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2004 Field Season: Day 13

July 5, 2004

The sweeping and sieving of loose sediment continues below Site AD5/7-1, where we discovered the early human cranium. There's a lot of interest in the geography of this site and in what it tells us about this particular individual.

Sifting below Site AD5/7-1

In earlier periods of time, at sites you may have heard of like Olduvai, Hadar, and Lake Turkana, there are lots of fossil human bones that researchers have discovered. These bones are usually found buried along the ancient lake margin or in stream channels - in other words, in the lowlands where sediments are deposited and accumulate in layers. We long wondered, why is Olorgesailie so different? From 1986 to 1996, we had dug in nearly every area of the lowland basin, in almost every kind of sediment. We were happy about our discoveries regarding early human behavior, environmental change, and the animals and vegetation that lived with the early humans. But no hominin fossils had been found - and that includes all the way back to Glynn Isaac's excavations here in the 1960s and the Leakeys' digs in the 1940s.

While Olorgesailie seemed strange in this way, it wasn't so strange for the mid-Pleistocene time period recorded here. In fact, not a single early human fossil in East Africa had been securely dated to this period between about 1 million and 600,000 years ago. There are nice fossil specimens from Ethiopia known from the ends of this time period. But why were there no hominin bones preserved and found so far in the sediments with all the handaxes at Olorgesailie and elsewhere?

At least for Olorgesailie, I think we've found an explanation. On a previous day, I mentioned that the hominin discovery site occurred along the slope of a volcanic ridge that extended up towards Mount Olorgesailie. The fact it was found at this location helps support what I call the highland hypothesis - an idea we suggested in a paper published in 1999. We developed this hypothesis as a result of many years of finding animal fossils and stone tools, but not finding any hominin fossils. About 10 years ago, we were drawing a map of the diverse kinds of volcanic rock in the highlands. We found basically all of the local types of rock from which the toolmakers made handaxes.

From this work, it dawned on me that the hominins had been really active on Mt. Olorgesailie and the nearby volcanic ridges. Could it be that this is where they primarily lived? And since their bones weren't found on the lake margin or in stream channels with the fossil animals and stone tools, could the hominins have been living and dying in the highlands?

The idea of sleeping on the higher ground rather than next to water seemed an attractive idea. Lakes, ponds, and stream channels in the African bush are good natural sources of water and plant food during the day. But at night they turn into really great places if you want to be hunted down as prey! The water margins attract the big and small predators that like to hunt in the dark of night. Even today at Olorgesailie, we often go to sleep hearing hyenas, jackals, and sometimes lions growling and whooping off in the distance during their nighttime prowls. Anyway, early humans could get food in the lowlands - that's where they left the chipped stone tools and other evidence of their activities. And, unlike earlier hominins, they could have avoided the favored hunting areas of other predators if they got to higher ground at night.

That sounds smart. But how could we test this hypothesis to see whether it's correct or not? The problem is that it's difficult, if not impossible, to find fossil remains on the high ground because no sediments are deposited there. The one possibility involved the sediments deposited along the ridges that linked the highlands and the lowlands. Over time, sediments built up along the sides of the ridges, and if anyone or anything had died while walking on a ridge, some parts of the body could have fallen or been washed into the sediments along the sides.

So, in 1996, I decided to excavate along two of the volcanic ridges. We found hundreds of fossils and stone tools nestled in among the cracks and crevices in the rock. We kept at these excavations for a couple years, and had many fossil bones to clean and look at back in Nairobi, which took up until last year. Then, in 2003, we finally cleaned a fossil that had been almost completely encased in sediment. We had excavated the block of sediment earlier, but all we had was an edge-on view of a thin piece of the braincase, not much like the thick braincases of early humans known in this time period from Asia and Europe. This specimen was obtained from yet a third ridge that we hadn't explored much before. It's the ridge where Site AD5/7-1 is now located. And the fossil inside the sediment turned out to be the brow ridge of the first hominin ever discovered at Olorgesailie. The thin piece of braincase was a small part of the frontal bone that rises up from the brow. As I mentioned in the July 2 dispatch, the skull is of a remarkably small individual.

The hominin site, AD5/7-1.
You can see in the photograph where the man in the white shirt is standing - that's the level at which the hominin was found. In fact, when we returned last year, we found more pieces of the braincase at that level and on the erosion slope just below. Above where the man is standing is the now-buried top of the volcanic ridge. In our excavation this season, we're down to the boulders at the base of the ridge, which you can also see in the first photograph, taken from the top of the ridge looking down over the excavation.

Finding this fossil along the ridge supports the idea that this individual was on or near this ridge leading between the highlands of Mt. Olorgesailie and the lowlands. But I think we'd have to find more fossil hominins along this and other ridges before we feel comfortable with the highland hypothesis. For now, it just seems like a good idea - especially since it motivated us to excavate in these unusual places along the ridges.

An interesting side note, which didn't make it into the paper published last week in Science: The Olorgesailie fossil has carnivore bite marks on the left brow ridge. It's interesting to imagine that maybe here was an individual that didn't quite make it to the highlands one night. But it's also possible that a carnivore had scavenged part of this unfortunate individual's body after he or she had died of other causes.

For now, our excavation crew feels a great deal of affection for this fossil. We just call it "our little guy."