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The increase in brain size in our species over the last 3 million years has been the subject of numerous articles and books. Over that time period, brain size increased from our ancestor Lucy, all the way to today. Many stories are proposed to explain how and why it exactly happened. The explanation is the same ol’ one: Those with bigger heads, and therefore bigger brains had more children and passed on their “brain genes” to the next generation until all that was left was bigger-brained individuals of that species. But there is a problem here, just like with all just-so stories. How do we know that selection ‘acted’ on brain size and thusly “selected-for” the ‘smarter’ individual?

Christopher Badcock, an evolutionary psychologist, as an intro to EP published in 2001, where he has a very balanced take on EP—noting its pitfalls and where, in his opinion, EP is useful. (Most may know my views on this already, see here.) In any case, Badcock cites R.D. Martin (1996: 155) who writes:

… when the effects of confounding variables such as body size and socio-economic status are excluded, no correlation is found between IQ and brain size among modern humans.

Badcock (2001: 48) also quotes George Williams—author of Adaptation and Natural Selection (1966; the precursor to Dawkins’ The Selfish Gene) where he writes:

Despite the arguments that have been advanced, I cannot readily accept the idea that advanced mental capabilities have ever been directly favored by selection. There is no reason for believing that a genius has ever been likely to leave more children than a man of somewhat below average intelligence. It has been suggested that a tribe that produces an occasional genius for its leadership is more likely to prevail in competition with tribes that lack this intellectual resource. This may well be true in the sense that a group with highly intelligent leaders is likely to gain political domination over less gifted groups, but political domination need not result in genetic domination, as indicated by the failure of many a ruling class to maintain its members.

In Adaptation and Natural Selection, Williams was much more cautious than adaptationists today, stating that adaptationism should be used only in very special cases. Too bad that adaptationists today did not get the memo. But what gives? Doesn’t it make sense that the “more intelligent” human 2 mya would be more successful when it comes to fitness than the “less intelligent” (whatever these words mean in this context) individual? Would a pre-historic Bill Gates have the most children due to his “high IQ” as PumpkinPerson has claimed in the past? I doubt it.

In any case, the increase in brain size—and therefore increase in intellectual ability in humans—has been the last stand for evolutionary progressionists. “Look at the increase in brain size”, the progressionist says “over the past 3mya. Doesn’t it look like there is a trend toward bigger, higher-quality brains in humans as our skills increased?” While it may look like that on its face, in fact, the real story is much more complicated.

Deacon (1990a) notes many fallacies that those who invoke the brain size increase across evolutionary history make, including: the evolutionary progression fallacy; the bigger-is-smarter fallacy; and the numerology fallacy. The evolutionary progression fallacy is simple enough. Deacon (1990a: 194) writes:

In theories of brain evolution, the concept of evolutionary progress finds implicit expression in the analysis of brain-size differences and presumed grade shifts in allometric brain/body size trends, in theories of comparative intelligence, in claims about the relative proportions of presumed advanced vs. primitive brain areas, in estimates of neural complexity, including the multiplication and differentiation of brain areas, and in the assessment of other species with respect to humans, as the presumed most advanced exemplar. Most of these accounts in some way or other are tied to problems of interpreting the correlates of brain size. The task that follows is to dispose of fallacious progressivist notions hidden in these analyses without ignoring the questions otherwise begged by the many enigmatic correlations of brain size in vertebrate evolution.

Of course, when it comes to the bigger-is-smarter fallacy, it’s quite obviously not true that bigger IS always better when it comes to brain size, as elephants and whales have larger brains than humans (also see Skoyles, 1999). But what they do not have more of than humans is cortical neurons (see Herculano-Houzel, 2009). Decon (1990a: 201) describes the numerology fallacy:

Numerology fallacies are apparent correlations that turn out to be artifacts of numerical oversimplification. Numerology fallacies in science, like their mystical counterparts, are likely to be committed when meaning is ascribed to some statistic merely by virtue of its numeric similarity to some other statistic, without supportive evidence from the empirical system that is being described.

