Two million years from now, a single tooth might well be all that's left of you. Tooth enamel is the hardest substance in the human body, packed dense with mineral elements such as calcium and strontium. While a softer bone might succumb to the vicissitudes of time, a tooth is pre-fossilized -- ready to be preserved for the ages.
A single tooth might not seem like much for an anthropologist to go on, but researchers at the University of Colorado are finding creative ways to unlock the wisdom buried in the teeth of ancient human relatives. By combining geology, chemistry and ecology they are starting to paint a picture of what life was like for the hominin species of southern Africa millions of years ago. "To know anything about fossil organisms is a big, big deal," said Matt Sponheimer, professor of anthropology at CU. "In many cases, all we have is a few teeth, maybe a couple of bones from different parts of the body." Sponheimer directs a research program that makes the most of what little fossil evidence exists for early hominins. With colleagues from around the world, he has spent the last several years investigating what ancient hominins ate and how they moved about the landscape. And thanks to some miracles of chemistry, all it takes is a handful of teeth. Much of the research turns on isotopes: different versions of a single element. Take strontium, a calcium-like element that gets fixed in teeth during childhood. One isotope of strontium occurs naturally in bedrock. A second, slightly heavier isotope is created by the radioactive decay of a different element, rubidium. As the rock ages, more rubidium decays, so older geologic areas have a higher ratio of heavier strontium to lighter strontium. The principle is similar to carbon dating. But while carbon isotopes can place a fossil in time, strontium isotopes can place that fossil in space -- beyond the site where the organism died. "Unlike carbon, strontium isotopes tell you about where on the landscape an individual lived," said anthropologist Sandi Copeland, a CU professor who joined Sponheimer's research team as a post-doctoral student. Bedrock eventually breaks down into soil, allowing both isotopes of strontium to be taken up by plants. Animals who eat those plants during their early lives end up with a strontium isotope ratio in their tooth enamel that reflects the geology of the area where they grew up. "The strontium isotope ratio just goes right up the food chain," Copeland said. Earlier this month, Copeland and Sponheimer published a study of strontium isotopes in fossil teeth from two hominin species, Paranthropus robustus and Australopithecus africanus, found in caves in South Africa. The researchers looked at adult teeth of various sizes, knowing that the larger and smaller teeth probably represented males and females, respectively. By comparing the strontium ratio of each tooth to the ratio found in plants near the cave sites and in other nearby geologic regions, they discovered that almost all the male hominins grew up in the caves where they died, while more than half of the females grew up elsewhere. The results provide the first evidence of dispersal patterns in these species, suggesting that the females left their birth groups upon reaching sexual maturity. The same pattern is seen in chimpanzees and many human groups, but not in other primates. It's tempting to see that as evidence for further similarities between early hominins and ourselves, but Copeland is reluctant to make assumptions. "I certainly wouldn't rule out the possibility that (this behavior) evolved independently for different reasons," she said. In fact, other recent work from Sponheimer's group has shown that anthropologists know even less about our ancient relatives than they thought. In a study published last month, Sponheimer and colleagues revealed evidence that may overturn prevailing wisdom on Paranthropus boisei, or "Nutcracker Man." As the nickname suggests, paleontologists had assumed that this species used its large teeth and powerful jaws to crack nuts and chew through seeds and hard fruits. Sponheimer's group put that notion to the test by analyzing the carbon isotopes found in P. boisei fossil teeth. In addition to their use in dating fossils, carbon isotopes can reveal what kind of plants an organism ate. Nut- and fruit-bearing trees contain a low ratio of heavy to light carbon isotopes. If P. boisei really was a nutcracker, the carbon ratio in the fossils would be similar. But the researchers found that the ratio in the teeth was actually much closer to that of grasses. "Our results don't say that they never crack nuts," Sponheimer said. "But what it does suggest is that the predominant part of their diet was for something other than hard fruits and the like." According to Sponheimer, the carbon isotope ratio suggests that P. boisei's diet resembled a zebra's more than a chimpanzee's. "That's really pretty stunning," he said. "It suggests that looking at our closest living relatives provides a very misleading model for those organisms, and suggests that we have to be really careful about using those kinds of analogies in making interpretations about behavior in fossil organisms." The picture should become clearer as more data accumulates, especially thanks to the newer techniques Sponheimer's group is using. He described the analysis of carbon isotopes to determine diet as "beyond reasonable doubt," but said the study using strontium isotopes to track movement patterns was a first step in a new direction. While researchers can't yet draw broad conclusions about hominin behavior, strontium analysis expands the possibilities for further research. "It's allowing us to ask completely new questions," Sponheimer said. "For instance, do we see evidence that individuals that come from different places, are they eating different things?" He said he would also want to examine species in the genus Homo, which are more closely related to modern humans than the species studied so far. "Was early Homo using the landscape in ways that are obviously different?" he said. "That would be predicted. Do we, in fact, see that?" Only teeth will tell.