Scholars have been debating the conditions that led to the evolution of humans' large brains and unique behaviors, including complex stone tool use and language, for more than a century. One influential theory proposes that these features emerged in response to seasonal resource availability in Africa over the last five million years. Geological and paleontological data suggest that changing rainfall patterns might have produced more arid African ecosystems and novel foraging behaviors, although approaches for documenting ancient precipitation patterns remain rudimentary. Under the direction of Dr. Tanya Smith, Daniel Green aims to improve methods of seasonal reconstruction with new laboratory techniques and experimentation, facilitating research on ancient climate change during key periods in human evolution.
Climatic patterns may be reconstructed through chemical analysis of teeth, which record environmental chemistry during formation and remain unchanged after death and fossilization. Tooth increments, analogous to rings in trees, form every day during childhood from ingested minerals that are in equilibrium with local water sources. As seasons pass, the composition of isotopes (atomic variants) change, and teeth record these changes. Importantly, isotopes in teeth record seasonal changes that can be related to time markers, facilitating precise estimates of environmental change. This project will improve climate reconstruction by precisely mapping the geometry and timing of tooth mineralization. A combination of cutting-edge synchrotron imaging and scanning electron microscopy of developing sheep teeth will yield mineral density change over time. This project then tests these results using animals subject to an isotopic water switch, experimentally demonstrating the relationship between tooth chemistry and changes in water that approximates seasonal variation.
Ultimately, a concrete understanding of how environmentally sensitive minerals are added to teeth will improve seasonal reconstruction, and will allow one to investigate if marked climatic change accompanied the development of unique human anatomy and behaviors. This project will train graduate and undergraduate students, and enhance collaborations with scholars in Europe and Africa. Results of this research will be broadly disseminated in professional journals and meetings, and presented to public audiences.
Seasonal rainfall and resource availability are critical determinants of largescale ecosystem structure, and are thought to have shaped unique human behaviors as early as the first stone tool production. Scientific efforts to reconstruct past seasonal rainfall patterns rely on teeth, which grow incrementally and record climatic information as heavy or light oxygen isotopes (atomic variants ingested in water). Previous efforts to reconstruct seasonal patterns have been hampered by incomplete knowledge of tooth formation, and by a lack of experimental studies demonstrating that teeth contain precise climatic information. Domesticated sheep teeth are similar to those of other herbivores, which are abundant in the fossil record and a potentially rich source of information about past seasonal rainfall regimes, climates, and contexts of human evolution. This study quantified mineral density over time in developing teeth using high-resolution synchrotron x-ray imaging from sheep that died at known ages and of natural causes. Synchrotron x-ray imaging is especially powerful in this context because it allows quantitative measurement of enamel mineral density at resolutions as low as 13 microns. To harness our x-ray data we employed a mathematical technique called Markov Chain Monte Carlo, recreated the history of mineral density increase for over 12,000 locations in the tooth crown, and built a comprehensive model of tooth growth. While previous research suggested contradictory and competing models for herbivore tooth mineralization, we found that teeth mineralize in two waves that are temporally distinct and progress using different geometric orientations. We produced a system for predicting past rainfall patterns using isotopes measured from teeth by combining our mineralization model with previous research linking environmental water chemistry with blood and tooth isotope ratios. We tested these predictions by raising our own sheep in Concord, Massachusetts, and experimentally delivering them isotopically light water collected from the Beartooth Mountains in Montana. Water from Montana is distinct from Boston water – and useful as an experimental water source – because most heavier oxygen isotopes have fallen as rain by the time moist air masses moving over North America reach the state. We devised a special protocol to measure isotope ratios in body water and found that sheep body water came to wholly reflect new drinking sources within two weeks of drinking it. Then, we devised a method of sampling isotopes from tooth enamel on a microscopic scale using dicing machines originally designed for manufacturing computer microchips. With this method we demonstrated that predictions from our model are observed in real sheep, though with higher variability than predicted by our model. Our work is the first to comprehensively reveal the geometry and timing of tooth mineralization in a large herbivore, and allows oxygen isotope chemistry in drinking water – a reflection of seasonal rainfall and climate – to be inferred from modern and ancient teeth. This research is another important step towards understanding changing climates, past ecosystems, and our own evolutionary origins in a dynamic environment.
