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2004 Field Season: Day 10
July 2, 2004
One thing is that very little is known about what the early humans in Africa looked like between about 1 million and 600,000 years ago, despite all the stone tools they left behind. In fact, the Olorgesailie fossil is the only one definitely dated to that 400,000-year gap in the African fossil record. Our find gives a little hint of what the skull looked like. Our specimen is smaller than any of the other hominin fossils known from this time range in Asia and Europe. This is rather odd, since the handaxes we find at Olorgesailie can sometimes be really large. Either these hominins were very powerful despite their size, or there were also larger individuals in the local population capable of powerful flaking of the rock.
The fossil's also interesting because it represents the ancestors of those populations from which our own species, Homo sapiens, arose. So the Olorgesailie fossil may reflect the roots of our own immediate ancestry - the ancestors of all living people. Most scientists consider our species to have evolved in Africa, probably around 200,000 years ago, and then spread to other parts of the world.
So the main thing I'm asked concerns the importance of our find for human evolution. One thing's for sure - this one fossil is not going to explain a lot of what we'd like to know. The Olorgesailie fossil could belong to the species known as Homo erectus, which first evolved about 1.7 million years ago. I think this species evolved intriguing variations and different mixtures of physical traits among the widely separate groups. This is one way to explain our discovery - the skull is of an individual from an East African population of Homo erectus. Another possibility is that Homo erectus was confined to Asia, where it is best known, and that other variations of early humans in Africa and Europe belonged to different but related species.
I used to think there were multiple species that lived between about 1.5 and 0.5 million years ago. Several other species besides Homo erectus have been proposed. For example, one called Homo antecessor comes from a small collection of fossils at Atapuerca, Spain. Another, called Homo cepranensis, is based on only a single cranium discovered at the site of Ceprano, Italy. But when I compared the Olorgesailie find to these other fossils, I noted that the Olorgesailie cranium shows similarities to H. erectus - but that all of the fossils in this period possess some unique traits and others that cut across so-called species lines. I find the variability in the skulls (and parts of skulls) impossible to divide neatly into separate lineages that existed for any substantial time, like Homo erectus in Asia did.
I think what's going on during the mid-Pleistocene is that populations tended to be pretty small in numbers and fairly localized in their movements. (We know this at Olorgesailie given that the toolmakers carried different types of stone only over short distances of several kilometers, without any evidence of exchange of rocks between distant groups.) On occasion, the populations spread and interbred across wide regions. But other times, they became isolated possibly for hundreds of generations, and so developed their own unique combination of physical features. Then, as populations moved, partly in response to dramatic environmental change during the mid-Pleistocene (even in Africa with its moist/arid cycles), the isolation broke down, or populations faced extinction. I don't see anything but H. erectus being certain in this time period. But it leaves up in the air whether the other variations, like Olorgesailie, that depart from the H. erectus standard in China and Indonesia, truly represent different species, or simply short experiments - populations that evolved in isolation for some time but didn't become different enough to prevent interbreeding. Or maybe they just failed, becoming extinct after a short existence. This is a more complicated picture than portrayed in the usual argument over how many species there are. But I think it offers a more realistic view of the population biology of that time, prior to the origin of H. sapiens.
In sum, the Olorgesailie fossil is part of wide range of physical variation in the skulls of hominins between about 1.7 and 0.5 million years ago. This variation makes it difficult to assign the Olorgesailie fossil to any one species. And it suggests that the process of evolution involved a lot of mixing of traits as populations spread, became isolated for a while, came into contact again with other groups - repeatedly for hundreds of thousands of years. For the time being, I would see the hominin population at Olorgesailie as part of a single, highly variable species, with both large and small (possibly male/female) adults.
OK, well after spending the day answering such questions, I left Nairobi to reach Kampi Safi just in time for dinner. I was told that Jessica, who just turned 13, tried her hand this morning at learning to excavate, but found it a bit slow to her taste, so she returned to camp and her preferred habit of throwing rocks off the cliff. Under one rock she turned up this lovely specimen of local wildlife. It may sound odd, but it's really nice to be back in the natural world of Olorgesailie!