A just-so story, of course. We can’t know whether anything like this really happened, or even if it did, how often such scenes took place before familiarity bred courage and the first attempt on a megacarcass was made. If things happened that way, the result of that attempt must have been successful enough to be repeated, and once the strategy was established, everything had to change.
For the new strategy unleashed a cascade of changes.
Those changes may well have been more rapid than the changes brought about by geographic separation. It seems reasonable to suppose that changes driven by animals’ own purposive actions would move faster than changes due to genetic drift or even to changed selective pressures. It was precisely such sequences of rapid change, followed by long periods of apparent evolutionary stasis, that gave rise to the theory of punctuated equilibrium.
WHY EQUILIBRIA GET PUNCTUATED
For decades this theory has remained controversial. Now, as with so many things, niche construction theory puts a new light on it by supplying an explanatory mechanism.
It is, and remains no matter what anyone says, a fact of life that most species appear rather suddenly in the fossil record and thereafter change relatively little, if at all, until they go extinct. This fact was what led Stephen Jay Gould and Niles Eldredge to launch, in 1972, their theory of punctuated equilibrium—that evolution moves in spurts interrupted by long periods of stasis. I call it a theory because everyone else does, but in fact, if it’s even a theory at all, it’s one at the lowest level of theories—a descriptive generalization that does no more than summarize a set of facts. Real theories at least try to explain what they describe. But all the brouhaha pro and con punctuated equilibrium was about whether it really happened or not. No one seemed to notice that no cause for it had ever been suggested by Gould or Eldredge or by anyone else.
Indeed, the alternative to punctuated equilibrium, put forward by Richard Dawkins, didn’t have an explanatory mechanism either. Dawkins declared himself in favor of what he called “variable speedism.” Sometimes evolution went very fast, sometimes slower, sometimes very very slow, so slow you might think it had stopped until you remembered your dogma: like Monty Python’s Norwegian parrot, evolution hadn’t ceased to function, it was just taking a nap. Evolution never stops!
The trouble was, Dawkins didn’t have anything that would explain why the speed of evolution varied from one time to another. But any theory worth its salt that sets out to describe a set of effects is expected also to provide a mechanism sufficient to cause those effects. For example, Alfred Wegener’s theory of continental drift made perfect sense even in the absence of any mechanism, but everybody in the business rejected it. (It didn’t help that Wegener was a meteorologist, not a geologist.) His theory was nonsense! Have you ever known a continent that drifted? What power on earth or in heaven could make it drift?
Wegener’s problem was that he never came up with a mechanism to explain why continents drifted, just as Gould never came up with a mechanism to explain why evolution should alternate between rapid change and stasis. Then plate tectonics was discovered, and everyone suddenly saw that continents just couldn’t help drifting.
In the same way that plate tectonics explained continental drift, niche construction explains the otherwise inexplicable stop-go-stop of evolution. A species goes merrily along its way, happily settled in its old niche. Then something in the environment changes; survival demands that a new niche be constructed, real fast. But once that niche has been constructed, when new species and new niche fit like a hand in a glove, what are you going to do? Expand the hand and burst the glove? You stay the way you were, as long as the niche lasts.
Niche construction theory also explains why, since the last common ancestor of humans and apes, there have been so many speciations in our line and so few in the ape line. The ape branch lived in an unchanging environment and stayed happy in the niches it already had. Our branch was forced, at first, and chose, later, as its capacities broadened through successive constructions, to construct more and more new niches. That’s why it changed so much and so fast (something else that, before niche construction theory, nobody could explain). Successive niche constructions meant we could evolve in place, without waiting for geographic separation to trigger the speciation process. The process of niche construction was what drove our successive speciations and made us what we are.
But between construction jobs there were long spells of unemployment. That’s why our forebears used the same old hand ax for a million years.
THE OPTIMAL FORAGING STRATEGY
Let’s now look at this particular speciation, the one that turned bone-crackers into hide-cutters, a little more closely.
The problem is that niche-constructed speciations are triggered by changes in behavior, and behaviors don’t fossilize. All we can do is infer them from bones and tools, plus what we know of the habitat—climate, terrain, vegetation, and so on.
The habitat dictated foraging behaviors. It imposed two constraints, constraints that pulled in opposite directions.
On the one hand, the risk of predation called for an increase in the size of foraging groups, for protection. We see this effect in other terrestrial primates, such as the various species of baboon that travel everywhere in large groups and roost at night in still larger ones. The smaller the group, the greater the risk of predation. To forage singly or in pairs would have been even more disastrous than foraging in small groups.
On the other hand, the widely scattered and unpredictable nature of food sources made large-group foraging the least efficient mode for our ancestors. True, they were omnivores like baboons, but they couldn’t digest grass, and baboons can. That means baboons can usually find enough to eat in a relatively small area; human ancestors couldn’t. Suppose that initially they tried to forage in a large band, but found that even the largest area they could cover in a day supplied only enough food for a fraction of the band. They would then have had to break up into smaller groups.
