May 16, 2009  -- Scientists say a 47-million-year-old fossil found in Germany may be a key link to explain the evolution of early primates, and, ultimately, modern human beings.

The fossil, of a young female that probably resembled a modern-day lemur, is described as "the most complete primate fossil ever found." It is small -- with a body about the size of a raccoon -- but it has characteristics found in later primates and in humans.

It has, among other things, opposable thumbs, similar to humans' and unlike those found on other modern mammals. It has fingernails instead of claws. And by examining the structure of its hind legs (one of which is partly missing), scientists say they can see evidence of evolutionary changes that would eventually lead to primates standing upright.

Remains From a German Caldera

The find is being published next week in the online journal Public Library of Science - One (PLoS One for short). The cable channel History, which bought the North American documentary rights to the find, provided exclusive details to ABC News.

"She is a transitional species showing characteristics from both the non-human (prosimians and lemurs) and human (anthropoids, monkeys, apes and man) evolutionary lines," said the producers in a statement reviewed by the authors of the PLoS One paper.

It was discovered by amateur fossil hunters in 1983 in a mile-wide crater called the Messel Shale Pit, not far from Frankfurt. Scientists speculate that at one time the pit was a volcanic caldera, where animals got caught and their remains unusually were well preserved. The amateurs recognized that they had a very good fossil, but they did not recognize its potential importance.

The location, a UNESCO World Heritage Site, has been the source of many other fossils from the Eocene Period, about 50 million years ago.

The fossil was shown by collectors to Dr. Jorn Hurum of the University of Oslo, who brought in colleagues from Oslo and from the Senckenberg Research Institute in Frankfurt to authenticate the find. They bought the specimen so that it would not disappear into private hands.

Examination of the fossil went on for two years. They have called the specimen Darwinius masillae. The researchers were able to keep their work secret until word leaked out in the last week.

Fossil May Be Evolution 'Link'

Hurum's team said 98 percent of the fossil's skeleton appears intact. It has a long, curling tail. X-rays show it was young; it had baby teeth and, beneath them, adult teeth forming to replace them. There also was evidence that the animal had once broken its wrist.

Evolution Debate

It is hard to say what the find may do to the modern understanding of evolution. The fossil, for one thing, is far older than any of the human ancestors other scientists have reported finding in what is now eastern Africa. "Lucy," probably the best-known African fossil that is generally accepted as pre-human, is somewhat more than three million years old.

This year happens to be the 200th anniversary of the birth of Charles Darwin, originator of the theory of evolution by means of natural selection. Most scientists, following Darwin, believe that humans and modern apes had common ancestors, but the fossil record has had gaps in it.

The fossil is to be unveiled Tuesday, when the paper is published in PLoS One and a news conference takes place in New York. The fossil will be shown on "Good Morning America" to coincide with the announcement. History plans to air its two-hour documentary, titled "The Link," on Monday, May 25.