[...] before long the skull came to the attention of S. H. Haughton, one of the country’s few formally trained paleontologists. He reported his findings at a 1915 meeting of the Royal Society of South Africa. “The cranial capacity must have been very large,” he said, and “calculation by the method of Broca gives a minimum figure of 1,832 cc [cubic centimeters].” The Boskop skull, it would seem, housed a brain perhaps 25 percent or more larger than our own.

The idea that giant-brained people were not so long ago walking the dusty plains of South Africa was sufficiently shocking to draw in the luminaries back in England. Two of the most prominent anatomists of the day, both experts in the reconstruction of skulls, weighed in with opinions generally supportive of Haughton’s conclusions.

The Scottish scientist Robert Broom reported that “we get for the corrected cranial capacity of the Boskop skull the very remarkable figure of 1,980 cc.” Remarkable indeed: These measures say that the distance from Boskop to humans is greater than the distance between humans and their Homo erectus predecessors.

What Happened to the Hominids who were Smarter than Us?

I'm strongly inclined to defy the data -- true superintelligence should have just dominated our ancestors -- but given the expense of large skull size (primarily in difficult birthing) it also seems profoundly unlikely that a lineage would see expansion like this that wasn't buying them something mentally.