Some balance had to be struck between the pressure to increase group size (predation) and the pressure to shrink it (scarcity of food sources).
The balance would have been different for low-end scavengers and high-end scavengers. In the absence of hard evidence, let’s see how far logic will take us in determining what foraging strategies were best.
Consider range size for the two species. To be more specific, consider day-range size, since there is no necessary connection between the area a group may cover over the course of a year and the ground they may be forced to cover in a single day, if they are to find food enough to survive.
Now consider the relative distributions of bones and recently dead megafauna. Bones last indefinitely; in the absence of any other species of bone-marrow extractors, bones that had lain for months or even a year or so could potentially be exploited. In contrast, the meat of dead megafauna lasts a few days at most. It follows that, on any given day, there will be, in any given area, more bones than there are dead megafauna.
This is what makes catchment scavenging possible. A smaller area is required for it, hence a smaller day-range. If animals can subsist on a smaller day-range, they can afford to increase their group size and thus maximize safety from predation. Baboons with their grass went this route; we may reasonably assume that bone-cracking hominids did the same. But high-end scavenging demands a much larger day-range, for two reasons. First, hominids have to cover a much wider area if they are to locate large carcasses. Second, if they fail to find a carcass (as may have happened on a large majority of days) they have to revert to omnivory to keep alive, and even for an omnivore, savanna food sources are far from plentiful.
Accordingly, it would make no sense for a large group to forage together. Most days they wouldn’t get enough for all of them to eat. The only strategy would be to break up into smaller groups. For instance, suppose there was a foraging band of forty individuals that might meet with other bands at night for added protection. If the fir
st band split into groups of, say, eight individuals, it could cover five times as much ground in the same time.
We have no hard evidence that our ancestors foraged in this way, and it’s difficult to see how there could be any, or even what hard evidence might consist of, in this case. But the balance of probabilities is strongly in favor of the kind of strategy I’ve described. It was, after all, only a version of the fission-fusion foraging strategy common among primate species generally. And it sets the stage for a unique adaptation, one that emerges quite naturally from the situation our ancestors found themselves in, once they’d discovered that tools they could easily manufacture could turn them from humble gatherers of the leavings of others into active competitors with the most savage among the beasts.
FASHIONS IN FORAGING
It’s ironic, really, when you consider the history of our prehistory, how it’s been colored by swings in cultural fashion. In the still male-dominated seventies, man the hunter reigned supreme. The fact that, until well under a million years ago, we didn’t have the weapons to hunt big game with was studiously ignored. Then with feminism came woman the gatherer, who supplied the bulk of the diet in terms of fruits, berries, edible veggies, and the like. The fact that, in a Pleistocene savanna, you’d never get enough of these foods to keep you alive was, again, studiously ignored. Nobody ever lets facts get in the way of a culturally appropriate theory.
Meantime, man the hunter had been demoted to man the scavenger. Many men were upset. What a comedown for those prehistoric heroes! No wonder that when both dentition and arguments from gut size indicated that, from around two million years ago, meat formed a significant part of hominid diet, man the hunter came surging in again on a macho backlash.
What were supposed to be the options for paleontologists were starkly put by Craig Stanford, chair of anthropology at the University of Southern California: “Did bands of early humans courageously attack and slaughter large and dangerous game, or did they nervously creep up to decomposing, nearly stripped carcasses to glean a few scraps of meat and fat?”
Well, neither. Those bands engaged in what has been called “aggressive scavenging” or “power scavenging,” a third option that Stanford seems to have been unaware of. And here’s where the irony comes in. These protohumans were engaging in an activity far more risky, requiring far more macho hardihood than merely (however courageously) hunting big game. True, the latter would have put them in competition with major carnivores, but not normally in direct combat with them. Power scavenging did just that.
And, in a final ironic twist, women most likely did this macho thing too.
We’ve finally come to the part where I said, at the end of the last chapter, that I’d have to go past the bounds of the certainly known and the all-but-certainly inferable. I could avoid that; I could give you a prosaic summary of how things might have gone, with all the “perhaps”es and “probably”s and “could have”s that academic modesty dictates. Instead I want you to live these moments—I want you to imagine yourselves, at this most critical juncture in our evolution, out there with the forerunners of humanity. I want you, in so far as that’s possible, to experience it as they did.
THE MAGIC MOMENT APPROACHES
Imagine that we’re together, you and I, in this small group, eight of us, drawing breath in the scant shade cast by a thorny tree. Apart from a small rabbit-sized creature, a few lizards, and a handful or two of withered figs that we squabbled over, we haven’t eaten today and it’s getting on for noon. As we sit, briefly, we scan the rolling patterns the wind makes in the tawny grasses, and the white-on-white sky where motionless streaks of cirrus fail to impede the burning sun, watching for any sign that there might be food out there.
Suddenly one of us lets out a shout and stands erect, pointing. There to the west, not too far away, a circling vulture has come into view, followed by another, then a third. We begin to jabber excitedly, pointing not only toward the sky but to one another, and gesturing. Then we begin to move.
