(*) Note: for convenience, I use the term "brain size" instead of "cranial capacity". Because the brain does not fill the cranial cavity, the brain size is smaller than the cranial capacity, but the latter value is, obviously, the only one that can be determined from a skull.
Figures for the average brain size of modern humans tend to vary between sources, but a typical value is 1350 or 1400 cc (cubic centimetres). The following figures should convey a feel for the normal range of variation in human skulls. Burenhult (1993) states that the 90% of humans fit in the range 1040-1595 cc, and that the extreme range is 900-2000 cc. S.J. Gould, in "The Mismeasure of Man", reviewed a 19th century study by Morton of 600 skulls which ranged from 950 to 1870 cc (and 25% of this sample was of small-statured Peruvians, so the figure of 950 cc is, if anything, lower than it might be for 600 randomly selected humans). Morton also catalogued his skulls by race, with the lowest average for any racial group being 1230 cc.
Various sources, some of them creationist, give lower limits for human brain size of 900 or 830 cc. The prominent British anatomist Sir Arthur Keith in 1948 gave 855 cc as the lowest known human brain volume (compared with 650 cc as the then highest known brain volume for a gorilla). Normal humans with even smaller brains have been found, but they are very rare. Microcephalics, who are subnormal in intelligence, can be as low as 600 cc, but this is a pathological condition and such skulls cannot be considered normal.
Hrdlicka (1939) examined the extremes of brain size in the 12,000 American skulls stored in the U.S. National Museum collections. Of these, the smallest 29, or fewer than 1 in 400, ranged from 910 to 1050 cc. Hrdlicka states that the smallest skull in this collection, at 910 cc, appears to be the lowest volume ever measured for a normal human cranium. The low volume skulls were not primitive or aberrant in any way; their small volume was merely a result of the smallness of the entire skull. So although the extreme lower range of modern human brain sizes does overlap that of Homo erectus, their skulls are very different: in H. erectus, the brain case really is smaller in relation to the rest of the skull. In small modern humans, the skull proportions are normal and the brain size is small only because the skull is small. (Compare the Turkana Boy skull and a modern human here.)
Compare the above figures with the 5 measurable Java Man skulls. These average 930 cc, less than the minimum of the 600 modern skulls cited above, with the smallest being 815 cc. Moreover, unlike modern humans with low brain sizes, these skulls are very robust, with flattened braincases and large brow ridges.
These figures also show how extraordinary the Turkana Boy is. As an adult, he would have been around 183 cm (6'0") tall, large even by modern standards. Modern men of that stature would be expected to have a larger than average brain size, but the Turkana Boy's estimated adult brain size of 910 cc is smaller than all but a fraction of 1% of modern humans of all sizes and both sexes. For comparison, 900 cc is a typical brain size for a modern child of 3 or 4 years weighing 15 kg (33 lbs).
Creationist Marvin Lubenow (1992) states that the lower limit of human cranial capacity is 700 cc, a much lower figure than anyone else. His source is Races, Types and Ethnic Groups by Stephen Molnar. Molnar says that "there are many persons with 700 to 800 cubic centimeters", but provides no source for this information, and none of his sources appear to do so either. In fact, one of his sources contradicts Molnar (and Lubenow). Tobias (1970) says that according to Dart, "apparently normal human beings have existed with brain-sizes in the 700's and 800's" (maybe Molnar's claim is a mis-statement of this), and that the smallest cranial capacity ever documented is 790 cc.
This strongly contradicts Molnar's claim that "many" modern humans have a cranial capacity below 800 cc, and Lubenow's derived claim that anything above 700 cc is a "normal" value. Instead, it appears from a variety of sources that values below 900 cc are exceptionally rare, and values below 800 cc virtually nonexistent.
Even if exceptional humans were found as low as 700 cc, it is still implausible for Lubenow to claim (p.162) that ER 1470, at 750-775 cc, is "well within the normal human range". (One might equally validly claim that an adult height of 122 cm (4'0") is well within the normal range on the grounds that some people are only 107 cm (3'6") tall.) Such cases, if they even occur, are obviously exceptionally rare, and the probability of finding a fossil human skull with such a small brain is essentially zero. It is far more probable that 1470 was a fairly typical member of its population. This is what we find: other habilis fossils, very similar to 1470, are even smaller, and well below Lubenow's lower limit of 700 cc.
Chimpanzees have a brain size between 300 and 500 cc, with an average of 400 cc. Gorillas have an average brain size of 500 cc, with large individuals going up to 700 cc, or even 752 cc in one reported (but unverifiable) instance. Hominids are best compared with the similar-sized chimpanzees than the much larger gorillas.
Lubenow states that "the crucial element is not brain size but brain organization. A large gorilla brain is no closer to the human condition than is a small gorilla brain". Lubenow's point is correct. If evolution is true, transitional creatures with brain sizes between 650 and 800 cc must have existed, but finding a skull with such a brain size does not prove that its owner was a transitional form. To be a convincing transitional form, a skull should not only have an intermediate brain size, but also an intermediate morphology.
This is exactly what is found in some H. habilis fossils. While there are no habiline fossils for which both brain and body size can be measured, it is fairly clear that they were smaller than humans, and many times smaller than male gorillas, the only apes with comparable brain sizes. Nor do H. habilis skulls have the crests and bone ridges found in large ape skulls. In addition, the insides of their skulls show many modern features (Tobias 1987). They are both larger and more modern, internally and externally, than the skull of any comparably sized ape.
Between species, average brain size, when a corrective formula for body size is applied, is a fair indicator of relative intelligence. The results are approximate, because they depend on which formula is used, and also on brain and body size, both of which are difficult to estimate for most fossil hominids. However it seems australopithecines were roughly as smart as, or probably a bit smarter than, chimps. Homo habilis and erectus were intermediate between chimps and modern humans. Walker and Leakey (1993) and Tobias (1987) have good overviews of attempts to estimate the relative intelligence of hominid species.
The following graph from McHenry (1994), plotting brainsizes against time, shows a general trend towards increased brainsize over time for the hominids:
Burenhult G. (1993): The first humans: human origins and history to 10,000 BC. New York: HarperCollins.
Hrdlicka A. (1939): Normal micro- and macrocephaly in America. American Journal of Physical Anthropology, 25:1-91.
Lubenow M.L. (1992): Bones of contention: a creationist assessment of human fossils. Grand Rapids,MI: Baker Books.
McHenry H.M. (1994): Tempo and mode in human evolution. Proceedings of the National Academy of Sciences, USA, 91:6780-6.
Tobias P.V. (1970): Brain size, grey matter and race - fact or fiction? American Journal of Physical Anthropology, 32:3-31.
Tobias P.V. (1987): The brain of Homo habilis: a new level of organization in cerebral evolution. Journal of Human Evolution, 16:741-61.
Walker A.C. and Leakey R.E. (1993): The Nariokotome Homo erectus skeleton. Cambridge,MA: Harvard University Press.
This page is part of the Fossil Hominids FAQ at the talk.origins Archive.
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