Forty years ago today, a young American paleoanthropologist named Donald Johanson made the discovery of a lifetime in the arid badlands of Ethiopia’s remote Afar region: a 3.2-million-year-old skeleton of a small-brained creature that walked upright like we do. It was a primitive hominin, a member of the human family. To scientists it is known as AL 288-1, a female specimen of the species Australopithecus afarensis. But most people know her as Lucy. I recently sat down with Johanson, founding director of the Institute of Human Origins at Arizona State University, to talk with him about that fateful day and the science that has grown out of his extraordinary find. (Full disclosure: I wrote a book about Lucy with Johanson a few years ago). An edited transcript of our conversation follows.

KATE WONG: How did you find Lucy?

DONALD JOHANSON: It was November 24, 1974. We were in our second field season of research at Hadar. We'd been there in '73 when I found a [fossil hominin] knee joint, but we couldn't tell to what species the knee joint belonged or whether it was perhaps even a new species, because we were dealing with older strata than anyone had worked in in Eastern Africa, except for some exposures in Southern Ethiopia, where isolated teeth were found. This time we were really hoping to find something much more impressive and complete.

I wasn’t particularly keen on going out that day, but Tom Gray, my graduate student at the time, who was in charge of mapping, wanted to go back to a fossil locality we had found previously so he could plot it precisely on our map. So we went back to the site, up on a little plateau, and marked the locality. And then we walked around and we looked for fossils. I always keep my eyes on the ground--that's the only way you find things there. Tom was on my left side, and I glanced over my right shoulder and saw this perfectly preserved elbow end of a forearm bone, the ulna. I considered whether it could have been a monkey elbow-we had found fossils of baboons, colobus and other monkeys in the region--but it didn’t have the extended flare on the back of it that monkey elbows have. I knew it was a hominin elbow.

Tom was very doubtful about my assessment. But then I saw shards of a skull next to his foot. And damn it if the find didn’t turn out to be a partial skeleton. Looking up the slope we could see these pieces of bone just glistening and coming down in almost a straight line--the chunk of jaw, a femur and so on. We collected a couple pieces and brought them back to camp. Tom was supposed to be very cool, quiet, and calm, but he couldn't help honking the horn, driving into camp saying, "Don found the whole damn thing!"

Of course this was a watershed moment in my career. I knew right away that the skeleton was older than three million because the pigs and elephants from that level were over three million. Up to that point in the history of our field of paleoanthropology, all of the human ancestor fossils older than 3 million could fit in the palm of your hand, and none were diagnostic enough to be able to say what species they were. We had both cranial and postcranial remains.

It's a very tiny specimen. I thought it was in the Australopithecus group, largely because of the brain size, which you could estimate from the occipital bone. The arc of that bone was so small you couldn't put anything bigger in there than a grapefruit. And there were features of the teeth, particularly the pre-molar, that were suggestive that this might be a new species. But that decision wasn’t really made until December of '77, when we said, "This is it, we've got to fish or cut bait. It is a new species. Let's define it.” We called it Australopithecus afarensis, in honor of the Afar people of the Afar region of Ethiopia.

KW: How did Lucy become such an icon?

DJ: I knew from the beginning that she would be important. But in hindsight, she also got the right nickname. A member of the expedition suggested if she was a female, as we suspected, why not call her Lucy, after the Beatles song “Lucy in the Sky with Diamonds,” which was playing on my little black Sony tape deck that night after we found her. With the throwaway line, "Why don't you call her Lucy?" came total commitment from everyone on the team by breakfast the next day. "When are we going back to the Lucy site?” people asked. “How old do you think Lucy was when she died?” Immediately she became a person.

I think that’s part of what led to her becoming such an icon, giving her this affectionate name that people could identify with. When they saw photographs it wasn't a chunk of jaw or even a skull staring with empty eye sockets--it was the visage of an individual.

This has been particularly effective in drawing young children into this field. Kids write to me all the time saying, "We're doing a project on Lucy. Do you think she was married? What did she eat?"

KW: Over the years you’ve actually been able to answer those questions, at least with regard to Lucy's species, if not Lucy herself.

