The Desert Laboratory sits on Tumamoc Hill overlooking the city of Tucson, Arizona. Rainfall amounts here have been above normal this winter and the desert is in bloom. I am sitting in the office of Dr. Jay Quade, from where the view of both Tucson and the desert is spectacular. A faculty member in the Geosciences Department at the University of Arizona, Dr. Quade has recently returned from his latest trip to Ethiopia with samples of tuff – a rock of compacted volcanic ash barely distinguishable from soil – important for dating sedimentary layers in the region. Why does the tuff need to be dated? Somewhere between layers of datable tuff lies the dates of important events in the origins of modern humans.
With a climate similar to Tucson at its warmest, Gona Western Margin in the region of Afar in Ethiopia on the African continent has become a hot spot for scientists looking for fossil evidence of hominids, those evolutionary precursor primates related to modern humans. Dr. Quade is a member of the expedition that recently announced in the journal Nature the second discovery to date of fossil remains of Ardipithecus ramidus. This species may be one of the last common ancestors of humans and chimpanzees.
“I know “ramidus” means “root” in the local Afar language,” explains Dr. Quade, “and that’s the basic idea…is that this is near the base of the human tree, in fact, down in the root system somewhere.” The sample data used by the team for their Nature article “Early Pliocene hominids from Gona, Ethiopia” suggests the fossils are nearly 4.5 million years old. This is significant because around that time the hominid branch splits, leading to chimpanzees and modern humans. In fact, Ardipithecus ramidus has both human and chimpanzee characteristics.
“If you were to encounter these creatures in the street today, they’d look like apes. You’d say ‘that’s an ape,’ except for one thing…there is some evidence that they were bipedal – in other words, upright walking.” That bipedal ability is shared with modern humans. However, Ardipithecus ramidus likely had the brain size of modern chimpanzees.
Dr. Quade was asked to join the research team in 1999 during a panicked telephone call. The original geologist was no longer available. With the expedition about ready to begin, Dr. Quade agreed to head for Ethiopia. He brought to the team the ability to work with fine layers of sediments and trace these layers over long distances, in order to find samples that can be dated by current dating procedures. He also had experience in reconstructing paleoenvironments from the little evidence available in the geologic record. As the team began to explore As Duma in Gona, Dr. Quade was able to point the way to promising sites. Should interesting fossil fragments be found, he could then help restrict their age.
Why Ethiopia? “It’s clear that Africa is the cradle of mankind. When you look at the fossil record within Africa and then outside of Africa for, say, the Pliocene period, 4.5 million years ago, or 6 million years ago, the late Miocene, our antecedents are showing up in deposits in Africa, and mostly in Ethiopia, and they’re not showing up outside of Africa. So we know that Africa itself is the cradle, but we don’t really know where. You have to keep in mind that Ethiopia is famous for its fossils not necessarily because that’s where man developed, down in the Ethiopian Rift, in these big river valleys that occupy the Ethiopian Rift, but it is were they’re preserved.”
The geologic record is preserved in the basin formed by the rifting of Ethiopia. The land is being ripped apart by tectonics, and in a few million years the Red Sea will intrude further into the African continent. There are plenty of rivers in the region to deliver sediments to lower lying areas. The fossil fragments found tend to be in sediments deposited near lake shores. (Go to Page 2.)