(Continued from Page 1.) Meanwhile, volcanoes in Ethiopia lay down layers of ash, pumice and other fragments through frequent eruptions. As this material begins to cement over time, it becomes tuff. Tuff sticks out from other layers because it is so light in color. The river and volcano activity are preserved in the varying layers of sediments that make up rock sequences in the region. In the background image the tuff is visible as light tan layers and the sequence is capped by a dark layer of basalt. Researchers can start to build a chronology of events by analyzing rock layers above and below the layer that contains fossil fragments from Ardipithecus ramidus, thus narrowing in on an age for the fossils.
Two types of absolute dating were of particular importance for this research. Samples of tuff from layers above and below the discovered fossil fragments were dated using radiometric dating, specifically Argon-40/Argon-39 dating. While the ash itself cannot be dated using this technique, ash nearer volcanoes may contain tiny plagioclase (a mineral) crystals that formed when magma spewed by the volcano rapidly cooled, trapping a particular amount of potassium inside individual crystals. Over time some of this potassium decayed into argon. By measuring the amount of argon to original potassium, and knowing that the decay rate is constant, an age can be determined for the sample. (This process is described in more detail in “An Interview with Geologist Jay Quade – Tools of Time.”) Combining the radiometric data with paleomagnetic data (also described in “Tools of Time“) Dr. Quade’s team was able to determine that the Ardipithecus ramidus fossils are between 4.51 and 4.32 million years old.
The article in Nature is not the end of story. While he could not speak specifically about what has been found, Dr. Quade indicated that his recent trip back to Ethiopia was fruitful. “There is much more complete evidence now. Various groups have found it but it’s unpublished. So stay tuned. […] It’s an ongoing story and there’s a lot happening.”
While the story about human origins continues to unfold, Dr. Quade is continuing other lines of research, including refining the use of the geologic record to reconstruct ancient climates and environments and studying the Atacama Desert in Chile as a Mars analog.
A few million years of evolution have led us from Ardipithecus ramidus trying to survive the violence of the Ethiopian environment to Homo sapiens sapiens – modern humans – also trying to survive the violence of the same region (see “An Interview with Geologist Jay Quade – Geology, Famine and War.”) But there is a huge difference between these two species, evident in the professional and personal passion of scientists like Dr. Quade. Modern humans also return to Ethiopia to sift through rock fragments and study rock layers because they are passionate about their interests and curious about our origins. We reconstruct our past to build a better future by using our higher brain functions, a result of the sometimes violent evolution of the primate family, to pursue knowledge.
The beckoning fossil fragments have something to tell us. That we listen is the direct result of evolution at work. Our response to this new knowledge will indicate where humanity is heading.