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The Second Brain Boom

What kicked in, about 750,000 years ago?

The first brain boom started with the advent of the Homo spinoff, 2.5 million years back.  Over the next 1.5 million years, brain size doubled.  Because body size was also growing (and that alone will increase brain size), this doubling in size is not as impressive as it originally looked.

            Plotting the skull sizes from all the hominid species against time, it is apparent that brain size started growing more rapidly about 750,000 years ago.  And not much of this second brain boom can be attributed to a parallel enlargement in the body itself.

            So what’s going on here?  No one knows because the data are so sparse.  It’s easy to produce a list of things that had to get started sometime in the last 7 million years.  Indeed, there are a hundred differences between the great apes and humans and most cannot be pinpointed in time.  Paleoanthropology is like a jigsaw puzzle with lots of pieces missing – and those pieces that you dig out of the ground are so “fuzzy around the edges” that they seem to fit in a number of places.

The australopithecines endured until about a million years ago, looking more and more heavily built, like the gorillas.  When they died out, it left Homo erectus as the only hominid game in town.  Starting at about 800,000 years ago, Homo antecessor was found in Spain, but no one knows whether it evolved there or in Africa – or what it was doing differently.  Homo erectus carried on elsewhere.

            While toolmaking doesn’t seem to change much at this time, several other parts of the puzzle do.  This is about when the ice age climate rhythms are modified.  The major drivers are well known.  The tilt of the earth’s axis changes from 24.6° down to 22.0° and back over a cycle lasting 41,000 years.  The month of the earth’s closest approach to the sun, when we get about 10 percent more energy, is currently in early January but it will drift around to July in another 12,000 years or so, depending on what the other planets are doing (that’s why it varies from a 19,000- to a 26,000-year cycle).  And, for similar reasons, the shape of the elliptical orbit around the sun changes from rounder to more elongated over cycles that mix a minor 100,000-year and a stronger 400,000-year component.  The three rhythms combine to produce a complex fluctuation.

            The global amount of water tied up in ice sheets can be estimated from sea-floor cores, although this does not give a very complete picture of changing climate.  Up until about 750,000 years ago, the successive meltoffs of ice were about 41,000 years apart, dominated by the tilt cycle.  More recently, the period between major meltoffs has been closer to 100,000 years and the amplitude of the cycle has been greater.

            Since the astronomical factors are unlikely to have changed strength or rhythm, and the astronomical 100,000-year component is relatively weak, the change is probably a matter of an alteration in how the earth resonates with the driving rhythms.  For example, it takes time for ice sheets to become heavy enough to depress the earth’s crust, and when they melt it takes time for the surface to spring back up.  As for why the rhythm shifted when it did, no one knows but the most recent reversal of the earth’s magnetic poles was about 780,000 years ago (and some people think we are getting close to another reversal now).

            The increase in the height of the swings suggests that other things might be going on, such as more instability of the sort associated with the big abrupt climate changes.  The paleoclimate records aren’t sufficiently good to see very much in detail except the last ice age, but at least one massive type of abrupt climate change, the Heinrich events associated with the collapse of the Hudson Bay ice sheet, can be seen as far back as 1.1 million years ago.

More demanding hunting techniques are one possibility for why brain size starts increasing faster. Perhaps techniques changed from running down prey to projectile predation.  It could be a shift from side-of-the-barn throws at large targets (say, herds bunched up while visiting the waterhole) to more accurate throws involving individual prey animals at greater distances.  It is only the accurate throws that make a lot of demands on the brain, compared to what great apes can do.

            All throws require some planning during get set, and so even the inaccurate throws might instill some preparatory  traits that could carry over to preparing for other ballistic tasks (say, clubbing and hammering).  But accurate throws really have to tune up this neural machinery.  If my Law of Large Numbers analysis in The Cerebral Code proves correct, you have to borrow some inexpert areas of the brain temporarily to assist the expert areas, much as the amateur audience assists the expert choir in singing The Hallelujah Chorus.  So the get set prelude involves a major amount of reassignment of cerebral resources, quite unlike most cognitive tasks.  Once you start refining accuracy, however, there is that long growth curve where payoff increases with each redoubling of distance achieved with accuracy.

Protolanguage is my other candidate for what might fit the jigsaw puzzle for this period.  While the burst of creativity about 50,000 years ago was originally credited to the onset of language, the last half-century of research on language origins suggests that language is a two-step affair – or, rather, a ramp of improvements followed by a big step up to structured language.  Perhaps the ramp was rising about 750,000 years ago and the step is what comes 50,000 years ago.

