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The Loom

December 30, 2003

Why The Cousins Are Gone

paranthropus.jpg They say that history is written by the winners, but if that's true, then natural history is written by those who can write. Our ancestors split from the ancestors of chimpanzees some 6 or 7 million years ago, and since then they've given rise to perhaps twenty known species of hominids (and potentially many more waiting to be discovered). Today only our own species survives, and only ours has acquired the intelligence to learn things about the distant past--such as the fact that we are the product of evolution. Our survival and our intelligence sometimes blur together, with the result that a lot of the research on human evolution (and most of the popular accounts of it) revolve around what makes our own lineage unique and successful. All the other branches of hominid dynasty become our foil--the losers who, through their extinctions, reveal what is most glorious about ourselves. As a way of thinking, this is both unfair and foolish. We become satisfied with our own false assumptions about other hominids, and may miss some lessons they have for us. Exhibit A: our ancient thick-headed cousin Paranthropus.

Paranthropus, which existed from about 2.5 to 1.5 million years ago, was among the first hominids to be discovered by paleoanthropologists. In 1938, a young South African schoolboy led Robert Broom to a spot where he had found fossils of jaws and teeth. Broom dug up more pieces of the skull and realized that it belonged to some kind of ape. A closer look revealed that it was more like humans than chimps or gorillas. For example, the hole at the base of the skull was far forward like humans, suggesting that the creature could walk upright. But compared to the other hominids that had been found at that point, Paranthropus was peculiar for its big frame and its massive jaws and teeth. If paleoanthropologists had to pick a hominid that looked like our direct ancestor, Paranthropus was definitely not it.

Over time, as more hominid bones emerged, Paranthropus solidified its reputation as a dead end of human evolution. The conventional wisdom ran like this: until about 2.5 million years ago, most hominid species were runty, small-brained apes that were unusual only for spending a fair amount of their life on the ground. But then the climate became drastically cooler and drier. This change drove hominids into two major branches. On one branch was Paranthropus, a five-foot tall creature with molars as thick as your thumb and buttressed jaws. On the other branch were the earliest members of our own genus Homo. They were shorter, and their teeth and jaws were small. Over time their teeth and jaws got even smaller, while their brains got bigger. Around 1.5 million years ago the climate went through yet another change. The planet got so cool that it slipped into cycles of Ice Ages, altering the African landscape over thousands of years rather than millions. Many mammals became extinct as a result, and Paranthropus went with them. Homo, meanwhile, had evolved to the point where it now stood six feet tall, could make sophisticated stone tools for scavenging meat, and was even beginning to venture out of Africa altogether.

So why Homo and not Paranthropus? Evolutionary biologists have theorized for a long time that there are two directions in which organisms can evolve: they can become specialists or generalists. It's astonishing just how specialized some species can get. Think for example, of lice that live only on humans. In fact, they come in two species, one that lives only on human hair and one that lives only on the human body. Think of the aye-aye of Madagascar, a primate armed with a hideously long index finger it uses to fish out insects from hollow trees. Specialists, the theory goes, thrive only in times and places of tranquility, in which evolution can fine-tune life to fit very narrow niches. Generalists, on the other hand, can live anywhere on anything. Think of rats, cockroaches, and the like. During good times, new specialist species may emerge and thrive. But when some environmental catastrophe hits, the jack-of-all trade generalists are the ones equipped to adapt and to survive.

Paranthropus and Homo, paleoanthropologists generally agreed, were classic examples, respectively, of specialists and generalists. Paranthropus evolved the ability to crush seeds and other hard plant matter, losing the ability to feed on anything else. Homo meanwhile had to search for any food it could find, whether it was honey, tubers, or carcasses. Homo survived thanks to its generalist skills--which depended in part on its growing brain. We are what we are today, then, thanks to our generalist ancestors.

This conventional wisdom is widespread. (The web site for the TV series Walking with Cavemen offers a version here.) But just how accurate is it? The overall notion of generalists and specialists seems to hold up pretty well. A couple weeks ago in Science, English scientists reported how they had watched specialist and generalist bacteria evolve in their lab. Some of the specialists that emerged lived only on the surface of their microbial broth. But as they became more fit in their narrow niche they lost the ability to adapt to other niches. But what about the particular case of Paranthropus and Homo?

Bernard Wood and David Strait, two paleoanthropologists, took a critical look at the evidence to date--everything from isotopes in fossil teeth to species ranges. You'd expect certain things from a specialist--it should only eat a few foods, for example, and it should be more prone to give rise to new species (but those species should tend to go extinct faster than generalists). When a specialist's environment changes, it should follow its food to a new range.

All told, Wood and Strait looked at eleven different predictions. In most cases, the evidence ran counter to the idea that Paranthropus was a specialist, and many of the remaining cases, the results were simply ambiguous. "On balance," they write in a paper in press at the Journal of Human Evolution, "Paranthropus and early Homo were both likely to have been ecological generalists."

At first their conclusion looks patently absurd. The very sight of Paranthropus seems to tell you that it's dead end. But Wood and Strait point out that looks can be deceiving--especially looks based on nothing more than bones. The howler monkey, for example, has massive intestines that allow it--unlike most other primates--to eat leaves. But howlers only eat leaves during part of the year; they also eat fruits and flowers. Their intestines don't cut down their options--just the opposite has happened. Likewise, Paranthropus's huge jaws and teeth may have allowed it to crush seeds, but there's no good evidence to suggest that this anatomy prevented it from eating other food. A big bite can help you eat a lot of things. And if you want to rely on the shape of the face to decide who's a generalist and who's a specialist, then you might well conclude from the shrinking jaws and teeth of Homo that our ancestors were the specialists.

Wood and Strait are pretty agnostic about what should take the place of the conventional wisdom, but they do make a few suggestions. Perhaps the climate change 2.5 million years ago led Paranthropus and Homo into two different kinds of generalist ways of life. Paranthropus broadened its diet as it turned its head into a nutcracker. Homo meanwhile broadened its diet with tools and meat. The mystery of why we survived and Paranthropus didn't becomes dark and deep. We can't give credit toour wonderful brains for making us generalists able to live anywhere. Being a generalist doesn't seem to have been a guarantee for survival. (And Wood and Strait also point out that Paranthropus's brain actually expanded over its million-year dynasty, showing that it also had some potential upstairs.)

Today we believe that our technology has made us into the ultimate generalist, transcending nature itself, and perhaps even the planet. Paranthropus looks on our happy beliefs from its oblivion and wonders.

Posted by Carl at 10:12 PM
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