The 4.4 million year old Aramis site in Ethiopia preserves, alongside the hominid Ard. ramidus, a multitude of colobine monkey fossils. Leslea was part of the team working on the monkey teeth and jaw bones there in 2000, when bad weather and good luck entered the picture. A massive rainstorm flooded the rivers that the team would have had to cross to get to their field site — so they were forced to explore a location, which they’d repeatedly driven by, closer to their camp. They arrived at Asa Issie and immediately thought, “This is just like Aramis” — and then they started finding the hominids.
Asa Issie is indeed much like Aramis, though 300,000 years younger. The sites are just five or six miles apart, both were once woodland environments, and both preserve a remarkably similar assemblage of animals, including colobine monkeys and hominids. But the researchers were struck by differences in fossils found at the two sites. At Aramis, the male colobine monkeys would have weighed about 40 pounds — but at Asa Issie, the male colobines are much larger and would have weighed around 80 pounds. Most intriguingly, the hominid found at Aramis is Ardipithecus ramidus, while the hominid found at Asa Issie is Australopithecus anamensis. While there are many gaps in our knowledge of these two species, some big differences between them are already clear from the fossils. Ramidus sports teeth more appropriate for a diet of soft foods: thinner enamel and smaller molars. Anamensis, on the other hand, probably had a more varied diet with some tough foods, as suggested by its thicker enamel and particularly large molars relative to its front teeth.
Asa Issie and Aramis are quite close together in terms of time (just 300,000 years apart), space (just a few miles apart), and habitat (same sort of environment, same sorts of animals) — and yet, the hominids occupying each don’t overlap: no ramidus were found at Asa Issie and no anamensis were found at Aramis. In fact, no ramidus and anamensis fossils have ever been found in the same rock layers; all ramidus fossils pre-date all anamensis fossils by more than 300,000 years. Why?
To the Asa Issie team, all that (along with physical similarities between ramidus and anamensis bones) suggested that the two were not just relatives but instead formed a single lineage, with Ard. ramidus evolving directly into Au. anamensis. Australopithecus is currently regarded as the genus ancestral to our own genus, Homo — so if Leslea and her colleagues are right, it would place Ardipithecus squarely within our own ancestral lineage.
Dig deeper. Learn more about the teeth of other members of our family tree in What large teeth you have!.
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