence.
[1]http://en.wikipedia.org/wiki/Artificial_cranial_deformation
Well, define "intelligence".
Evolution seems to favor technology very strongly, and our models imply that it would. But more detailed senses for example isn't as strongly favored.
evolution seems to favor volume over quality, not to say there aren't exceptions but roaches and rats seem to be doing pretty well.
Possibly there was very little advantage being a genius would give you back then? Especially if you count the life expectancy.
If everyone had a spear and you were the genius that invented the bow and arrow, then one day later, everyone would copy you.
Also, an older society of average people would have created more technology than a newer society of geniuses. Or even if a society of thousands developed, they would be able to create more advanced weaponry than a village of 40 geniuses. So maybe the group of humanoids with the genetics that made them a super social species with the predilection for massive groups would give them a huge advantage over a genius group without the genetics to seek massive societies.
A city of 10,000 people all with "IQ" of 90 that has been around for 500 years would annihilate technologically a village of 40 all with "IQ" of 150 which has been around for 100 years.