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Fossil Find Sheds Light on ‘Early Evolutionary Steps’

October 1, 2009 at 12:00 AM EDT
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A 4.4-million year old fossil, discovered in Africa has opened a window onto humans' "early evolutionary steps," according to the group of scientists responsible for the find. Ray Suarez reports.
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RAY SUAREZ: Science has long taught that humans evolved from a species similar to modern-day chimps and gorillas, but researchers announced new findings today that call that belief into question, suggesting the line of evolution may have been more complex.

The research was published in the journal Science and chronicled in a new documentary produced by the Discovery Channel. Scientists say the species lived more than 4 million years ago and could be a common ancestor for humans and chimps.

It was nicknamed Ardi, short for Ardipithecus ramidus. The finding was of a female fossil, thought to be 4 feet tall and weighing about 120 pounds.

The story of the discovery began in 1992 at a site in Ethiopia’s Afar rift, about 140 miles northeast of Addis Ababa. At the time, teeth and other small fragments of a skeleton were discovered. It would take more than 15 years of work — digging, reconstruction, and computer simulation — to confirm that an earlier ancestor had been discovered.

C. OWEN LOVEJOY, Kent State University: It took years as a consequence to look at all the details of things like the wrist bone to figure it all out, because we’ve never seen anything like this before.

RAY SUAREZ: Until today, anthropologists had thought Lucy, a skeleton dating back more than 3 million years, was the oldest ancestor known to humans. She, too, was discovered in Ethiopia.

But at a press conference today, scientists said Ardi came first. C. Owen Lovejoy was part of the team.

C. OWEN LOVEJOY: If you were to ask someone on the street today, “What did an early ancestor of humans look like?” they would probably say, well, it would look like Lucy and, before that, it would look like a chimpanzee. What the fossils that are being described in Science today will tell you is that both of those conclusions are very incorrect.

RAY SUAREZ: As shown in this animation done for the Discovery Channel, Ardi was an agile tree climber, like chimps and apes, but she was more inclined to walk upright on two legs.

First phase in human evolution

C. OWEN LOVEJOY: What Ardi does tell us is that, in the hominid line, our first phases of evolution were a mixture of upright walking on the ground and what we call "palmigrade," or careful climbing in the trees.

Now, that's something very different than what you see in chimpanzees. Chimpanzees and gorillas are also very terrestrial, but they've modified their hands and feet so much that they utilize a totally different form of locomotion called knuckle walking. And Ardipithecus tells us that that kind of locomotion never occurred in the human line.

RAY SUAREZ: The skeleton is being unveiled in Ethiopia today.

Here to tell us more about Ardipithecus is Tim White, a paleoanthropologist from the University of California, Berkeley, and a leader of the research team.

Professor White, what does this discovery do to shake up the timeline and what we thought we knew about the origins of early hominids?

TIM WHITE: Well, actually, the discovery is more than a skeleton. It's a very intact skeleton. It has hands, feet, head, many parts of the body that are very important in understanding our evolution, and it's one of 36 individuals all found in one geological horizon at 4.4 million years ago.

It's just one species of many species found in this horizon -- giraffe, rhino, shrews, bats, birds, seeds, pollen. It takes us almost like a time capsule back into a past well beyond the Lucy species where we had no information at all. So it's really illuminateda formerly completely dark era of human pre-history.

RAY SUAREZ: Well, you mentioned you had a pretty intact skeleton. How do you extrapolate from that skeleton the things that we saw in that report, a being walking through a forest instead of walking on all fours, standing upright and then climbing into the trees? How do you know that stuff from the bones?

Using science to explain history

TIM WHITE: Well, the first thing you do when you find a time capsule like this with that kind of evidence is assemble a team, a team of experts. And this particular team, the announcement we're making tomorrow in Science, is 47 total people, specialists in everything from the soil isotopes to the plants to the shrews to the bats to the hominids.

And then we piece it together, slowly and carefully, because this is world heritage. We may never find a skeleton as intact as this one. So every piece was extracted, mirror imaged, computer, high resolution was used to look inside the teeth of this creature.

We've examined its biology in detail. And by doing that, it provides a kind of a foundation or a substrate. We brought in Jay Matternes, who's the world's leading natural history artist, to create these restorations and then turn them from his charcoal drawings that are accurate in detail to the millimeter based on the bones, turn them and digitally render them, and then put them into this world that all of these experts have been able to reconstruct from their work on similar fossils of the plants and the animals that lived at the same time.

RAY SUAREZ: The tremendous advances that you describe help us see Ardipithecus, but it sounds like there's also been a lot of advances in the science of paleoanthropology, so there's just a lot more that you can know from a molar, let's say.

TIM WHITE: We have the kind of technology today that we did not have 20 years ago, so now, when one finds a broken and fragmented skull like the one that we found, this is not the original. The original is national heritage. It's in Ethiopia. It was announced there earlier today in Addis Ababa.

This is a stereolithograph. This was a series of pieces that were imaged in Tokyo and then put together three-dimensionally to create this replica of what the skull -- the original one looked like when restored. We couldn't have done that 20 years ago.

On the original teeth, the samples of the enamel, when that enamel was developing, an isotopic signature based on the plant community was preserved in that enamel. We're able to go in and -- and extract that signal that's 4.4 million years old. So we're able to understand the biology in a detail that we've never really been able to do before.

RAY SUAREZ: Whenever I've interviewed people in your line of work, there's never an end, really. When you find something that opens up all this new information, there's still things that you need to know and still things you're looking for. What would help you out that's lurking in the soil of Ethiopia still somewhere?

'A mission into the deep past'

TIM WHITE: Well, if you'd have asked that question back at the time that Lucy was found -- and Lucy opened up a wonderful world of 3.2 million years ago -- I would have said, if we just have some success in the rocks older than 4 million years, we can maybe find a skeleton. Maybe we can find an animal community. Make we can recover a lot of data.

So we went out there because we were curious to explore that unknown part of the past. Just as we send planetary missions into deep space, this was a mission into the deep past, into Planet Earth's past, and into our past.

And so by recovering these data, all of this evidence, we've opened up a new window, a new chapter in human evolution. What chapter lies beyond? It's out in the desert in Ethiopia.

RAY SUAREZ: But are there limits? This is a 4.4-million-year estimate. Beyond that, are the things that you find likely to be just too degraded, too fragmentary, too hard to find?

TIM WHITE: No, we just need the right conditions of preservation. We got fortunate this time. An ancient river floodplain embedded this community, including all of its inhabitants. What we're looking for are older and older rocks in similar circumstances where the remains of the plants and animals can become fossilized.

So we hold out hope. As a paleontologist, that's all you can do, hope and take the best possible team of experts into the field.

The other great news for African scholarship is that it is African scholars who are now leading this research, and that gives me a great deal of hope, because they're able to go out into sites and expand the number of people in the field, expand the science being done, and we're going to get more and more data from the deep past. And that's really what it's all about: getting the evidence, the evidence that Darwin didn't have.

RAY SUAREZ: Professor White, thanks a lot.

TIM WHITE: You're welcome.