Fossil Implies Our Early Kin Lived in Trees, Study Says

Hillary Mayell
for National Geographic News

November 21, 2002

The discovery of a fossil skeleton of a 56-million-year-old tiny mammal indicates that our early ancestors were tree-living fruit eaters, and is helping scientists to understand the early evolution of primates.

"One of the big unanswered questions in mammalian vertebrate paleontology is what did primates evolve from, where, and why," said Jonathan Bloch, a paleontologist at the University of Michigan Museum of Paleontology.

The earliest true primates, called euprimates, first appear in the fossil record 55 million years ago in North America, Asia, and Europe. Euprimates as a group includes humans, monkeys, and apes. The question is what happened during the 10-million-year stretch between the extinction of the dinosaurs, roughly 65 million years ago, and the first appearance of primates.

The nearly complete fossil skeleton of a Carpolestes simpsoni, found in the Clarks Fork Basin of Wyoming, may begin to answer some of those questions.

Carpolestes is a member of an archaic group of primates called plesiadapiforms. Plesiadapiforms lived from about 65 to 45 million years ago.

The characteristics that separate primates from other mammals are a large brain; the ability to grasp, which requires opposable thumbs and big toes; the ability to leap; eyes in the front of the face rather than on the side; and nails instead of claws.

Because Carpolestes combines features of the earlier plesiadapiforms with primate-like features, it begins to answer the question of what order these traits evolved, and for what purpose. The study is published in the November 22 issue of the journal Science.

Evolution of Primates

There have been three hypotheses to explain why and how primates evolved.

In one view, grasping ability with nails, and forward facing eyes evolved together to enable the animals to prey on insects found at the base of trees.

A second hypothesis is that grasping with nails, forward facing eyes, and the ability to leap evolved together in order to move through the branches of trees rapidly.

It has also been suggested that grasping evolved first to allow the animals to eat the fruit on small branches, and forward facing eyes evolved later to enable them to prey on insects.

The Carpolestes, which weighed about 4 ounces (100 grams), had a long tail, and a body about 14 inches (35 centimeters) long, shared some, but not all of the characteristics of modern primates, and thus can be viewed as a transitional animal. It had very primate like teeth that were highly specialized for eating flowers, seeds, and fruit. The opposable big toe gave it a grasping ability that indicates it spent most of its time climbing trees.

Carpolestes also had a nail on its big toe, but its eyes were not forward facing, and it did not have the bone structure that would allow for specialized leaping, like some of the earliest primates.

Bloch and his co-author Doug Boyer conclude that Carpolestes spent most of its time clinging to tree branches and eating fruit, rather than spotting prey or leaping for its dinner. Boyer has been working with Bloch under a National Science Foundation grant to study plesiadapiform skeletons from Wyoming and the origin of primates.

The authors speculate that as the diversity of fruits, flowers, leaf buds, and nectar increased in the Paleocene, 65 to 55 million years ago, Carpolestes took to the trees to exploit a new food source and to avoid competition with early rodents.

A small group of scientists, led by Robert D. Martin at the Field Museum in Chicago, has argued for a primate origin date of 85 million years ago, based on statistical modeling. There is no fossil evidence to support the contention, and most paleontologists rely on the fossil record to piece together the story of evolution.

Clarks Fork Basin, where the fossil was found, is particularly rich and has yielded many nearly complete and almost fully articulated Eocene (55 to 34 million years ago) fossils. The researchers are expanding their search to include Bighorn Basin in Wyoming, and Crazy Mountain Basin in Montana. The quarries are expected to produce fossils that will further scientific understanding of the evolution of primates.