02月18, 2010

饥饿进化的人类

The spear point marks the arrival of meat on the dinner plate

Meat, Evolution And Us: Confessions of a Hungry Ape

There are plenty of moral and environmental arguments for vegetarianism. On the moral side, we know that animals want to live, (as do plants, of course…) and that killing them contradicts that desire. Even the best farms are inhumane on some level, then, because they end with killing, and there is no way to do that politely.

On the environmental front, our making over of vast tracts of land for our domesticates has been a death sentence to innumerable wild species. Even species we had not discovered yet. E.O Wilson, perhaps the most eminent of biologists, has written that it is not a stretch to imagine a not-too-distant future in which the planet is host only to us and the five species we like to eat.

But what are we meant to eat? By “meant to,” I mean what did our bodies evolve to eat? For that, surely, must be exhibit A in the what-to-eat debate? Anthropologists talk about the “mismatch theory,” which argues that we were evolved for one environment and set of habits, yet live in a totally different one with totally different habits. Most of our current diseases, including heart disease, diabetes, cancer, arthritis, to name just a few, are “mismatch diseases,” caused by this…mismatch between evolutionary background and current circumstances.

Food is high on the list of ways in which we are mismatched. For example, we evolved to crave sugars. Why? Because — in their unprocessed form — they are an important source of energy. But, and it is a big “but,” we did not have much access to sugar in the Paleolithic (when we were busy doing this evolving). Honey was occasionally available. So was fruit — seasonally and regionally. Now we have highly-processed glucose and sucrose, fructose and lactose in everything we eat. We have Twinkies. And we can buy them by the case.

But what about meat? There is no doubt that we are omnivores. Many of us are vegan for decades, and do just fine. Others consume large amounts of meat protein and also do fine. Within both of those categories are people who do not do so well, but there are good ways to be a carnivore, or a vegan, and bad ways.

I am going to argue here that animal protein played a massive role in our evolutionary history — a formative role, in fact, and that without it our evolutionary path would have looked quite different, and quite possibly would have ended altogether long before the invention of the wheel, or even the bi-faced hand axe.

The place to start looking for evidence here is the morphology of early hominids. In geographic terms this means specifically East Africa, about 2.5 million years ago. Until roughly this date the hominids that inhabited parts of the African continent were probably mostly plant-eaters. The emphasis here should be on the words “probably,” and “mostly.” The reason for this is that when tools show up in the fossil record (that is to say stones which appear to have been purposefully shaped) we can safely assume that the creatures doing this were using them to get at marrow and strip flesh from bones, in other words they were after animal protein. Before this technology was developed, eating any meat was more of a challenge.

Scientific consensus for when this happened seems to be around 2.5 million years ago, and this associates it with the being known as Homo Habilis, or “Handyman.” This could have happened earlier, and if so it would have been associated with a slightly different type of hominid (or what some more laid-back scientists allow us to call “ape-men”). Australopithecus, or “southern ape,” shows up in the fossil record about 3.5 million years ago. The best- known example of this creature is of course Lucy, so named for the song “Lucy in the Sky with Diamonds” playing on the anthropologists’ radio when she was discovered in 1974. Lucy was certainly more ape than human, only about 3–4 feet tall, and quite hairy, with a sloping, chinless face and not much of a nose.

For further information on her diet, you’d need to look at Lucy’s teeth. Dental remains allow us to get an idea of her diet. In this case, her dental “microwear” suggested that she was mainly a plant eater, able to manage harder foodstuffs in droughts — bark, stringy plants, etc. Her consumption of animal protein, however, was probably limited to small mammals and insects.

So far, then, from Lucy through Habilis, we (I use the term loosely) were not really “carnivores.” We might have been occasional scavengers, not eating meat in quantities or regularly, and certainly not killing anything big ourselves for the pot. And there were no pots.

But lets look more closely at Homo Habilis. Her invention of stone tools (we’re not talking about a whole chest of ’em here, just a couple of variations — the “Swiss Army knife of the Paleolithic”) was a game-changer. Previously, hominids like Lucy were kind of vulnerable. Here’s the linguist Dereck Bickerton describing the situation: “It is more than likely that some of our ancestors suffered the ignominious fate of being eaten by weasels. Some of them, its pretty certain were eaten by birds.” They were largely helpless, in other words, and relatively small, in a very dangerous world of large predators.

