A 47-million-year-old primate fossil, a purported "missing link" between primates and humans, was unveiled this week in New York. The fossil, formally called Darwinius masillae but nicknamed Ida, could, due to it being an essentially whole skeleton, shed light on the construction of the primate family tree, says an expert on primate evolution at Washington University in St. Louis.
Tab Rasmussen, Ph.D., professor of anthropology in Arts & Sciences, studies primate evolution by drawing on two major lines of evidence: the fossil record and the comparative study of living primates. One goal of this research is to trace the actual course of primate evolution. More importantly, detailed studies of primate evolution can provide insight into the evolutionary process itself. He has been particularly interested in major evolutionary transitions, such as primate origins and anthropoid origins.
Rasmussen explains the significance of the find in the following piece:
The new specimen of Darwinius is truly remarkable. Over the last 150 years of paleontological collecting around the world, thousands of fossil primates have been found representing many hundreds of species, but the new find is the first complete skeleton. Usually we are lucky to find a few fragments of jaws or teeth, or isolated limb bones. Only infrequently do we get partial limbs or multiple bones from a single individual. In Darwinius, we have essentially the whole skeleton, with impressions of body outlines and stomach contents as well.
Some of the bones least likely to be found and properly identified from typical fossil sites are small bones of the feet, hands, fingers, tail, and so forth. In Darwinius, we can judge precisely the proportions and arrangements of these parts. Hands, feet and tail are critical for active arboreal movements, and Darwinius shows us that it had prehensile hands, powerful grasping feet, and a long, counterbalancing tail, all contributing to the efficiency of arboreal running and leaping.
Because Darwinius died at a immature age, the specimen is also very helpful in reconstructing the life history of the animal. The sequence in which deciduous (baby) teeth and adult teeth erupt allows researchers to estimate the rate of development, whether an animal grew up fast like a rabbit, or slow like a human being. It turns out Darwinius had a medium growth rate, slower than most other mammals its size, but faster than modern apes. In this way, it was similar to squirrel monkeys and some lemurs. The individual represented by the new fossil had broken its wrist before dying, possibly from a fall. The stomach contents reveal fruit and leaves.
Darwinius lived at a time (47 million years ago) not too long after the very earliest true primates appear in the fossil record. It provides us with the best look ever at such early primates. The world was much warmer then, and tropical zones extended far north into North America, Europe and Asia. The very widespread Eocene rainforests of the time held a remarkable diversity of primates. The Eocene was truly the golden age of primate evolution, judged by global diversity and abundance.