A natural history of the human mind: tracing evolutionary
changes in brain and cognition by Chet C. Sherwood,
Francys Subiaul and Tadeusz W. Zawidzki; J. Anat. (2008) 212, pp426â??454

From sensation to cognition (PDF) by M.-Marsel Mesulam, Brain (1998), 121, 1013â??1052

You left out the artistic revolution of ca40-50KYA (Cro-Magnon), which was not associated with any increase in average brain size. Indeed, the “Possible drop in brain size ca 30,000” might have been associated with it.

I think my point about the limits of birth canal width and brain size still stands, but will have to spend a while staring into space while ironing out wrinkles left by this incredible shrinking anomaly before I dare to raise the question again…

Perhaps I should erase all my notes from Dr. Shlain’s intriguing but somewhat flaky book. (The blog he attempted on it, alas, was one of the most troll-ridden I’ve ever seen.)

A. Brains grow 20% to Homo Habilis/late australopith size at the same time that stone tools show up. The stone tools are not impressive. This is 2.5 mya

B. Brains triple in size, bodies double in mass, complexity of tools goes from breaking rocks to fashioning Acheulean hand axes, range of hominids goes from tropical and subtropical wooded savanna in Africa to inlcude semi arid regions, warmish temperate regions, and possibly more forested regions across all of Africa and most of southern Eurasia and occassionally father north, from Britian to East Asia (and Flores!???) at about 2.0 mya.

Brains go from 900 to 1100 CC’s to 1100 to 1500 CC’s, modest expansion of geographical range, significant expansion of range of habitats, one more order of magnitude of tool complexity (prepared platform technology) during the period of 500,000 years ago to 250,000 years ago. Earliest pre-Modern humans or actual modern humans, Neanderhtlas, etc. during and just subsequent to this period.

No change in human brain size, fully modern human geographical and habitat range, varying toolkits indicating fully modern range of technology with respect to the brain part (the technology gets to evolve on its own, of course, after this) from 250,000 to the origins of agriculture.

Possible drop in brain size ca 30,000

Probable drop in brain size in agricultural population starting 10,000

Predicted drop in brain size any day now unless the simulus package works.

I doubt the item below is the only place I’ve read that claim, but it’s what I can find at present:

â??Linguist Derek Bickerton noted â??the two most shocking facts of human evolution: that our ancestors stagnated so long despite their ever-growing brains and that human culture grew exponentially only after the brain had ceased to grow.â??â?

– Leonard Shlain, Time, Sex and Power: How Womenâ??s Sexuality Shaped Human Evolution, pg 363

The way the regions of the cerebral cortex appear to work includes a complex transformation of a number of continuous scalar functions in n_(1,2,3,4…) dimensions (the inputs) into an output (continuous) scalar function in n_o dimensions. Each pyramidal cell in a particular population in layers 2-3 provides a sampling of the output function at a particular location. There are multiple populations of pyramidal cells, each of which presumably performs a different calculation (of a different output).

Input axons from other cerebral regions, the thalamus, and sometimes other parts of the CNS similarly carry samplings of the input functions. The “White matter” of the cerebral hemispheres is made up of the myelinated axons carrying signals between regions. The “gray matter” is contains the cell bodies, unmyelinated axons, dendrites, synapses, extensive blood supply and other supporting systems. (See my earlier links for pictures.)

For example, in “The Astonishing Hypothesis“, Crick describes the way in which inputs to the V1 region of the visual area, in which each axon carries a signal representing the likelihood that there is a spot of a particular type (size, color, etc.) at a particular point of the retina, are transformed into an output in which each axon carries a signal representing a bar of a particular type (size, orientation, color, etc.). This is a simple transformation to describe, and one of the few we seem to know about, but it is plausible that every region of the cerebral cortex performs similar transformations.

Consider the impact of increasing the number of inputs or outputs (pyramidal cells) for V1. Both the underlying white matter, and the cortex would increase in volume with increasing number of myelinated axons, cell bodies, and cross connections. But the “intelligence” of the calculation would not increase, only the precision.

Hypothetically, (or, perhaps speculatively,) a similar growth of overall brain size might support an increase in precision of some brain function(s) without any real increase in intelligence, while actual intelligence increases might be limited to improvements to the calculation algorithms each nerve cell performs.

A perfect example of the above, IMO, would be vocabulary. Although it couldn’t be proven at this stage, it seems highly likely to me that the ability to remember large numbers of independent, arbitrary words is independent of any of the various abilities to work with them. This means that an increase in brain size might well be explained by a simple need for expanded vocabulary.

The large increase in brain size that may have been enough to make birth dangerous is probably associated with the largest shift ever in technology, as well as (percentage wise) the largest expansion ever of hominds to new ranges and into new habitats.

Other domesticated mammals have a great deal of difficulty with childbirth (and that’s a very good question, by the way) but I don’t think we have the kind of data that makes this comparable for a number of reasons, including the fact that none of the mammalian domesticates are bipedal.

(and other reasons)

To pull numbers out of the air, let’s say that a slightly larger brain increases maternal mortality by 1%. To persist, the mutation that increases brain size would have to confer a survival advantage of >1%.

Most of the proposed advantages of larger brains don’t seem to fit the incrementalist model of natural selection: language would require a group of smarter hominids to be effective, tools found so far show no corresponding improvements, sexual selection would more likely favor females who preferred smaller-brained jocks, etc.

Moreover, the selective pressures against large brains would have increased dramatically in the later stages of the process: is there any non-domesticated mammal which has such difficulty with childbirth as homo sap?

My hypothesis is that some strange set of ecological circumstances propelled our ancestors through a particularly demanding evolutionary bottleneck over hundreds or thousands of generations, but my speculations on same haven’t even reached the level of worthiness for an end-of-thread comment on a freewheeling blog.