DJ: We now have close to 400 specimens of Lucy's species from Hadar--a treasure trove for scientific study. We know from stable isotope studies of her teeth that she was pretty much a vegetarian. She probably ate grasses as well as fruits and leaves. I suspect she also ate things like termites and small vertebrates. She may have spent time at the lakeshore gathering turtle and crocodile eggs, too--fossil turtle eggs and crocodile eggs have both turned up in pretty much the same stratum where Lucy was found.

Afarensis lived in multi-male, multi-female groups, probably not markedly different from what we see in chimpanzee troops in Africa today. Interestingly, though, afarensis shows reduction in the size of the canine teeth. In chimps and gorillas males use their large canine teeth to fight. Some people have interpreted the smaller canines in afarensis as evidence of reduced intragroup aggression, so that males were not necessarily competing as aggressively for access to females. Reducing intragroup aggression is advantageous because intragroup trust and commitment become heightened.

KW: You have argued that afarensis occupies a pretty important place on the family tree--ancestral to our genus, Homo. How did you arrive at that conclusion?

DJ: I would say that Lucy’s species is the last common ancestor of two different adaptive radiations that were subsequent to her. It’s always very difficult to make a one-to-one causation between climate change and species change, but we do know that when afarensis disappeared from the fossil record at about 3 million years, very soon thereafter there was a general cooling and drying of Eastern Africa. Some descendants of Lucy in Eastern Africa evolved through things like Australopithecus aethiopicus, which had a projecting face like afarensis, but also possessed a whopping crest atop its head and hugely expanded back teeth. These kinds of adaptations to herbivory foreshadow other features in another species, Australopithecus boisei. So I think that the afarensis-aethiopicus-boisei line is a traceable lineage in East Africa.

On the other hand, afarensis was not too specialized to be an ancestor of Homo, too. Once you get things like Australopithecus africanus in South Africa, they are already pretty specialized and look like ideal ancestors for the South African species A. robustus. But afarensis had a generalized nature to it, so I think it also gave rise to Homo. At Hadar, Homo traces back to 2.4 million years ago.

KW: One proposed alternative to that scenario has come from the recent discovery in South Africa of a previously unknown hominin species, Australopithecus sediba, by Lee Berger of the University of the Witwatersrand and his colleagues. They have suggested that this species could have given rise to Homo.

DJ: Australopithecus sediba is dated to 1.9 million years ago, which is 500,000 years after Homo in eastern Africa. It doesn’t look like there are any specimens of Homo that old in southern Africa. I think sediba is just another Australopithecus species, one that has nothing whatsoever to do with Homo. It may very well have been an offshoot of Australopithecus africanus.

What is interesting about the new South African fossils is they have a strange amalgam of features, combinations of what seem to be classically Australopithecus traits and some that are perhaps reminiscent of Homo. I think what we're looking at as we develop a more complete fossil record is the great inventiveness of natural selection. The old view of linear evolution along one lineage has long been abandoned by most scholars. But we are seeing combinations of features that startle us.

KW: Over the past couple decades a number of other new hominin species have come to light. Some of them even shared the planet with Homo sapiens within the past 50,000 years. Might Lucy’s species have encountered other kinds of humans, too?

DJ: There are a couple of contenders for other species during the reign of afarensis from 4 million to 3 million years ago. One of them is Kenyanthropus [a 3.5 million- to 3.2 million-year-old skull from Kenya]. There are some specialists who think that it's really just a very distorted afarensis skull. But it certainly is possible that there was a parallel species of human there.

The other contender is represented by this fossil foot that was found in the Waranso-Mille area of Ethiopia’s Afar region by Yohannes Haile-Selassie of the Cleveland Museum of Natural History. Unlike Lucy’s foot, in which the big toe lined up with the other toes, this foot had a divergent big toe, which would have been good for climbing trees. At 3.4 million years old, the owner of this foot lived right smack in the middle of afarensis' reign. The specimen has been referred to the species Ardipithecus ramidus [best known from the 4.4-million-year-old partial skeleton dubbed Ardi], which itself has been linked to the older species Ardipithecus kadabba. This could be a lineage that was parallel to Australopithecus. So we shouldn't be surprised if we see an increasing number of species that were overlapping in time.

KW: As we said at the start of this discussion, this is the 40-year anniversary of the discovery of Lucy. Why have you and your colleagues continued to go back to Hadar for all these years?