There’s a hill a mile or so away, low, but it should be high enough for us to see what the vultures can see. We move toward it at a steady, loping run. The sun beats down on us and we’re starting to get thirsty, but that’s par for the course. We can run like that all day if we have to, not too fast, a steady pace that eats the miles. In perhaps ten or fifteen minutes—minutes? Who knows what they are?—we reach the summit of the hill, lie down, and begin to crawl. Parting the grasses, we look down, and there . . .
There, in a patch of marshy ground, lies the corpse of a huge deinotherium, a prehistoric elephant. Its hide is intact still, but other scavengers have arrived already—lionlike or tigerlike creatures, some larger than those of modern times. A pack of big protohyenas prowls the perimeter of the scene. The vultures drift on the wind, tightening their circles. One or two of them land, taking off quickly as a saber-tooth lunges at them. One lands near us, stares curiously at us with limpid and strangely innocent blue eyes.
We look at one another. We break and run, you one way, I another. Nobody tells us which way to go. If one goes one way, another goes a different way. There’s nothing so few of us can do in the present situation. We need numbers, as many as we can get. And we need them now.
HOW DO YOU GET PRELINGUAL PRIMATES TO DO STUFF?
When I give talks on language evolution, I often start by asking my audience to imagine that, instead of a hundred or two members of our own species, the hall is filled with similar numbers from any other primate species. “You will sit here,” I tell them, “unless you have urgent business or are outraged by something I say, for the next fifty minutes or so, and you will not move or utter a sound until I finish speaking. What do you suppose would be your chances of getting an equivalent number of apes to stay like that for fifty seconds, let alone minutes?” The answer, of course, is zero.
Language is, among many other things, an unparalleled instrument of social control. There’s no coercion involved; I can’t have you arrested if you leave, or talk, though if you start throwing things security will probably come in. Cultural norms and expectations take care of it all. But without language, those norms and expectations would not exist.
The biggest problem facing our ancestors as they ran in search of recruits was therefore that of getting those recruits to act as a unit. They had somehow to convince members of the other subgroups that made up their band (and probably also members of nearby groups belonging to other bands) that they should stop whatever they were doing and go after some mysterious target that they could not see, hear, or smell. Why should they? Energy is at a premium in all lifestyles but today’s. Until there were machines with power sources external to the animal, to spend energy on anything that didn’t have an immediate payoff—whether in terms of calories or mate access or rank within the group—was purely wasteful and potentially suicidal. And primates with smaller brains than these ancestors of ours were fully able and willing to deceive one another. Suppose a group’s just discovered, say, a nest of wild bees, and is trying to figure out how best to access the honey with a minimum of stings. Suddenly you come running up to them, gibbering, pointing, and waving wildly. You seem to want them to go with you. What for? Why should they?
It looks like the only way you could get them to go with you would be by telling them what you have found—several days’ or even weeks’ supply of the most nutritious food around. But you have no language. What can you do?
You are in exactly the situation the ant is in when it finds a large dead caterpillar that it can neither consume alone nor move. Actually you are in a worse situation, because ants are eusocial and you aren’t. Ants are all siblings, but with more genes in common than any primate pair has. On top of which, their brains are too small for them to have minds of their own. Yet they still need to know what they’ll be going for, get a sample of it, before they’ll move.
You can’t give a sample here because of a catch-22—you can’t get near the carcass without the help you’re tryi
ng to recruit. You can’t lay a scent trail to it because our much more remote ancestors lost their capacity to deposit and read smells, even in urine, when they took to the trees.
All you can do is imitate whatever species the dead animal belongs to, imitate the sound it makes or the way it moves, or mime some prominent feature of its anatomy.
But that’s iconic, you say, and language is symbolic.
Well, even today our vaunted, sophisticated faculty of language isn’t ashamed to use iconicity when that’s the only way to go. What did the Saramaccan, a group of escaped slaves in Suriname, do when they saw hummingbirds? For most of the animals they met in the tropical rain forests of South America, they could find some analogue among the African animals they’d known back home, so they could give the new animals African names. But there’s nothing like a hummingbird in the Old World, and they’d escaped before they’d had time to learn what massa called it. So the word for “hummingbird” in the Saramaccan language is vumvum—a conventionalized version of the sound a hummingbird’s wings make as it hovers before a flower, a typical piece of iconicity.
Nor do we disdain iconicity when “helping” children to acquire language with words like “bow-wow” and “baa-lamb.”
Far more important than what kind of signal was used is the fact that using any kind of signal to indicate an animal carcass at perhaps several miles distance that you’d seen some hours ago would be the first clear case of displacement outside the hymenoptera. Some who write about language evolution make far too much of arbitrariness—the fact that, in today’s languages, words hardly ever look or sound like the things they refer to. But the same is true of many ACS signals—a large majority of alarm calls, for example. On the other hand, outside of bees and ants, no ACS signal achieves displacement.
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