Earth was once truly a planet of the apes. In the Miocene Epoch, 24 to 5 million years ago, some 30 species lived in Africa, Asia, and Europe. One of those species, anthropologists believe, gave rise to all modern apes and humans. Although the scant fossil evidence has made it difficult to single out that ancestor, recent discoveries in East Africa are bringing researchers a few steps closer to finding out what it might have looked like.
Two fossil apes have emerged as particularly strong candidates for spots on the human-ape family tree. As early as 20 million years ago, says paleoanthropologist Laura MacLatchy of the State University of New York at Stony Brook, a surprisingly modern-looking ape lived in the forest canopy of Uganda. The ape’s enigmatic fossils, first discovered in the 1960s near the village of Moroto, consist of a jutting jaw and primitive teeth that resemble those of other early apes, but the vertebrae are strikingly advanced. They had attachment points for muscles that helped the vertebrae resist flexion, suggesting that the 90-pound Morotopithecus had a spine that could have supported an upright posture--a disconcertingly modern attribute in such a primitive animal. It was this odd sample of features that no one knew what to do with, says MacLatchy.
Over the past few years, MacLatchy has revisited Moroto and discovered new Morotopithecus fossils possessing other modern traits, including thighbones and part of a shoulder blade. The thighbones outwardly resemble many other primitive-ape femurs, but a cross section revealed that they consist mostly of a hard outer layer of bone, or cortex, surrounding a thin inner core of what was once marrow. The femurs of modern orangutans have a similar structure, and MacLatchy thinks this feature relates to how these apes move.
Climbing slowly and deliberately the way an orangutan does puts a lot of stress on limb bones, she says. The cortical area is very thick in these animals. Morotopithecus may also have been a cautious climber. MacLatchy found further proof of Morotopithecus’s ability to navigate the treetops in the socket where the arm bone attaches to the shoulder blade. Morotopithecus’s socket was cup-shaped, like that of chimpanzees and orangutans, suggesting that it had good shoulder mobility. Morotopithecus is the first example of an ape able to hang and swing from branches; other early apes scurried along branches on all fours.
The 20-million-year-old Morotopithecus fossils push back the emergence of the modern ape body plan by at least 5 million years. If we saw Morotopithecus climbing around in the trees, we would recognize it as similar to apes today, says MacLatchy. Whereas with another ape from that time, like Proconsul, we would see that as something different.
While Morotopithecus is clearly too old to be the last common ancestor of humans, chimps, and gorillas--dna studies suggest that our hominid forebears diverged from apes 5 to 7 million years ago--its modern characteristics suggest that it is probably a cousin of all living apes, including orangutans and small Asian apes called gibbons. That Morotopithecus was a fairly large ape suggests that apes did not necessarily start out small, like gibbons, and evolve a larger body size over time, as some researchers have thought. Our work supports the view that apes started out big, and gibbons are a specialized, shrunken version, says MacLatchy.
The second ape vying for a branch on the human family tree lived about 6 million years after Morotopithecus and may have been one of the first apes to descend from the forest canopy and live mainly on the ground. Kenyapithecus has a perverse mixture of features, says paleoanthropologist Monte McCrossin of Southern Illinois University.
When first described about 30 years ago, Kenyapithecus was linked to humans because of the shape of its upper jaw and palate. But in the late 1980s, McCrossin and Brenda Benefit, also at siu, discovered that Kenyapithecus had a specialized, buck-toothed incisor just like that of a modern, partly ground-dwelling South American monkey called a saki, which uses its incisors to crack open hard seeds and nuts.
McCrossin and Benefit also discovered a part of the shoulder joint whose shape suggested that Kenyapithecus couldn’t hold its arms above its head--its arm was better designed for running on the ground like a saki than for swinging on tree branches. Despite its modern appearance, Kenyapithecus’s skeleton looked more like that of a semiterrestrial monkey than of a modern ape, McCrossin argued. Kenyapithecus, while it had features that mimicked modern apes, probably wasn’t a relative. It was at best a member of an archaic ape lineage that left no modern descendant, McCrossin concluded.
That’s where things stood for about ten years--until McCrossin changed his mind. Last summer he discovered new Kenyapithecus fossils that have made him rethink its position in ape evolution. Working on a large excavation on Maboko Island in Lake Victoria in Kenya, McCrossin and Benefit uncovered an upper arm bone and a foot skeleton of Kenyapithecus, slightly younger than the ones McCrossin had found previously. Unlike the earlier shoulder-joint fossil, the newly discovered specimen suggested that a slightly more recent Kenyapithecus could actually hang in trees.
I still haven’t resolved in my head what this means, says McCrossin, but it may be more arboreal than the older forms. We may have caught Kenyapithecus going from life on the ground, which was revolutionary in itself, to becoming more suited for life in the trees. The foot skeleton shows that like chimps and gorillas, Kenyapithecus could walk flat-footed on the ground and rotate its feet sideways to grasp a tree trunk for climbing.
These features, together with its modern-looking jaw, face, and teeth, make Kenyapithecus the most likely ancestor of humans, chimps, and gorillas, says McCrossin. I’m in the awkward position of proving myself wrong, he says, but I’m happy to be wrong. This new evidence shows that I thought the archaic features were more important than they really are.