letters to nature sulphides can have different Os isotope compositions. Care must therefore be exercised in interpreting whole-rock PGE and Re/Os data without detailed information on these sulphides. • Starch grains reveal early root crop Received 29 February; accepted 23 August 2000. horticulture in the Panamanian 1. Snow, J. E. & Schmidt, G. Constraints on Earth accretion deduced from noble metals in the oceanic mantle. Ncííurc 391, 166-169(1998). tropical forest 2. Lorand, J. P., Gros, M. & Pattou, L. Eractionation of platinum-group elements in the upper mantle: a detailed study in Pyrenean orogenic peridotites. /. Petrol. 40, 951-987 (1999). Dolores R. Piperno*, Anthony J. Raneret, Irene Hoist* 3. Rehkämper, M. et al. Non-chondritic platinum-group element ratios in oceanic mantle lithosphère: & Patricia Hanseiit Petrogenetic signature of melt percolation. Earth Planet. Sei. Lett. 172, 65-81 (1999). 4. Borisov, A., Palme, H. & Spettel, B. Solubility of palladium in silicate melts: impÜcations for core * Smithsonian Tropical Research Institute, Box 2072, Ancon, Balboa, formation in the Earth. Geochim. Cosmochim. Acta 58, 705-716 (1994). 5. Jagoutz, E. H. et al. The abundance of major, minor and trace elements in the earth s mantle as derived Republic of Panama from primitive ultramafic nodules. Proc. Lunar. Planet. Sei. Conf. X, 2031-2050 (1979). t Department of Anthropology, Temple University, Philadelphia, 6. Mitchell, R. H. & Keays, R. R. Abundance and distribution of gold, palladium and iridium in some Pennsylvania 19122, USA spinel and garnet Iherzolites: implications for the nature and origin of precious metal-rich intergranular components in the upper mantle. Geochim. Cosmochim. Acta 45, 2425-2445 (1981). Morgan, J. W. Ultramafic xenoliths: clues to earths late accretionary history. /. Geophys. Res. 91, Native American populations are known to have cultivated a large 12375-12387(1986). number of plants and domesticated them for their starch-rich O'Neill, H. S. C. The origin and the early history of the Earth•A chemical model. Part 2: The Earth. underground organs'. Suggestions^'^ that the likely source of many Geochim. Cosmochim. Acta 55, 1159-1172 (1991). Meisel, T., Walker, R. J. & Morgan, J. W. The osmium isotopic composition of the Earth's primitive of these crops, the tropical forest, was an early and influential upper mantle. Nature 383, 517-520 (1996). centre of plant husbandry have long been controversial''"^ because Pattou, L., Lorand, J. P. & Gros, M. Non-chondritic platinum-group element ratios in the Earth's the organic remains of roots and tubers are poorly preserved in mantle. Nature 379, 712-715 (1996). archaeological sediments from the humid tropics. Here we report Shirey, S. B. & Walker, R. J. The Re-Os isotope system in cosmochemistry and high-temperature geochemistry. Anna. Rev. Earth. Planet. Sei. 26, 423-500 (1998). the occurrence of starch grains identifiable as manioc {Manihot Bulanova, G. P., Griffin, W. L., Ryan, C. G., Shestakova O. Ye. & Barnes, S. J. Trace element in sulfide esculenta Crantz), yams {Dioscorea sp.) and arrowroot {Maranta inclusions fi^om Yakutian diamonds. Contrib. Mineral. Petrol. 124, 111-125 (1996). arundinacea L.) on assemblages of plant milling stones from Guo, J., Griffin, W. L. & O'Reilly, S. Y. Geochemistry and origin of sulfide minerals, in mantle preceramic horizons at the Aguadulce Shelter, Panama, dated xenohths: Qifin, southeastern China./. Petrol. 40, 1125-1149 (1999). Hart, S. R. & Ravizza, G. E. in Reading the Isotopic Code (eds Basu, A. & Hart, S. R.) 123-134 between 7,000 and 5,000 years before present (BP). The artefacts (American Geophysical Union, Washington DC, 1996). also contain maize starch {Zea mays L.), indicating that early Lorand, J. P. & Conquere, F. Contribution a l'étude des sulfures dans les enclaves de Iherzofites à horticultural systems in this region were mixtures of root and seed spinelle des basaltes alcalins (Massif Central et Languedoc, France). Bull. Minéral. 106, 585-606 (1983). crops. The data provide the earliest direct evidence for root crop Dromgoole, E. L. & Pasteris, J. D. in Mantle Metasomatism and Alkaline Magmatism (eds Morris, E. & cultivation in the Americas, and support an ancient and indepen- Pasteris, J. D.) 25-46 (Special Paper 215, Geological Society of America, Washington DC, 1987). dent emergence of plant domestication in the lowland Neotropical Szabó, C. S. & Bodnar, R. J. Chemistry and origin of mantle sulfides in spinel peridotite xenoliths from forest. alkaline basaltic lavas, Nograd-Gomor Volcanic Field, northern Hungary and southern Slovakia. Geochim. Cosmochim. Acta 59, 3917-3927 (1995). The site (Ag-13) is located on the Pacific coastal plain of Central Mackovicky, M., Mackovicky, E. & Rose-Hansen, J. in Metallogeny of Basic and Ultrabasic Rocks (eds Panama approximately 17 km from the sea'. Today the region Gallagher, M. J., Ixer, R. A., Neary, C. R. & Prichard, H. M.) 415-425 (Inst. Min. Metí., London, receives 1,600 mm of precipitation annually, distributed on a 1986). highly seasonal basis, and has a mean annual temperature of Lorand, J. P. Are spinel Iherzolite xenoliths representative of the abundance of sulfur in the upper mantle? Geochim. Cosmochim. Acta 54, 1487-1492 (1989). 