DJ: Well, we had a nine-year hiatus during the Ethiopian revolution, but we've continued to go back to Hadar because erosion continues. Fossils erode out onto the surface that may have been buried only a half inch or less under the ground. There are always new things appearing. And after the end of the series of expeditions in the '70s we still didn’t have a skull of A. afarensis. This was a nagging realization to all of us. We had a composite skull put together from several different individuals, but we didn’t have a complete skull from a single individual. Skulls are where some of the more important and diagnostic changes in anatomy occur, because of diet predominantly, but also brain size increase and so on. So going back in the '90s, our goal was to find a skull. Fortunately there was a male skull found and there is now a fairly complete female skull as well.

KW: A few years ago researchers announced the discovery of cut-marked animal bones from a site very near Hadar called Dikika, which had previously yielded a stunning skeleton of an afarensis toddler. What do those cut marks suggest about Lucy’s species?

DJ: One of the major behavioral features of humans is omnivory. At some stage in the human past our ancestors began to expand their traditional vegetarian diets to depend more regularly on meat. If you are going to support an important organ as large as our brain you want to have a foodstuff that's high in amino acids and energy, calories, et cetera.

Stone tools, useful for processing meat, have been found as far back as 2.6 million in Ethiopia. But we now have hints that butchery began before then. Zeresenay Alemseged of the California Academy of Sciences found a small number of 3.4-million-year-old mammal bones at Dikika that had incisions on them. These were examined very closely by Curtis Marean, an archeologist at the Institute of Human Origins, and others, who concluded they were intentional cut marks made by the sharp edge of a stone. Now if that's true, it means that afarensis was already beginning to experiment and manufacture stone implements. This pushes back by 800,000 years the dawn of culture, when hominins began to purposely fashion tools out of rocks to acquire a new food source, which was meat. It doesn’t necessarily mean they were hunting, but they could have certainly been very effective scavengers, especially if they were vigilant about learning what carnivores do with their kills, like leopards taking their meals into the trees. When that leopard was away the hominins might take advantage of that.

At the moment we're finishing a program to systematically look at all of the fossils that were collected in the '70s and '90s from Hadar to see if we can document more cut marks.

Could it be that culture reaches that far back in time? Well, other things are getting pushed back in time that we thought didn’t happen until later in evolution. And we do know that in West Africa there are certain groups of chimps that use a hammer and anvil technique to break open nuts. It's not a huge leap to see a real a-ha moment when one of those proto-humans breaking nuts like that cut his or her finger on the edge of a sharp flake that came off and thought, "Oh!" It could be that simple.

KW: What are the burning questions about human evolution that remain to be answered?

DJ: The origin of Homo is what is really gripping the paleoanthropological community today. My guess is that Homo arose somewhere between 2.4 and 3 million years ago. There are teams from the Institute of Human Origins that have obtained permits to work in areas where geological deposits of those ages are exposed, and they have found fossils. They're not as fossil-rich as Hadar, for example, or many of these other sites. But I think it's only a matter of time before they begin to find fossil hominins. That will tell us something about that time range and perhaps provide the link between afarensis and the Homo line.

The question of tool-making is another one that is of particular interest to everyone, because ultimately one of the major things that distinguish humans is our ability to be infinitely inventive. Chimps have some level of culture. A chimp can strip the leaves and twigs off a stick to make a tool for catching termites. But it’s never going to decorate that tool, like a human might.

What's most interesting is that for a long time hominins made only very rudimentary tools. Our species has not been around for very long, if you consider the age of Homo sapiens at 200,000 years. Culture started like a long, heavy freight train that's just chugging out of the station, and then when it picked up speed it was like all of a sudden we're landing on the moon, and people are carrying in their hip pockets computers that used to occupy a quarter of a city block in New York City. What were the forces that caused anatomically and behaviorally modern humans to all of a sudden make quantum leaps?

That is one of the most unique features of humans, and I think ultimately it's made possible by the acquisition of symbolic thought and language. Cooperation is key, too. Try giving one person the task of building a rocket to put a man in space. That takes cooperation beyond comprehension, even after it's made.

Examining the evolutionary roots of what really makes us Homo sapiens is one of the current objectives of the Institute of Human Origins. And so our research is bookended by the emergence of our own genus and the refinement of that genus to become the most influential creature on the planet.

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Scientific American special issue on human evolution (September 2014)