IRA FLATOW, host:

This is TALK OF THE NATION: Science Friday. I'm Ira Flatow.

A bit later in the hour, we'll be talking with Michael Pollan, author of a new book, THE OMNIVORE'S DILEMMA. But first, last week, two new discoveries filled in crucial gaps in the evolutionary record, and now we've got another one.

Anthropologists say they have uncovered new fossils that provide the best evidence yet that Lucy -- remember our three-million-year-old ancestor, famous fossil -- well, the best evidence yet that Lucy evolved from an even older ancestor, one that lived more than four million years ago. So this new find helps fill in the million-year gap between these two groups of hominids. And amazingly, all of these fossils, all three groups that I'm talking about, were found in the same region of Ethiopia in an area called the Middle Awash.

Joining me now is the co-director of the team that discovered the new fossils. Tim White is Professor of Integrative Biology, co-director of the Human Evolution Research Center, University of California at Berkeley. He joins us by phone from his office. Welcome back to Science Friday, Dr. White.

Dr. TIM WHITE (University of California, Berkeley): Hello, Ira.

FLATOW: How exciting is this? Is this a true missing link? You know, is that an overused word?

Dr. WHITE: It's way overused. In fact, it's a link that's not missing anymore because we found it. And it's one of many we've found in this region.

FLATOW: Why is that, I mentioned the three, why is that such a rich region?

Dr. WHITE: Well, what's happened over the last six million years is with the rotation of the Arabian Peninsula away from Africa, a phenomenon known as plate tectonics, and the pulling of the Somali Plate away from Africa, it's resulted in a depression known as the Afar Depression or the lowlands of Ethiopia, and over six million years, lakes and rivers have been forming in this triangular depression.

And as the sediment accumulates around these lakes and rivers, the plants and animals who died in that region have their remains covered and buried, and now because of the plate tectonics that continue today, these ancient sediments are brought up to the surface of the ground where we can recover the fossils of all of these organisms, including our own ancestors.

FLATOW: Wow. Tell us about this ancestor, the fossils that you found. What body parts are changed? Why do you think it fits right in that missing gap there?

Dr. WHITE: Well, the great thing about this place is it's become a sort of a Grand Canyon of human evolution, but instead of the Grand Canyon, which is a deep, narrow ravine with steep sides, the sediments of the Middle Awash are quite soft.

And so what happens is you have the geomorphological evolution of a badland topography, and it's a desert region today, which helps us because there aren't a lot of plants around to obscure finding the fossils that come out, but then this badlands topography, we can go out and look at given time horizons, and the time horizon at 4.1 million years ago, being almost a million years older than Lucy, is a very crucial one.

And so by going out there and focusing with this big team that's an international team of scientists supported by the National Science Foundation, we've been successful at finding many different horizons. But this 4.1 is a key one because it's the earliest species of a genus known as Australopithecus.

And the teeth that we found in this species are more primitive than the ones we see in later species but more evolutionarily advanced than the ones we see in even older rocks in the same study area. So we have this ability in the Middle Awash to track human evolution through time, through six million years of time.

FLATOW: So they fill in this gap between Lucy and the older ancestor. How far apart now is the gap once you put this piece in?

Dr. WHITE: Well, it, every time you fill a bigger gap, it creates two smaller gaps.

FLATOW: Right.

Dr. WHITE: We've now created a gap in-between this species, anamensis, and an earlier one that we found at 4.4 million years ago, a creature known as Ardipithecus ramidus, and so we have about a 300,000-year period, and the question that this discovery brings to the front is what kind of evolution happened?

Was it a splitting evolution that led to this new genus and species, Australopithecus? Or did we have what we call phyletic or straight-line evolution fairly rapidly during this 300,000-year interval? And so we're able to look, because of the excellence of the evidence, we're able to look at evolution in a very fine-grained manner.

FLATOW: Where does Lucy fit in to us? Is she on a direct line to us?

Dr. WHITE: Well, it's probably best to think of human evolution as in three stages. There's an early stage, represented by Ardipithecus, from about four million years ago to six million years ago. That is right after our line split from the line leading to modern chimps.

These are quite primitive creatures. They live in woodland environments in Eastern Africa. After four million years ago, with the evolution of the genus Australopithecus, we get a little bit of speciation. Lucy is the best known of those species of Australopithecus. She lived about 3.2 million years ago in the same region of Ethiopia.

And shortly after that, a species known as africanus, which is a sister species of the Lucy species, appears in Southern Africa, and then you have the evolution by two and a half million years ago of very specialized species of Australopithecus that eventually go extinct.

So we have a little bit of radiation in that part of human evolution, but starting about two and half million years ago, we have the third phase of human evolution. That's our own. That's our own genus Homo. That phase eventually culminates in Homo sapiens, our own species, that we also found in the same mile deep stack of sediments in the Afar at about 155,000 years ago. So our own species is a relative latecomer but with clear evolutionary roots that go all the way back to six million.

FLATOW: Do you, what would seal the deal on connecting, you know, filling in, as you say you created two more gaps because you put one in the middle, what do you need to find, or is, and do you think you might find it there in that great spot?

Dr. WHITE: Well, I'm sure it will be found in the Afar. It's, Ethiopia's just a wonderful country, wonderful people and great geology and fossils. We have local volcanoes that give us this timeline. When you go to get this fossil evidence, you want to put it just like at a crime scene. This is not a crime scene. It's more of an evolution scene and you want to create a timeline. So by dating these volcanic rocks, we know how old things are, and we've already identified some rock bodies that go back beyond six million. We've not found hominid fossils from there yet.

So that's one of the gaps that we haven't yet filled, and we're also working right now, in fact, the last field season, we were in an area of rocks that looked like they're gonna date to about 2.7 -- actually, the rock samples are in the lab in Los Alamos and in Berkeley right now --about two and a half, 2.7 million, and this is when our own genus originated and we see the very first stone tool technologies in the record. So this is another place we want to focus on because it's a kind of a phase transition between Australopithecus and Homo.

FLATOW: Yeah, that would be post-Lucy and closer to us.

Dr. WHITE: Exactly.

FLATOW: Yeah. So you haven't got any tidbits you could share with us, yet.

Dr. WHITE: Oh, we've got the tidbits but we like to get our dating in order and get our publications up there before we talk about these things.

FLATOW: Oh, details, details. Well, I wanna thank you very much for taking time to talk with us about it, and congratulations and good luck to you, and reserve that spot for coming back next time.

Dr. WHITE: All right, thanks a lot, Ira.

FLATOW: Thanks a lot. Tim White, who has made this interesting new find. He's Professor of Integrative Biology, Co-Director of the Human Evolution Research Center at the University of California at Berkeley.

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