23 July 2004
I mentioned a few days ago the fossil animals found in the Olorgesailie basin, and that they differed from their living relatives. There is another aspect to the change in animal species that I didn't mention - through time, certain species disappeared from the valley, not necessarily because they went extinct and were replaced by another similar animal (like Elephas recki replaced by the living African elephant Loxodonta africana) but because the environment changed dramatically.
We first noticed this pattern when we compared the species discovered in Upper Member 1, Lower Member 7 and Members 10 and 11. The predominant species in Upper Member 1, dated to 990,000 years old, were zebras. In Lower Member 7, at 900,000 years old, the two dominant species were the large baboon Theropithecus oswaldi and the zebra Equus oldowayensis. In Members 10 and 11, roughly 650,000 years old, large wild pigs and hippopotamuses topped the species lists. Throughout these members there were relatively few antelope, known collectively as bovids.
These widely varying patterns of species seemed very bizarre until we compared them to collections of species from other East African sites. The comparison shows that Olorgesailie fauna wasn't any stranger than those combinations of game animals found in other regions between about 1.2 million and 400,000 years ago, where we see the same pattern of breakup and reassembling of species mixes. What could explain these shifts? The main hypotheses proposed by ecologists are that climate and the availability of grass may control which grazing species are found together. Or that competition controls which species occur together, such as when one dominant grazing species controls the availability of grass to other species. Or another idea is that those animal species that are most widespread, such as elephants and impala, are most likely to occur together in any given region. Yet none of these factors seemed to explain the dramatic changes in the dominance from one set of grazing species to different combinations of species through time at Olorgesailie and elsewhere in Africa.
Why were the animal communities at Olorgesailie so changeable? The key, it seems, is that the species were responding to environmental change. The sediment layers here show many large-scale shifts in the landscape - from times dominated by lake, to times when the lake dried up, and other times when volcanic ash changed the chemistry of the soil and killed off all the grass. This meant that the various combinations of species gathered (assembled) in the basin for a period of time and then broke apart (or disassembled). The reason why they disassembled is that many of the animal species were forced to leave the basin during a widespread shift in the availability of grass and water. At one point, the environment encouraged one grouping of animals, but then the environment changed and some of the old species didn't become as dominant as before when they migrated back into the region.
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The type of drastic environmental shifts that drove this process can be seen all over the Olorgesailie basin. In the photograph you can see a thick layer that forms a ridge in the side of a hill. This layer was formed when the lake that filled the basin dried up abruptly and completely. Above and below this layer are layers of diatomite, showing the presence of a large, fresh-water lake in the valley. Animals that relied on the lake would have had a hard time when it disappeared, and would have either died off or simply migrated away from Olorgesailie. But then, later, as the lake returned, a new grouping of species would have repopulated the area.
But what does this pattern of local species change tell us about hominins? Well, the tools made by the hominins appear in all of the layers that contain animal bones. The stone tools indicate that the hominins were able to endure the changes in habitat. Maybe they were able to remain in the Olorgesailie region when big climate changes occurred or volcanic eruptions changed the landscape. Or perhaps they were able to quickly recolonize the basin soon after any of these large habitat shifts occurred. As long as there was plenty of water (which there was in the Olorgesailie basin at those times), the hominins were able to live, probably helped by the versatile handaxes we described yesterday.