But Habilis had not discovered weapons — only tools. So how did a rock with a somewhat sharp edge help matters? No, it could not kill a leopard, or hyena. Its benefit was more circuitous, but it looks something like this: The small amount of animal protein gleaned by eating bone marrow and scraps of scavenged meat led to a feed-back loop along the following lines: more meat=bigger brains. Bigger brains=more meat.

Taking advantage of meat protein was also a much more efficient way of acquiring energy for those who could get it. Daniel Lieberman of Harvard University, and his colleagues, have determined that full-time vegetarians like the australopithecines would have needed to go through 15 million “chewing cycles” in a life time. That is to say they spent almost all day chewing tough things like tubers. This is how gorillas spend their time, deriving enough energy for their large bodies from relatively inefficient foodstuffs. Luckily this works just fine for Gorillas, who are able, in their natural environment, to find said foodstuffs, and have enough time in their schedules to chew it. Anyone who has tried a raw diet will attest that it requires that you stay at the table, chewing, long after the cooked-food eaters in the family have moved on.

The energy available from meat, especially pounded, and later cooked, was ounce for ounce considerably greater than that of vegetation. As we added meat to the diet, we therefore had time for activities beyond chewing.

While we started out as wholly amateur carnivores, as this line of hominids evolved, nature selected for bigger brains. This was a game changer. The biologist E.O. Wilson considered the evolution of the human brain the most important development in human history, as it allowed for everything downstream of it, most notably the social cooperation that is touted by evolutionary biologists, among others, as being the key human attribute.

This developing of bigger brains was never a no-brainer (excuse the pun), however. If it had been, all mammals would be brainiacs by now. As it was, only the hominid line showed extraordinary growth in brain size over the millennia. The pay off was long-term, but it was significant. The down sides were also considerable. The brain is an energy-hungry organ. Accounting for some two percent of our body weight it consumes about 20 percent of our daily energy. This means it needs extra calories. And the head that is required to store such an organ must be large, causing severe risks for childbirth, especially in a species with the kind of narrow hips that bipedal creatures have. Notwithstanding these risks, nature selected for large brains because those individuals with them tended to survive to reproductive age, and this is the only criterion that matters in evolutionary terms.

The Habilis brain averaged 650 cc, versus the 3–400 cc of earlier australopithecines. Habilis survived for hundreds of thousands of years. But by 1.8 million years ago we see a new face on the block. This one is a little more “human” in appearances. Homo Ergaster, or “working man,” was a different species, and recognizably closer to modern humans. As the historian Daniel Lord Smale puts it: “If they chose to have sex in public you would feel shocked or embarrassed rather than giggly.” This creature was about 6 foot tall, fully upright, a little less hairy, and they were working the stone tools effectively, even creating a refinement of the bone-basher, a bi-faced hand axe, which scientists call the “Acheulian” hand axe, from Acheul in France where early examples were found. They were distributed in a massive area from Africa to Europe and western Asia.

The spread of this tool is relevant because this is the species which many believed first left Africa, our ancestral birthplace, and started colonizing other parts of the globe. Diet continued to be of vital importance to their brain and body function, and meat was central. Ergaster, still feeding off of roots, tubers, berries and nuts, was also scavenging meat more widely than its predecessors, and possibly even cooking it, which allowed digestion of tougher materials and the release of more nutrients.

The social consequences of cooking have recently been showed to have been vital in the making of human social life as well — the hearth being perhaps the earliest human institution — and cooking itself created pair bond relationships and wider social connections. Apart from the morphological changes — height, facial features, etc. — Ergaster also exhibited social and behavioral changes. Clearly they had undergone evolutionary shifts. “Whatever selection pressures favored these shifts,” say Lieberman et al, “they would not have been possible without increased meat consumption combined with food processing technology.”

