Transport yourself to the Madagascan forest 1,000 years ago. Tramping along, you notice a giant creature hanging by long, hook-shaped hands and feet from a tree branch. It lumbers along in slow motion, much like a sloth. But while a typical sloth weighs 12 pounds, this one tops 120. And unlike a sloth’s rodentlike face, this animal’s countenance is vaguely human, with big, intelligent eyes. It climbs down the trunk and crawls on its belly to a nearby tree. There an even odder animal sits, pawing at low branches. It looks similar to the first animal, with the same face and body. But at 500 pounds, it is as big as a bear.
These curious creatures have recently come to occupy a pivotal place on our very own family tree. While sloths are relatives of armadillos, these beasts, called sloth lemurs, were primates. They’ve been known vaguely by fragments of fossil remains for 100 years. Since 1982, however, a team of researchers from the United States and Madagascar, led by Elwyn Simons of Duke University, has been mining a mother lode of sloth lemur bones in a chain of remote caves in northwestern Madagascar. Not only are the bones giving them a handle on how lemurs evolved, they’re producing major cracks in some cherished old chestnuts about the earliest primates.
The evolution of sloth lemurs proves the rule that there are a limited number of ways for animals to make a living. Like sloths, sloth lemurs were big animals that ate leaves (a poor source of energy), and so it made sense for both animals to hang from branches rather than run on top of them. Because no major predators lurked on Madagascar, sloth lemurs didn’t have to worry about being attacked as they crawled from tree to tree. One species of sloth lemur evolved a body so heavy that it couldn’t hang anymore (the seated monster you saw in the forest). Several million years ago sloths did the same thing: fossil remains indicate that some sloths came down from the trees and evolved into 10,000-pound behemoths.
When sloth lemur bones were first found, paleontologists labeled them sloths. Subsequent fossil finds put the sloth lemur in the lemur family where it belonged. Yet the question of just how these animals fit into the primate family has long been a puzzle.
At first, researchers seized on the connection between the teeth of sloth lemurs and those of a radically different lemur, the indri. The indri uses its powerful thighs to push off from tree to tree like a ricocheting bullet. Indris have the biggest hind limb-forelimb ratio of any lemur, while the sloth lemurs have the smallest. Despite their similar dentition, the sloth lemur and the indri seemed to have as much in common as a Butterball turkey and a hawk.
For a time, sloth lemurs were dismissed as an evolutionary sideshow, while indris took center stage. In the mid-1960s researchers dubbed indris living fossils; they clung and leapt just as paleontologists imagined our earliest ancestors did 50 million years ago. This hypothesis soon became textbook dogma.
The new sloth lemur fossils, however, have shaken this family tree to its very roots. Bones discovered as recently as last summer have led to the construction of a revised tree--one that reveals sloth lemurs as the indri’s closest relatives. The branching pattern shows that indris are not living fossils; their skill at leaping and clinging is quite new. Both sloth lemurs and indris descend from a common ancestor that possessed neither the sloth lemur’s grappling-hook hands nor the indri’s bulging thighs--it was probably a small quadrupedal jack-of-all-trades that could jump or hang from a branch as the occasion demanded. Only later did some of its descendants specialize. The earliest primates probably resembled this common ancestor and not the indri.
Considering evidence that sloth lemurs have been extinct for only about 1,000 years, scientific admirers can be forgiven for indulging in a dream: It’s a paleontologist’s fantasy to go into one of these regions and find a living sloth lemur, says William Jungers of the State University of New York at Stony Brook, a member of the research team. After all, two extinct lemurs reappeared in the eighties, and zoologists have been scouting out unexplored forests this summer. It’s a remote possibility, says Jungers, but it’s fun to think about.