            Protolanguage is a distinction developed by the linguist Derek Bickerton from his studies of how unstructured pidgin languages are converted by children into fully structured creole languages.  You can see the same transition in the stages through which a child develops language.  First come words, then word combinations that have an additional meaning besides those of the component words.  Short sentences do not often require any notions of structuring to be understood, but once you try to double the number of words in a sentence, its meaning becomes quite ambiguous without some structural scaffolding that we call syntax.

            Still, you can say quite a lot with two-word sentences, compared to what other animals accomplish with their one-call-one-meaning vocalizations.  While the child’s passion for naming things might lead us to think that nouns are the big thing, the emotional expressions of other primates are a lot closer to verbs.

            Most words are a bit abstract, more categories than labels for an individual or a place.  Proper nouns are considerably more difficult, it seems, than categories.  For example, a brain-damaged patient might be able to name the make and model of cars in a series of photographs but be unable to pick out a picture of his own car.  So the evolution of language abilities might not be “naming the creatures” so much as a series of prompts for action, equivalents of “Let’s go” and “Look there.”

            Some such foundation is likely how protolanguage got started.  Unlike the onset of syntax (more in a moment), it certainly looks as if protolanguage progress could be gradual, with no really big steps needed, just a growing vocabulary and then two words paired for a third meaning.  It could have been going on for several million years, or it might have begun only 160,000 years ago with the advent of anatomically modern Homo sapiens.  No one knows.  But neocortex is all about forming new associations between concepts, and neocortical size expands more than fourfold between the great apes and modern humans.  So it would not be surprising if the novel meanings for word combinations might have promoted, or profited from, a bigger brain.

            Body postures communicate mood and intention (dogs communicate dozens), and arm or face posture sequences provide even more bandwidth for broadcasting your emotions and intentions.  Species-specific vocalizations get a big addition from culturally defined “words” (whether signed or spoken) whose learned meaning depends much more on context. 

            Word combination is just another example of context dependence, but it was likely an important step.  Yet with only protolanguage, you couldn’t say “Who did what to whom” much faster than you could pantomime it.  You’d have to make a series of short sentences rather than one compact structured sentence.  Pantomime tends to be appealing as an early stage but acting it out might not have been so common early on.  Pretense is involved, and playing a role (where your actions are to be interpreted as those of someone else) might not have come along until structured thought arose.  Still, you can do a lot without much pretense – say, pounding on something while looking at a third party might communicate to your friend what happened in her absence, just by simple association.

            It was originally supposed that coordinating hunts was a big early payoff for language – until it turned out that chimps had all the basic moves without using vocalizations.  Now it is supposed that much of the everyday payoff for language has to do with socializing and sexual selection, where “verbal grooming” and gossip become important players.  Again, it looks as if a gradual improvement ought to work, and that identifying starting times is likely to be less relevant than finding periods of more rapid progress.

            Words are tools in some sense and extend the realm of thought beyond the here-and-now.  There is one class of words that might have been particularly handy as protolanguage progressed in an era of hunting.  These are the “closed class” words (so called because, unlike nouns and verbs, it is so hard to invent a new one) that serve to orient you.  Some indicate relative direction (to, from, through, left, right, up, down) in the manner of vectors.  Words such as above, below, in, on, at, next to, and by serve to orient you relative to other objects.  In the brain, such spatial tasks tend to involve midbrain areas such as superior colliculus and the “where” specializations of the upper parts of both parietal lobes.  Because the frontal lobe tends to be involved with planning, perhaps the closed-class words for relative time (before, after, while, and the various indicators of tense) might live up there instead.

            While there is an elementary sort of directional “structure” involved here, it is not open ended in the sense of being expandable (in the manner of nouns and verbs).  The reason that it’s a closed class of words is that it is about coordinates, and you only need so many words for the four dimensions of space-time, even when you add relative terms to relate several objects and their movements (inside, beneath, alongside, atop).

            Such are not the type of recursion and nesting that constitutes the big step up to syntax – and, more generally, structured thought – with its open-ended nature.  Still, the acquisition of these “little words of grammar” would have made short sentences much more versatile and hominid mental life even less like that of the great apes.  With them, you could begin to order the world.

All humans do it.  Gossip, schmooze, chitchat, gab, talk, tattle, rap, banter, discuss, debate, and chew the fat.  Why?  To exchange information, share knowledge, criticize, manipulate, encourage, teach, lie, and self-promote.

             – Marc D. Hauser, 2000

Seeing has, in our culture, become synonymous with understanding. We “look” at a problem. We “see” the point. We adopt a “viewpoint.” We “focus” on an issue. We “see things in perspective.” The world “as we see it” (rather than “as we know it” and certainly not “as we hear it” or “as we feel it”) has become the measure for what is “real” and “true.”

             – Gunther Kress and Theo van Leeuwen, 1996