You’ll Never Guess Who Walked In!
Ardi redefines the branch between apes and hominins
The arboreal part of Ardi’s locomotion probably closely resembled that of Proconsul, an 18-million-year-old African genus. Proconsul was not a hominin but a primitive or ancestral ape near the divergence between the monkey lineage and the lineage of apes plus humans, and probably an early ancestor of Ardipithecus.
Like Ardi, both Proconsul heseloni and a larger species P. nyanzae are known from several partial skeletons. Both had grasping hands and feet, and fore- and hindlimbs roughly equal in length, features that are very important for an animal that moves quadrupedally in trees. Proconsul did not have any strong adaptations for jumping or leaping, so these animals probably moved slowly in the trees.
Two big differences between Proconsul and Ardi are body size and sexual dimorphism. P. heseloni females weighed about 8 kilograms, with males as heavy as 10.5 kilograms; P. nyanzae ranged from 27 kilograms for females to 40.5 kilograms for males. In contrast, Ardipithecus was larger, about 50 kilograms, with little apparent male-female difference in body size. Both Proconsul species and Ardi show many adaptations for slow clambering and quadrupedal locomotion in the trees, and none for knuckle-walking or brachiating. Proconsul is much older but gives clues about the anatomy from which something like Ardipithecus probably evolved.
Another striking resemblance between Proconsul and Ardipithecus lies in their preservation. Experiments feeding baboon carcasses to cheetahs carried out by C. K. “Bob” Brain, now retired from the Transvaal Museum in South Africa, showed that cheetahs leave a very particular pattern of bones uneaten. Cheetahs leave behind exactly those parts of the skeleton preserved in the Proconsul skeleton—teeth, cranial fragments, arms complete to the hands and legs complete to the feet—because cheetahs’ relatively weak jaws cannot break into the stronger and denser bones. Vertebrae, ribs and pelvises are almost completely destroyed. On this basis, Alan Walker of Pennsylvania State University suggested that the Proconsul partial skeletons had been eaten by an extinct carnivore analogous to the living cheetah.
The partial skeleton of Ardi shows the same pattern of preservation seen in cheetah kills and in Proconsul skeletons. Were there cheetahs in Aramis 4.4 million years ago? Not as far as anyone knows. But there were several catlike carnivores in ancient Aramis, including Dinofelis, which is an excellent candidate for Ardi’s killer.
Dinofelis is a false sabertoothed cat (its canines were flattened and only moderately long compared to those of true sabertooths) that lived between 5 million and 1.5 million years ago, probably in woodland habitats such as those where Ardipithecus lived. Dinofelis weighed two to three times more than a cheetah, yet its jaws were relatively weak because of the huge gape needed to use its elongated canines effectively. Also, Ardipithecus is about twice as large as a baboon and would have had more substantial bones. Though the research team concluded that most of the hominins and other animals they found were ravaged by hyenas after death, perhaps Ardi herself was attacked by a different carnivore, which accounts for her excellent preservation.