Modal IQ being much lower than IQs need for cutting edge scientific leave alone math performance seems to tell us something: cost of brains.
Other Homo
The extinct Homo specimens from Xuchang 1 & Xujiayao 6 (below) are believed by some to be East Asian Denisovans. They are estimated as having cranial capacities respectively of ~1800& ~1700 CC. Those are huge. For comparison, there is the Israeli Amud Neanderthal ~1740 CC. That’s AFAIK much larger than most Homo alive today. It is rather difficult to find raw volumetric data for modern Homo though I have read but found no confirmation of normal moderns with a size greater than that.
Modern humans
Source: TW
Cranial capacity is normally distributed (mean 1328, sd 138.34). Got 821 skulls from data of the early American phrenologist Morton. As noted before different measurement methods give somewhat different results but overall tend to be consistent. The data here is mean, when multiple measurements exist. The red lines on the right tail indicate the big-brained archaics & early sapiens, in order: Boskop: early African sapiens; Xujiayao-6: East Asian archaic; Amud-1: West Asian Neanderthal; Xuchang-1: East Asian archaics. The possibility that the east Asian archaics are actually Denisovans has been proposed, though we don’t know for sure.
It shows that the biggest modern sizes were already attained by 100,000K years in some groups. In Morton’s data the overall mean for sapiens is 1328 cc which is less than the average for Neanderthalis ~1500 cc. It has been proposed that neanderthalis had better eyesight & correspondingly more acute visual processing & perception than sapiens. This might have accounted for some of that extra brain. At the other end, archaic Homo naledi had a mean volume of 500-550 cc, close to Australopithecus
Phrenology
Source: TW
With phrenology falling out of vogue in the Occident, it is not at all easy to get a large dataset on human cranial volumes purely for research on the internet. One famous collection is that of the early American phrenologist Morton of 1225 crania of various peoples of the world. Morton, called by many to be a racist, was accused of inflating the volumes of the “white Caucasian” (or deflating that of others) by the Marxist scientist SJ Gould. While Morton had beliefs typical of Euro-ancestry Americans of the age (early 1800s), reexamination of his records shows no evidence for fudging of the data. There are different methods for cranial volumetry: Morton used two alternatives. Initially, he filled the skulls with pepper seeds and measured out their volume. Then he used lead pellets. His contemporary, a German Tiedemann who amassed his own cranial collection (not called a racist) filled skulls with millet seeds and weighed them rather than taking the volume – this is problematic for we do not have his seed density for converting to volume. Others have performed volumetry by filling skulls with water. More recently, people can do computerized volumetry from scans but such data is hard to easily access. The volumes of course differ by each method a bit but are mostly consistent.
We have a dataset of 460 of Morton’s crania. The most voluminous skull in his collection was a big-headed Dutch chap with 1786 cc. With phrenology falling out of favor in the Occident, the Morton collection which was once allowed for free public viewing was gradually made more & more inaccessible. Unfortunately, after the American upheaval of summer 2020 it was made entirely taken out of public view & the skulls of individuals of certain ethnicities might be physically buried.
A 1946 English phrenological study found a fellow with a 1800 cc skull. In India phrenology continued after the initial influences of the likes of the statistician R. Fisher& his H successors more or less in the old style of the Euro-american founders. One report mentioned a 1709 cc maximal volume. However, we have not yet found any published report with something bigger than that. From the recent Indian data assuming normal distribution it seem a 1/100 Indian males might have a cranial volume of 1700 cc or greater. An American survey in the 1970s suggests that 1/126 individuals (sex data not available to me) might have a 1700 cc or greater volume.
Hence, it does seem that East Asian archaic Homo possibly Denisovan had unusually large cranial volumes by modern Homo standards
Distribution of cephalic index, i.e., maximal breadth/maximal length*100 from Morton’s skulls. Cephalic index is not meaningfully correlated with cranial capacity but is of forensic value. In this data those with CI<70 tend to be from Africa or native Australians; CI>85 tend to be First Americans, predominantly from the tail of South America/Andes. A visual of cephalic index. Those with lower values will have longer heads and those with higher values broader heads.