Teethmarks

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Cold hard facts
Isotopes are good for more than tracking wild butterflies. How about shedding light on human evolution? When, for example, did our ancestors start walking upright? The evolution of bipedalism marked a key change in lifestyle made by none of the existing primates, our closest living relatives.
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In visible light (left) this rhinoceros tooth looks pretty uniform. But when illuminated with high-energy electrons (right), a white border and streaks show where different minerals entered the tooth since the rhino's death. An accurate sample must come from areas that have not changed since death. Courtesy Holly Reeser, University of Wisconsin-Madison department of anthropology.
The questions of when and why bipedalism developed is one of the most intriguing in human evolution. Almost as much as the large brain, bipedalism set us apart. It allowed us to use our hands for making tools and generally getting into trouble. Now comes evidence from ancient Africa, the birthplace of our ancestors the hominids, pointing to the relation between changes in climate and the origin of bipedalism.
How do we tell exactly what climate a creature lived in? If the fossils did not move after the animal's death, they can be dated by the rocks in which they are found. But fossils often wash away and are found mingled together in big heaps, making them tough to date. And lacking a date, you can't identify what climate an animal enjoyed -- or endured.
Isotopes, it turns out, offer a direct approach to determining the climate. Isotopes, you'll recall, are atoms of one element that can be separated based on their different masses.
Icy isotopes
A study by Margaret Schoeninger, a University of Wisconsin-Madison professor of anthropology, is using isotopes to examine Australopithecus anemensis, a likely human ancestor that walked upright across the landscape east of Lake Turkana in northern Kenya 4 million years ago. From looking at two oxygen isotopes in fossilized teeth, she's unraveling the global and local climates in which the teeth formed.
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Ocean water, which produces most of the moisture in the atmosphere, contains a mix of O¹⁸ [red arrows] and O¹⁶ [blue arrows].
Rain over the tropical ocean has a high proportion of O¹⁸.
Precipitation near the poles (the source of polar ice) has a higher proportion of O^16..

The oxygen isotopes under consideration, O¹⁶ and O¹⁸, both originated in water that, as Schoeninger explains, came mainly from the tropical ocean. (Diligent readers will recall that the hydrogen used in monarch butterfly migration also came from the ocean.)
After ocean water evaporates, water molecules containing the more massive O¹⁸ atom fall as rain sooner than water with the more common O¹⁶ atom. Thus rain in tropical regions has a high ratio of O¹⁸ to O¹⁶. As the water vapor moves toward the poles, the ratio of O¹⁶ increases.
During ice ages, lots of O¹⁶ is locked up in the polar ice. So during cold eras, rain everywhere on the planet has a higher concentration of O¹⁸.
And that's how an animal's tooth can tell about global temperatures when the animal lived. To this global picture, Schoeninger wants to add local details by comparing isotopes from animals that get water only from plants to isotopes from animals that drink from streams. In this case, the isotopes are separated by the process of transpiration in the plant.
Still concentrating? Good. We're getting to the meat. It was this type of study that is helping Schoeninger determine that the Australopithecus anemensis from northern Kenya probably lived in a kind of open woodland.
Today, the area is impossibly parched; some years yield absolutely no rain. But her study shows that 4 million years ago the landscape was more humid. Imagine a savanna dotted with trees -- an open woodland with more closed forests nearby.
That landscape is "the perfect place for the evolution of bipedalism," says Schoeninger. The earliest primates lived entirely in trees. But when the trees started to grow apart, she notes, "You have to walk to the next set of trees." That, in turn, freed the hands for carrying stuff and making tools. And sure enough, A. anemensis was one of the first bipeds to appear in the human family album.
One toothy grin
A different study (see "Isotopic Evidence..." in the bibliography) looked at the diet of a later hominid called Australopithecus africanus. While chimpanzees, our closest living relatives, eat mainly fruits and leaves, A. africanus may have eaten meat. Meat-eating is considered characteristic of the genus Homo, to which humans belong and which evolved from Australopithecus. Many scientists think eating meat allowed the rise of the energy-intensive human brain.
Don't start sharpening your barbecue skewers just yet -- that "meat" eaten by A. africanus may well have been grass-eating termites. And it's not certain that they ate meat at all.
The study, by Julia Lee-Thorp, at the University of Capetown, and Matt Sponheimer, of Rutgers University, examined carbon isotopes in the teeth of 3-million year old A. africanus fossils from South Africa. Eating grass or grass-eating animals leaves a different isotopic signature than eating most other plants. That's because grass uses an unusual photosynthetic mechanism that stores distinctive ratios of carbon isotopes in the tissue. The ratio is transferred to whatever eats the grass, and so on up the food chain.
It's unclear whether africanus ate grass, grass-eating animals, or both. Yet if it did eat meat, the information could support the theory that meat-eating was a precondition for big brains. Africanus, you see, had a small thinking machine, large brain developed sometime later.
At any rate, the research does indicate an important change in lifestyle, says Sponheimer, a graduate student in anthropology. "The most important thing is that they were not what we thought they were, [which was] basically fruit and leaf eaters. My interpretation is that early hominids had broadened their dietary base, and were looking to new resources."
A toothsome finding
Sponheimer says the area under study was probably shifting from a closed forest to woodlands and grasslands. Seeing a change in diet coincide with a change in climate supports the theory that environmental change produces behavior change. "If you are dealing with a climatic change, say increasing aridity and a shrinking forest," Sponheimer observes, "how better than to become a dietary generalist?"
The evidence supports the idea that africanus was a transitional species. It was clearly bipedal, but its brain was only slightly larger than a chimp's. "We're not talking about a mental giant," Sponheimer says. He thinks "it may have had a foot in both worlds," living partly in trees and partly on the ground, eating fruits and animals or grasses.
If using teeth to determine climate seems improbable, isotope studies are gaining momentum in the life sciences. Schoeninger, who's been active in the field for 20 years, says isotopes can track the movement of carbon dioxide in tree leaves, and even register stress in organisms. When animals are hungry, they break down, or catabolize, their tissue, which changes the ratio of nitrogen isotopes. Scientists are now measuring stable isotopes found in moose urine from Isle Royale in Lake Superior for evidence of starvation.
Are monarch butterflies stressed? What should be done to sustain their marvelous migration?

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