We could feel the depth of the history in the fossil vault of Kenya’s Nairobi National Museums. A high-domed ceiling keeps the interior cool, and the cabinets around the walls are filled with treasures from decades of hunting for fossil hominins. Louis and Mary Leakey used the museum, then known as the Coryndon, as a base of operations for nearly two decades before striking fossil gold at Olduvai Gorge, Tanzania, in the late 1950s. Their fossil finds were still housed here. Their son Richard followed in their footsteps, claiming his place in the pantheon of great fossil finders by leading his famed “hominid gang” on expeditions largely around Lake Turkana, in northern Kenya. On both the east and west sides of this tremendous, jade-colored lake, Richard, later joined by his wife, Meave, and their colleagues, had picked up and excavated hundreds of remains of hominins.
Some of those fossils represented very early branches of the genus Homo, and that was why we had come. Emma Mbua, curator of the collections, had kindly arranged for our team to view these wonderful collections on relatively short notice. We found ourselves seated around the large table in a laboratory adjacent to the vault—boxes of fossils set out in an orderly manner around the table, casts of the Malapa fossils distributed in a less orderly fashion. The work progressed well for almost a week, as we worked through the greater part of the East African fossil hominin record, carefully comparing every specimen with the relevant counterpart from Malapa.
Actually handling the original fossils attributed to early Homo held by the Kenya National Museums had me beginning to doubt my convictions. I had fairly convinced myself back in South Africa that the Malapa hominins belonged in our genus, Homo. Those first looks at the short Homo-like face of the Malapa fossil skull—so different from the long snouts of the South African australopiths—had put me in that frame of mind. The molar and premolar teeth of these fossils were mostly small, too—and didn’t those small teeth suggest an improved, more humanlike diet? We had worked with the casts of many of these fossils, but casts are often based on scientists’ reconstructions, and they can therefore be misleading. As I handled these original fossils, I was seeing more and more differences between the Malapa material and the fossils currently understood to belong in the genus Homo. Everyone else on the team was coming to the same conclusions.
There was that small brain—that certainly didn’t seem very Homo-like. Phillip Tobias had helped Louis Leakey define Homo habilis, focusing on the larger brains of the Olduvai Gorge skulls compared with those of the australopiths. Now, examining those very fossils, we could see that the cranial pieces of some of those habilis fossils curved gently, suggesting a brain size more like a grapefruit. Our Malapa skull was no bigger than an orange.
Still, here was a fossil skull of habilis found much later, at Koobi Fora, one of Richard Leakey’s trove of fossils collected in the 1970s. KNM-ER 1813 was the smallest habilis skull on record, with a brain size just over 500 cubic centimeters. That was still larger than our Malapa skull, but that face—with its short vertical profile—was not so different. Besides, because people had begun to accept the Flores skeleton as Homo floresiensis, brain size didn’t seem quite so important in defining Homo anymore. Maybe the Malapa skull wasn’t too small for Homo after all.
One thing was clear to all of us: Not one of the skulls, jaws, or teeth we were seeing in Kenya was a match for the features of those from Malapa. There were some strange ones there in the Nairobi museum vault: a skull with a small crest on its top for its big jaw muscles, but smaller teeth than any robust skull should have; a short, square Homo-like jaw with tremendous teeth; a classic habilis mandible with long, long wisdom teeth. We looked closely at every one of them, and each time we found that they just didn’t seem to fit what we had in the Malapa fossils. Our fossils were not what had been called Homo habilis, nor rudolfensis, nor did they really look like any of the pieces here that might belong to some form of Australopithecus. They were something new.
We carried out the work for days, debating back and forth among ourselves. Each day, our discussion would carry over into the evening, when we adjourned from the museum to a local bar, hashing out the day’s work over cold Tusker beers.
“Tomorrow, I want to perform an experiment,” I said one evening. “Let’s pull all of the characteristics out of our studies and list them as australopith-like or Homo-like, and go through each one by one. Also, let’s settle on what our definition of a genus actually is.”
This was easier said than done. Species were, in a way, pretty clear. In living animals, biologists look at species in terms of interbreeding: If populations can interbreed with each other in their natural habitat, they belong to the same species. Of course, with fossil animals, that’s not possible—if we wait for two fossils to interbreed, we’ll be waiting a very long time. So, paleontologists look at whether a fossil has unique features not seen in other fossils, and compare combinations of features with the skeletons of living populations of animals. These comparisons are not always simple, though, and they are made even harder with a fossil sample like those from Sterkfontein. With so many fragments that might cover thousands of years of time, one could imagine those remains representing a single evolving species.
But we were finding the Malapa case to be easier. There just weren’t any other fossils, neither from South Africa nor from East Africa, showing the same pattern of features we saw in these two skeletons. The fact that they were skeletons, not fragmentary jaws or a single skull, gave us confidence. The arms of these fossils were very long, and we had leg bones to show their relative sizes. We had, by now, a heel bone, and although the ankle and femur both clearly showed that the Malapa individuals were upright walking creatures, that heel had a very strange, twisted look, almost like that of a chimpanzee. The pelvis parts were not broad and flared like Lucy’s, but a bit more compact, like a human’s. It wasn’t just that we couldn’t match all the features of the skull, jaws, and teeth of Malapa to any other skull we had seen. If we had hypothesized that any of those other fossils, however fragmentary, represented the same strange mix of features found in the Malapa skeletons, people would have thought we were dreaming. No, clearly they came from a species new to science.
