The point of that post was to note how free-lance speculation about what a trait evolved 'for' is all too rampant in evolutionary biology, taken too seriously, and often without support or when it is very easy to develop (conjure up?) comparably plausible alternative stories.
Now, recently in the journal PNAS , a similar kind of story has appeared, concerning the evolutionary consequences of cooking and otherwise preprocessing food. Well, they say it better than we can, so here's the abstract to the paper:
Unique among animals, humans eat a diet rich in cooked and nonthermally processed food. The ancestors of modern humans who invented food processing (including cooking) gained critical advantages in survival and fitness through increased caloric intake. However, the time and manner in which food processing became biologically significant are uncertain. Here, we assess the inferred evolutionary consequences of food processing in the human lineage by applying a Bayesian phylogenetic outlier test to a comparative dataset of feeding time in humans and nonhuman primates. We find that modern humans spend an order of magnitude less time feeding than predicted by phylogeny and body mass (4.7% vs. predicted 48% of daily activity). This result suggests that a substantial evolutionary rate change in feeding time occurred along the human branch after the human–chimpanzee split. Along this same branch, Homo erectus shows a marked reduction in molar size that is followed by a gradual, although erratic, decline in H. sapiens. We show that reduction in molar size in early Homo (H. habilis and H. rudolfensis) is explicable by phylogeny and body size alone. By contrast, the change in molar size to H. erectus, H. neanderthalensis, and H. sapiens cannot be explained by the rate of craniodental and body size evolution. Together, our results indicate that the behaviorally driven adaptations of food processing (reduced feeding time and molar size) originated after the evolution of Homo but before or concurrent with the evolution of H. erectus, which was around 1.9 Mya.Apparently we began keeping the home fires burning way back at the beginning.
|Campfire, Wikimedia Commons|
If this is an adaptive explanation, that is, based on natural selection, for these findings, it implies that some individuals in the Australopithecine group caught and gobbled enough meat that they had a systematic reproductive fitness advantage because of their diet. We say 'some' individuals because if we had said the whole group we'd have crossed the line into the Never-Never Land of group selection. So the scenario had to be that sometime in pre-history George and Georgina Goomp, lonely and childless, gazed enviously at Don and Donna Drmilg, surrounded by their bevy of plump-bellied children, chewing dreamily on medium-well WildeBurgers.
Such stories conjure up the usual robust idea of reliance on meat, which presumably they could get because of their evolving mental powers, even though it is not clear that they had the wherewithal not just to hunt and scratch matches, but to hunt enough, regularly enough, to nourish on tasty viands often enough to have had such a reproductive advantage. Again, as with our previous commentary on the erotimanii, the argument seems to have been bolstered (somehow) by the absence of actual evidence for cooking at the time. The glamorous Man the Hunter is a hard canard to give up. Never mind the details!
Now we're having a bit of sport here, because this is a blog, not a journal. There is no doubt that humans do differ from other primates and our dentition, body size, and other traits are clearly part of that. The authors of the PNAS paper did a sophisticated job of analyzing the statistical strength and consistency of patterns of species relationships among us and our primate relatives, with patterns of morphology and feeding times. The data, methods, and analysis are clear. But whether our morphology inherently had to do with feeding duration or whether even if it did it was the result of natural selection, and whether that selection had anything to do with cooking are less than obvious. And that is where speculation or even pure story-telling begins.
Part of the evidence is the presence of small molar (back, grinding) teeth relative to the metabolic demands of a brain thinking away at a fevered pitch. But it is not enough to say that the Australopithecines have small teeth because they did not need big teeth to mash up tough plant material. There had to be an advantage to small molars. But what could that have been?
The subtle underlying idea is that small teeth were not sufficient for slow, grinding up of tough, low-energy plant foods that their hunting-capable, fire-discovering brains demanded. But the Australopithecines' brains weren't all that different from the brains of the more or less contemporary Hominids, who had big teeth. They did, it's true, eventually become extinct while we didn't. The common evolutionary theory is that everything must have got here by natural selection, and something not used will eventually mutate away because there would be no selective pressure to maintain the metabolic cost of keeping it. Thus, as the frying pan heated up, the teeth became smaller. That's the micromanagement view of natural selection, and may seem sensible on the face of it. We're not partial to that view, but even on its own terms there are problems.
Relaxation of selection would normally be expected to yield mutantly decrepit or lost structures, not the smaller but otherwise normal, fully functional, well-formed teeth that the Australopithecines had (and we have). The strict selectionistic idea is that maintaining such structures costs the individual and if it doesn't need them there's no selection pressure to keep them around, and mutations in tooth-building genes will eventually destroy them. There's not much evidence for mutational chaos in hominin molars as they evolved smallness, and in fact there are developmental genetic reasons why the dentition can become smaller without being degraded. Be that as it may, why wouldn't it be worth the trivial metabolic cost of making big molars, for the rainy day when you couldn't make a fire or the hunt was a bust? At least you could still munch on tubers. Once developed, teeth make very few energetic demands on the individual. That fall-back ability might, in fact, seem like an advantage!
Given these kinds of problems one can ask--one always should ask--whether some alternative hypothesis might provide as plausible an evolutionary explanation, and of course MT is right here, as always, with the answer:
Our ancestors evolved by eating insects!