26 °C. The potential vegetation is a deciduous tropical forest. Ballhaus, C. & Ryan, C. G. Platinum-group elements in the Merensky Reef. 1. PGE in solid solution in Excavations carried out in 1973-75 and 1997 uncovered the base metal sulfides and the down-temperature equilibration history of Merensky ores. Contrib. remains of three distinct human occupations. Zone D, the oldest, Mineral. Petrol. Ill, 241-251 (1995). Balhaus, C. & Sylvester, P. Noble metal enrichment processes in the Merensky Reef, Bushveld complex. is a yellow (inside the dripline) or red (beyond the dripline) silty clay /. Petrol. 41, 546-561 (1999). on top of weathered bedrock. It has uncalibrated "C dates of 10,725 Luguet, A. & Lorand, J. P. Minéralogie des sulfures de Fe-Ni-Cu dans les peridotites abyssales de la ± 80 BP (NZA-10930), 10,529 ± 184 BP (NZA-10930), 8,423 ± 79 BP zone Mark (ride médio-Atlantique, 20-24°N). C.R. Acad. Sei. Paris 329, 637-644 (1999). Li, C, Barnes, S. J., Mackovicky, E., Rose-Hansen, J. & Mackovicky, M. Partitioning of nickel, copper, iridium, rhenium, platinum and palladium between monosulfide solution and sulphide liquid: Effects of composition and temperature. Geochim. Cosmochim. Acta 60, 1231-1238 (1996). Table 1 Number and types of starch grains on milling stones from the Peach, C. L., Mathez, E. A., Keays, R. R. & Reeves, S. J. Experimentally-determined sulfide melt-silicate Aguadulce Shelter melt partition coefficients for iridium and palladium. Chem. Geol. 117, 361-377 (1994). Catalogue number Manioc IVIaize Dioscorea spp. Arrowroot other Totai n Fleet, M. E., Crocket, J. H., Liu, M. & Stone, W. E. Laboratory partitioning of platinum-group elements (PGE) and gold with application to magmatic sulfide-PGE deposits. Lithos 47, 127-142 Zone C, bottom (1999). Block 3 (42) 8 1 3 0 2 14 Burton, K. W, Chiano, P., Birck, J.-L. & Allègre, C. J. Osmium isotope disequilibrium between mantle Block 2 (26b) 1 2 0 0 0 3 minerals in a spinel-lherzofite. Earth Planet. Sei. Lett. 172, 311-322 (1999). Block 3 (38) 0 1 0 0 0 1 Alard, O., Pearson, N. J., Griffin, W. L., Graham, S. & Jackson, S. E. in Beyond 2000, New Frontiers in 1S3E(439) 0 7 0 0 0 0 Isotope Geoseience, Extended Abstract Volume 1-5 (Lome, Australia, 2000). Zone C, middle Kullerud, G., Yund, R. A. & Moh, G. H. Phase relation in the Cu-Fe-Ni, Cu-Ni-S and Fe-Ni-S systems. Block 2 (26a) 5 15 2 2 5 29 Econ. Geol. 4, 323-343 (1969). Zone C, top McDonough, W. F. & Sun, S. S. The chemical composition of the Earth. Chem. Geol. 120, 223-253 1S4E(350) 0 25 16 0 13 54 (1995). Zone B, bottom Supplementary information is available on Natures World-Wide Web site Block 2 (23) 0 4 0 0 0 4 (http://www.nature.com) or as paper copy from the London editorial office of Nature. Zone B, middie 1S6E(519) 0 3 0 0 9 12 Acknowledgements Zone B, top 3W1N(243) 0 7 0 0 0 7 We thank F.R. (Joe) Boyd, S. Talnikova, Y. Barashkov and W.J. Powell for providing ONOE (402) 0 7 0 0 0 7 samples, and N.J. Pearson, C. Lawson and A. Sharma for assistance with the analytical 1S5E (316-1) 0 3 0 0 0 3 facihties. We thank Y. Lahaye for comments on an earlier version of this Letter, and 3W7N(318) 0 2 0 0 1 3 R. Carlson and M. Rehkämper for comments on the final version. This is a GEMOC Block/geographic orientation indicates tlie excavation unit. Field catalogue number for each National Key Centre publication. artefact is given in parentheses. Ali artefacts except tool 26a, a boulder miiling stone base, are edge-ground cobbles. Artefacts with a blocl< number are from the 1973-1975 excavations, and Correspondence and requests for materials should be addressed to O.A. those with a geographic orientation are from the 1997 excavations. The single beli-shaped grain (e-mail: [email protected]). from tooi 26b is very likeiy to be from manioc. 894 A! © 2000 Macmillan Magazines Ltd NATURE I VOL 407119 OCTOBER 20001 www.nature.com letters to nature (NZA-9625), 7,061 ± 81 BP (NZA-9624) and 5,560 ± 80 BP (UCR- had a limited selection of modern comparative starches to identify 3462), all direct accelerator mass spectrometry (AMS) dates on domesticated plants. The 1997 excavations were designed to sediment phytoliths*'. The 5,560 ± 80BP date was run on a very increase the sample of milling stones and starch and to study the small sample that was recovered in a soil column directly under- context and chronology of archaeological starch grains in more neath a larger sample dating to 6,910 ± 60 BP.