Deciding on a genus posed a harder problem, because different biologists give different meanings to the term. Linnaeus invented the system that biologists still use to classify species, and he used the idea of a genus to mark species that look like each other and have similar habits of life—biologists today call this idea “adaptive grade.” A hundred years later, Darwin showed that species have common origins. To biologists, this means that the members of a genus should be related to each other—that they share an ancestor. On a tree of relationships, they belong to a single branch, what biologists call a “clade.” But some branches of the tree of life include species that adapt to their environments in different ways, and it’s not always easy to tell whether species that look similar are really closely related to each other. A species can evolve rapidly, gaining a new set of adaptations that relate to its environment in a new way, making it look different from its relatives. Grade and clade may not match.
In 1960, Louis and Mary Leakey had found the first specimens of Homo habilis at Olduvai Gorge. For more than 40 years since, it had stood as the earliest known member of the genus Homo, a species that set a new evolutionary course that led to Homo sapiens, people living today. When he enlisted Phillip Tobias and John Napier to define the new species, Louis Leakey had to decide on a boundary line separating our genus from its ancestors. They based the idea of Homo upon a common set of adaptations. Tool use, more humanlike hands, a larger brain, smaller teeth—all these things together indicated a new pattern of life, one that set our ancestors on a human path. If these scientists were right, these features that define the Homo pattern of adaptation should also define one branch. Grade and clade should be one and the same.
But in more recent years, some scientists have challenged this idea. What we knew about the body of habilis, with its small size, suggested that maybe this creature did not use the environment in the same way later kinds of Homo did. It could not have been routinely walking long distances and using large areas of the landscape in the way that the larger Homo erectus and later humans had done. The smaller brain of habilis pointed in the same direction. The way habilis interacted with its environment might have been a lot more like Australopithecus. And if habilis got reassigned out of the genus Homo, a lot of the more fragmentary fossils proposed as candidates for the earliest known members of the genus would have to go, too. Whatever their places on the family tree—which no one really could say for sure—the adaptive grade of these fossils was not the same.
So, understanding the origin of our genus was a big question in the study of human evolution, and we were caught in the middle of it. These Malapa fossils were what they were—whatever we called them wouldn’t change that—and whether they were placed in Homo or not, they would provide new evidence of what the ancestors of Homo had been like. But the name might shape the way other scientists test hypotheses about Homo and its origin—and the way they might respond when we revealed these new fossils from Malapa. The same features that argued for understanding the Malapa fossils as a new species actually created a big problem for us as we tried to assign them to a genus. The mixture of traits included many that had been found in Australopithecus fossils and many others that had been found in Homo fossils. How could we decide between the two?
The next day found me in front of a flip board, pen in hand, before the whole team. I created two columns: “Primitive” and “Derived.” Primitive features were those shared with the distant relatives and ancestors of Homo, whereas derived features were those shared with humans or closer human relatives, such as Homo erectus. “OK,” I said. “Let’s start at the head.”
One by one we went through the features. The tiny brain of the Malapa hominins, at 420 cubic centimeters, went under “Primitive.” The size of the teeth, “Derived.” And on and on. Down the body, to the foot, we explored and debated each bit of morphology before placing it in a column. We then repeated the exercise, but with two different headings: “Australopithecus” and “Early Homo.” During the course of making this list, we batted back and forth ideas about how to define a genus, and we finally decided that we would focus on the adaptive grade idea: physical characteristics and abilities, not common ancestors. It was, in a way, an inevitable choice. We had some of the best evidence across the entire skeleton for how these animals interacted with their environment. But despite their many similarities with some other species of Homo, we could not say for sure that they were close relatives on the family tree. That would take more evidence from other fossil species, evidence we didn’t have.
At the end of several hours, I stepped back, scanning the two long lists. I nodded to myself before looking to the group. “OK, I’m convinced,” I said. “It shares a lot of features with early Homo, but it’s not that. It’s Australopithecus.”
There was a sigh of relief. I think several members of the team thought I was going to push for these hominins being early Homo, just because it seems sexier than Australopithecus. The lists were nearly equal in length, but we could all see that this hominin was not a long-distance walker, and its long arms appeared to be adapted to climbing. With its small brain, I just couldn’t see placing it in the genus Homo. The evidence was what it was.
That evening, sitting at a restaurant, Steve Churchill asked me the question that I think was on the minds of several members of the Malapa team: “What would it take for you to put a hominin like this one into Homo?”
I thought for a moment of the long debates and discussions held over the last many months. “If it had long legs and humanlike feet, I could disregard the brain size.”
Little did I know that in just a few years, I would face exactly this question, but with an entirely new set of bones.