We confess we've never been to Africa, but it is likely that even 2 million years ago arthropods were everywhere, all year long. There are countless types of catchable insects for a varied diet. They're a mighty source of protein, have a nice snappy crunch to them, and if you don't get grossed out by wing bits between your teeth, they can satisfy the hungriest appetite. And, remarkably, while they are delicious fried and chocolate-covered, you don't have to cook them!
Our argument here is strongly bolstered, as in the case of our earlier explanation of the Australopithecine hand, by the complete lack of actual evidence for insectivory, which by precedent with other evolutionary stories seems to lock down our scenario tight as a moth's cocoon. It's not as romantic as heroically bringing down a giraffe for dinner, and since we in the west forbear the eating of these hexapods and octopods (much less centipedes!), it's easy simply to assume that our forebears didn't either. But there is little if any substance to such prejudicial judgments. And we haven't even mentioned nice, meaty worms!
Circumstantial evidence can certainly be used to formulate hypotheses in science, and there are precedents, such as the theoretical prediction of the existence of Neptune from oddities in the orbit of Uranus, and one could argue that the kinds of Platonic shadow-evidence we have in the fossil record may be analogous. Fair enough in principle, and the cooking scenario is certainly a plausible one. But it's not the only one, and we think it's safe to say that the the theoretical basis from which to extrapolate from circumstantial evidence in evolutionary anthropology is not quite as rigorous as physics.
Again, the point here is not when our ancestors began to cook their bacon, which certainly happened long ago, but the widespread lack of restraint before not only cooking up simple Darwinian explanations for complex traits, nor in marketing them with blaring trumpets to the hungry media who are always ready to splash a simple story to the eager public. This kind of evolutionary story-making is not new to our field by any means. Perhaps it's typical. It's a (legal) way to have fun and make a living. But it shouldn't be portrayed as science.
But on this note, it's time for another episode of....
The Misadventures of P'Qeeb: "Me smell bacon!"
Sniff. Sniff. P'Qeeb's ready nose (see earlier related comments here) caught his attention.
"Me smell bacon!" he said.
"Can't be bacon, " replied Ugmup confidently, "pigs not yet evolved."
"Me smell smoke, too," retorted P'Qeeb, irritated.
The two hulking paranthropine Hominids and their slow-witted cousin Slthmch crept carefully through the trees towards the enticing aroma. Then they were stopped in their tracks by a strange, loud buzzing sound.
"Sound like a bee--a very big bee," said Ugmup.
"Youp! Must have very big stinger!! Me go home," said Sthlmch, horrified at the images it conjured in his limited mind.
But the buzzing stopped, and all was quiet again. Even Sthlmch felt safer, and they moved closer to the puffs of smoke they could see drifting through breaks in the trees. With each step the savory aroma also grew stronger, drawing them forward. Finally, keeping well hidden, they were able to see the Australopithecines' camp. The males were circled around a fire, the females close behind them.
"That the smallest forest fire me ever see," said Sthlmch, "a forest fire on the ground!" And they realized to their surprise that this forest fire was burning not whole standing trees but only smaller branches. Who had ever heard of such a thing?
Soon Ugmup found the explanation: laying on the ground near the fire he spotted a ring of sharp small objects wrapped along an oblong piece of shiny material, all connected to a lumpy end that seemed to have a handle on it. Ugmup nodded knowingly, and pointed to this object, but could do no more, because their primitive language had not yet got a word for chain-saw. (but that at least explained the buzzing they'd heard)
The males squatting closely round this small forest-fire-on-the-ground each held a stick on which were impaled a few greenish globular objects sizzling just above the flames.
"What they doing?" asked P'Qeeb.
As they gazed in curious amazement at the tiny forest-fire, Ugmup's attention was naturally drawn to the females--because of their big, beautiful pair of .... hands. He saw that they were busily placing green leaves on something flat and whitish, and adding bright red juicy objects atop the leaves.
This previously unknown sight naturally confused Ugmup until he was struck by a flash of comprehension. He looked at the sun blazing high in the African sky. "It lunch time," he exclaimed, "and they making BLTs--beetle, lettuce, and tomato sandwiches!"*
"Right!" Sthlmch and P'Qeeb in whispered chorus. But although the sight and smell made them salivate desperately, they knew they were not powerful enough to attack the more advanced Australopithecines and seize their savory meal. So, rather than remain frustrated in hopeless temptation, they turned to wend their way back through the woodland, all as quiet, and as contemplatively as was possible with their rudimentary brains.
"Me still hungry....but very sad," remarked P'Qeeb after a while, as they walked along. His hairy stomach gurgled loudly.
"Why you sad?" asked Ugmup.
"Why P'Qeeba no make me forest-fire-on-ground? Why she no make me BLT?" asked P'Qeeb, a tear running down his cheek.
"Because....because she....she no have...." started Ugmup sympathetically. But then he stopped. He understood the reason, but could explain it no further because, as we said, they had no word for 'chain-saw'.
So they trod somberly through the trees and out into the savannah.
*NOTE: This is obviously only an allegory, not a true story. Tomatoes didn't get to Africa for another 2 million years.