for National Geographic News
Three South African scientists believe they have identified several predators that preyed upon human ancestors millions of years ago.
The potential hominid killers include Megantereon, an extinct saber-toothed cat with oversize fangs, the leopard, and spotted hyena.
Archaeologists Julia Lee-Thorp and Nikolaas van der Merwe of the University of Cape Town, and paleontologist Francis Thackeray of the Transvaal Museum in Pretoria, South Africa, believe these carnivores were stalking and killing early hominids on the South African savanna 2.5 million years ago.
Tracking the Past with Tooth Enamel
The team's findings are based on a study of the chemical composition of the tooth enamel of several prehistoric carnivores. Tooth enamel is composed mostly of calcium and phosphate, but includes small amounts of carbon. Stable carbon isotope analysis involves tracing the changes in the ratios of two naturally occurring carbon isotopes—carbon 12 and carbon 13—through the food chain.
Carbon 13 concentrations are higher in grasses than in trees and shrubs.
"In ecosystems with a warm growing season, most grasses use a C4 pathway of photosynthesis that leaves them relatively enriched in C13," says Lee-Thorp. "Trees, shrubs and herbs follow a C3 pathway that discriminates strongly against C13. The carbon isotope signature of these two groups is therefore distinct."
Farther along the food chain, the tissue and bone of animals that fed on grasses will also reflect more C13, while browsing animals that foraged on trees and shrubs show lower concentrations of C13.
Predators also reflect the carbon isotope ratios of their prey. To identify possible hominid predators, the researchers first ascertained what the hominid species excavated at Swartkrans, a site situated in the Sterkfontein valley about 25 miles (40 kilometers) northwest of Johannesburg, South Africa, were eating.
"Paranthropus robustus and the early Homo species, Homo ergaster, had a diet that reflected a mix of carbon isotopes, suggesting that they were omnivorous, which is similar to what modern humans are today," said Lee-Thorp.
Knowing the hominid carbon isotope ratios, Lee-Thorp and her colleagues were able to compare the ratios to those found in various carnivores found at the site. The team examined fossilized tooth enamel of leopards, lions, and spotted hyenas, in addition to three extinct species: Megantereon; Dino felis, a false saber-toothed cat; and Chasmoporithetes nitidula, an extinct hunting hyena.
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