1. Powis, T. G., Cyphers, A., Gaikwad, N. W., Grivetti, L. & Cheong, K. Cacao use and the San Lorenzo Olmec. Proc. Natl Acad. Sci. USA 108, 8595–8600 (2011).
2. Powis, T. G. et al. Oldest chocolate in the New World. Antiquity 81 (2007);
https://www.antiquity.ac.uk/projgall/powis314/
3. Henderson, J. S., Joyce, R. A., Hall, G. R., Hurst, W. J. & McGovern, P. E. Chemical and archaeological evidence for the earliest cacao beverages. Proc. Natl Acad. Sci. USA 104, 18937–18940 (2007).
4. Sheets, P. D. The Ceren Site: An Ancient Village Buried by Volcanic Ash in Central America 2nd edn (Wadsworth Publishing, Belmont, 2005).
5. Coe, S. D. & Coe, M. D. The True History of Chocolate 3rd edn (Thames and Hudson, London, 2013).