I've hooked you with the aliens, right? Good, because I'm not starting there. You need background info! (Which is being taken from an article by Discover**, by the way. And there is controversy that the humanoids in the article even existed as a separate group. Caveat lector.)
Once upon a time in Africa, there lived a group of Homo sapiens with large skulls and small faces. These people are known as the Boskops, named for the South Africa town where the first skull fragments were found in 1913. These are about the only fragments we've got, so calling them a separate group of hominids might be erroneous. They (he? she?) might just be someone with a really big head. For the sake of this post, I'm going to assume that the Boskops were a subspecies of human.
Or maybe they were aliens? After all, the classic image of an alien is the Roswell Type, right***? Short, greyish, slanting dark eyes, and giant heads that are mostly brain. More on this in a sec.
Boskop brains were about 25% larger than ours, at least. According to Discover:
So, basically what they're saying is that Boskops would have remembered more about places and would have had more ways to associate one thing with another. They'd have been able to process memories faster as well. (The article even speculates that they could have processed memories and retrieve memories at the same time, and that they'd be able to predict outcomes better than us.) Finally, we can probably assume pretty safely that most if not all Boskops had genius-level IQ. They certainly had superhuman brains.
Expanding the brain changes its internal proportions in highly predictable ways. From ape to human, the brain grows about fourfold, but most of that increase occurs in the cortex, not in more ancient structures. Moreover, even within the cortex, the areas that grow by far the most are the association areas, while cortical structures such as those controlling sensory and motor mechanisms stay unchanged.
Going from human to Boskop, these association zones are even more disproportionately expanded. Boskop’s brain size is about 30 percent larger than our own—that is, a 1,750-cc brain to our average of 1,350 cc. And that leads to an increase in the prefrontal cortex of a staggering 53 percent. If these principled relations among brain parts hold true, then Boskops would have had not only an impressively large brain but an inconceivably large prefrontal cortex.
The prefrontal cortex is closely linked to our highest cognitive functions. It makes sense out of the complex stream of events flowing into the brain; it places mental contents into appropriate sequences and hierarchies; and it plays a critical role in planning our future actions. Put simply, the prefrontal cortex is at the heart of our most flexible and forward-looking thoughts.
Sounds like they were better evolved than us, doesn't it, so why did they die off? Discover speculates that their ability to predict outcomes of actions would have paralyzed them, that larger brains equalled more trouble in the birth canal, or that there wasn't much advantage in having big brains back in the day, that being about to outthink regular humans didn't actually help. To this, I'd like to add the (sane) suggestion that the larger brains used up too much energy and the Boskops weren't able to acquire enough food in the long term to keep their brains working properly along with the rest of their bodies.**** Our brains need a massive amount of energy as it is.
And then there's the insane suggestion: they were aliens. I mean, there's the whole Chariots of the Gods theory, and all the urban legends and Hollywood films. (The image of a Boskop man has to be in the collective unconscious for a reason.) And if aliens are studying humans today, why wouldn't they have been studying us in the past?
Or perhaps the Boskop aliens were stranded on Earth, forced to use Stone Age technology to survive? Perhaps they interbred as an added survival measure. The last few paragraphs of the Discover article seem to indicate their genes didn't die off completely. This would mean we're at least partly alien (or some of us are).
One last thing: If we're going Chariots of the Gods here, why not go to an extreme and posit that the Boskop aliens selectively bred humans, created humans, or something along those lines? Why assume that since we've found all these other hominids and have fossil evidence of humans existing before the Boskop fossils died, that Homo sapiens came first? Maybe we just haven't found the super-ancient alien fossils yet.
*Hey, it could happen.
*** A.k.a. the Asgard from Stargate.
**** And here's where someone with more knowledge of hunter-gatherer societies, foraging, and the amount of food in ancient South Africa refutes me.