Dating rice remains through phytolith carbon-14 study reveals domestication at the beginning of the Holocene

Abstract

Phytolith remains of rice (Oryza sativa L.) recovered from the Shangshan site in the Lower Yangtze of China have previously been recognized as the earliest examples of rice cultivation. However, because of the poor preservation of macroplant fossils, many radiocarbon dates were derived from undifferentiated organic materials in pottery sherds. These materials remain a source of debate because of potential contamination by old carbon. Direct dating of the rice remains might serve to clarify their age. Here, we first validate the reliability of phytolith dating in the study region through a comparison with dates obtained from other material from the same layer or context. Our phytolith data indicate that rice remains retrieved from early stages of the Shangshan and Hehuashan sites have ages of approximately 9,400 and 9,000 calibrated years before the present, respectively. The morphology of rice bulliform phytoliths indicates they are closer to modern domesticated species than to wild species, suggesting that rice domestication may have begun at Shangshan during the beginning of the Holocene.

Keywords: Shangshan; chronology; phytolith-occluded carbon; radiocarbon dating; rice domestication.

Fig. 1.

Locations of archaeological sites.

Fig. S1.

Soil profiles of Shangshan (A), Huxi (B), and Hehuashan (C) sites.

Fig. 2.

Images and EDS analysis of phytoliths extracted from cultural layers. (A) Images of phytoliths; (B) optical microscopy of phytoliths; (C) SEM images of phytoliths; and (D) EDS analysis of phytolith surface. (The white boxes are EDS spectra of phytoliths, which are shown in Fig. S2.)

Fig. S2.

EDS analysis of phytolith surfaces. 1 and 4 are EDS spectra of elongate phytoliths; 2 and 3 are EDS spectra of bulliform phytoliths. EDS analysis results show that the predominant elements of the phytolith surfaces are silicon and oxygen, and weight percent ratios of Si and O vary between 0.6 and 0.8. No carbon was detected by the EDS spectra of the phytolith.

Fig. S3.

X-ray powder diffractogram of phytoliths. XRD patterns of phytoliths show no typical crystalline phases in the curve, except for two small peaks at 2.51 and 4.08 Å, indicating that the extracted materials did not fully crystallize. The XRD results demonstrate that the extracted materials are opal (SiO₂·n H₂O, here called phytoliths).

Fig. 3.

Calibrated 2σ probability distribution of radiocarbon assays on phytoliths (red) and other dating materials (gray). OxCal v4.2.4 (59); r;5 IntCal13 atmospheric curve (60).

Fig. S4.

Charcoal sample recovered from stratum 5 of Huxi (HX-5-C). (A) The whole sample before pretreatment. (B) The analyzed sample. There are some pieces that are lighter in color, although the sample received an alkali treatment (two times) to remove mobile humic acids in addition to the acid to remove carbonates. The sample is small (0.97 mg) and cannot receive any additional treatment for fear of it becoming too small. We cannot pick out the material or it will become too small.

Fig. S5.

Rice bulliform phytoliths. (A–D) More than nine fish-scale decorations; (E–H) fewer than nine fish-scale decorations. (Scale bars, 20 μm.)

Fig. 4.

Morphological characteristics of rice bulliform phytoliths from Shangshan, Hehuashan, and Huxi sites. HL, horizontal length (black circles); VL, vertical length of rice bulliform (black squares); black error bar represent ±1 SD; red diamonds represent the fish-scale decorations of rice bulliforms; SH-8 (9,417–9,134 cal yr B.P.), HHS-5 (9,121–8,992 cal yr B.P.), HHS-3 (9,030–8,762 cal yr B.P.), and HX-6 (8,406–8,221 cal yr B.P.).

Fig. S6.

Several pottery sherds tempered with rice husks from Shangshan.