While Deacon (1990a: 232) concludes that:

The idea, that there have been progressive trends of brain evolution, that include changes in the relative proportions of different structures (i.e., enlarging more “advanced” areas with respect to more primitive areas) and increased differentiation, interconnection, and overall complexity of neural circuits, is largely an artifact of misunderstanding the complex internal correlates of brain size. … Numerous statistical problems, arising from the difficulty of analyzing a system with so many interdependent scaling relationships, have served to reinforce these misconceptions, and have fostered the tacit assumption that intelligence, brain complexity, and brain size bear a simple relationship to one another.

Deacon (1990b: 255) notes how brains weren’t directly selected for, but bigger bodies (bigger bodies means bigger brains), and this does not lean near the natural selection fallacy theory for trait selection since this view is of the organism, not its trait:

I will argue that it is this remarkable parallelism, and not some progressive selection for increasing intelligence, that is responsible for many pseudoprogressive trends in mammalian brain evolution. Larger whole animals were being selected—not just larger brains—but along with the correlated brain enlargement in each lineage a multitude of parallel secondary internal adaptations followed.

Deacon (1990b: 697-698) notes that the large brain-to-body size ratio in humans compared to other primates is an illusion “a surface manifestation of a complex allometric reorganization within the brain” and that the brain itself is unlikely to be the object of selection. The correlated reorganization of the human brain, to Deacon, is what makes humans unique; not our “oversized” brains for our body. While Deacon (1990c) states that “To a great extent the apparent “progress” of mammalian brain evolution vanishes when the effects of brain size and functional specialization are taken into account.” (See also Deacon, 1997: chapter 5.)

So is there really progress in brain evolution, which would, in effect, lend credence to the idea that evolution is progressive? No, there is no progress in brain evolution; so-called size increases throughout human history are an artifact; when we take brain size and functional specialization into account (functional specialization is the claim that different areas in the brain are specialized to carry out different functions; see Mahon and Cantlon, 2014). Our brains only seem like they’ve increased; when we get down to the functional details, we can see that it’s just an artifact.

Skoyles and Sagan (2002: 240) note that erectus, for example, could have survived with much smaller brains and that the brain of erectus did not arise for the need for survival:

So how well equipped was Homo erectus? To throw some figures at you (calculations shown in the notes), easily well enough. Of Nariokotome boy’s 673 cc of cortex, 164 cc would have been prefrontal cortex, roughly the same as half-brained people. Nariokotome boy did not need the mental competence required by cotemporary hunter-gatherers. … Compared to that of our distant ancestors, Upper Paleolithic technology is high tech. And the organizational skills used in hunts greatly improved 400,000 years ago to 20,000 years ago. These skills, in terms of our species, are recent, occurring by some estimates in less than the last 1 percent of our 2.5 million year existence as people. Before then, hunting skills would have required less brain power, as they were less mentally demanding. If you do not make detailed forward plans, then you do not need as much mental planning abilities as those who do. This suggests that the brains of Homo erectus did not arise for reasons of survival. For what they did, they could have gotten away with much smaller, Daniel Lyon-sized brains.

In any case—irrespective of the problems that Deacon shows for arguments for increasing brain size—how would we be able to use the theory of natural selection to show what was selected-for, brain size or another correlated trait? The progressionist may say that it doesn’t matter which is selected-for, the brain size is still increasing even if the correlated trait—the free-rider—is being selected-for.

But, too bad for the progressionist: If the correlated non-fitness-enhancing trait is being selected-for and not brain size directly, then the progressionist cannot logically state that brain size—and along with it intelligence (as the implication always is)—is being directly selected-for. Deacon throws a wrench into such theories of evolutionary progress in regard to human brain size. Though, looking at erectus, it’s not clear that he really “needed” such a big brain for survival—it seems like he could have gotten away with a much smaller brain. And there is no reason, as George Williams notes, to attempt to argue that “high intelligence” was selected-for in our evolutionary history.

And so, Gould’s Full House argument still stands—there is no progress in evolution; bacteria occupy life’s mode; humans are insignificant to the number of bacteria on the planet, “big brains”, or not.