The on-the-ground reality of scavenging teaches us a lot about early human evolution. And it was harrowing. Dereck Bickerton has described what a scavenging operation might have looked like at around this time:

A large mammal, say a prehistoric elephant, has been located, dead. “Its hide is still intact, but other scavengers have arrived already — lionlike or tigerlike creatures, some larger than those of modern times.” Someone runs off to gather the tribe, all of them — men, women and children. “To go in without the women…would be like tying one hand behind your back.” Once everybody is ready, the men approach first — not having wombs, they are dispensable. They shout and wave their arms, hurl stones at the other animals. The women then advance, and go for the carcass. If the men are the body guards, the women are the technicians, carving slabs of meat off it with their hand axes and slicers. The danger is very real; people get hurt, maybe even killed. But the meat calories, as well as the bones for crunching or burning, represent weeks of resources for the tribe — a huge haul.

Ergaster’s braincase was about 800–850cc. And they put the extra processing power to use. Coordinating such a scavenging operation took brains, in particular the kind of brains that can use language effectively. Bickerton jokes about how for the most part his students sit and listen to him in the lecture hall. Try doing that with a bunch of chimps: Thus language as social control. The evolution of language allowed for the maintenance of increasingly large groups of people, and it is with numbers that the human population gained a survival edge in a dangerous world, eventually graduating from scavenger to hunter.

With the arrival of later species like the Neanderthals, hunting becomes a full-time job. Neanderthals were the successor species to those species that had left the African continent and spread out, eventually showing up in Europe. They were a very successful species, if measured in terms of millennia. For some 500,000 years they eked out an existence as carnivores, hunting primarily at the edge of the frozen wastes of the Ice Age. On the Tundra on the edge of the Boreal Forests was where the large animals lived, and so this was where Neanderthals made their homes. And they were big meat eaters, in both senses of the term. Their brains were also big, bigger, by many accounts, than ours, even, at around 1500cc.

Whereas once believed to have been our direct ancestors, it turns out, however, that Neanderthals were more like cousins, descending from a line of ape men that left Africa just under two million years ago, through several successive species. Homo Sapiens, on the other hand, while sharing ancestors with Neanderthals, first appeared in East Africa about 200,000 years ago. This is where all DNA evidence points, suggesting that all modern human populations are descended from populations in this region.

The state of our knowledge of the finer points of our evolution is still decidedly murky. For instance, one theory of the evolution of Homo Sapiens posits a near-extinction event about 70,000 years ago. Around this time Mount Toba, a “supervolcano” on Sumatra, erupted. This was the largest surface eruption the Earth had experienced for 400 million years. The debris disgorged amounted to 3000 times the amount seen during the Mount St. Helen’s eruption in 1980. It is likely that global temperatures plummeted between 15 and 20 degrees Fahrenheit, as the ash cloud filled with sky with over a billion tonnes of atmospheric dust, and a nuclear winter ensued which lasted some five years.

Among the traces of the contemporary human population archaeologists have found signs that radical changes occurred at this time. That Mount Toba erupted then is not in dispute. What is, however, is whether it was the cause of a massive drop in our species’ numbers. Retrenching to the southern tip of Africa, the human population, according to some experts, may have been as little as 10,000 globally at this point. But huge changes were afoot. Among them (to return to the question of meat) leaps in hunting technology. If there were doubts about hominid hunting prowess before now, they are over. Digs in the Blombos Caves of South Africa have revealed debris from this period, including shellfish and game from the surrounding countryside, as well as the earliest stone tools, including foot-long spear points, daggers and fish hooks. The bones represented there included giant cape buffalo, wildebeest, zebra and bush pig, among others, all either large and/or fierce, and requiring skill and cooperation to hunt. Putting all this evidence together revealed a picture of a human population that had made a transition to big-game hunters.

Meat, in other words, was central in the evolution of our species. Possibly even crucial. It is hard to imagine how we would have evolved the same way, or even survived as a species, as vegans or even vegetarians. These are positions we can maintain now, now that we have reached a certain evolutionary position.

But this, of course, doesn’t mean we should continue as carnivores. If meat eating early in our evolutionary journey allowed for brain growth, the brain took it from there, and cultural evolution gave us the ability to eat meat though large group cooperation and hunting technology. Selection pressure, especially if we faced at least one near-extinction event, in a reduced gene pool, made cultural and biological traits mainstream in the human population. But today, cultural evolution is exactly what has made it possible for us to lay off the animal flesh altogether. With the ability to supplement our diets with synthetic nutrients, and the ability to grow animal protein in the lab, the killing of animals may one day be history. But not because we were never “meant” to do it. Only because we have evolved out of that particular need.

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