Phytoliths as indicators of palaeoenvironmental change during the Middle Pleistocene Transition, in the So’a Basin, Flores

Excavations in the So’a Basin, central Flores, have provided some of the earliest evidence of hominin activity in Wallacea. Stone tool evidence suggest that the likely ancestors of the small-bodied Homo floresiensis had reached the island over 1 million years ago. Understanding the paleoenvironmental context of the arrival of genus Homo across the Wallace Line is crucial to understand the evolutionary context of Homo floresiensis.

The world of this early hominin, was in flux, undergoing climate shifts and major ecological change, driven by the Middle Pleistocene Transition (MPT). Observed between 1.2 and 0.7 Ma, the MPT is known to have led to the strengthening of global monsoon systems, as well as the increased intensity and duration of glacial cycles. The effect of these climate shifts on the small insular ecosystem of Flores is uncertain, obscured by a ~400 ka gap in the fossil record. In an effort to generate much needed palaeoenvironmental data for the So’a Basin during the MPT, phytolith analysis was conducted on 22 sediment samples from Tangi Talo and Mata Menge which provide a record of local vegetation spanning from >1.27 to >0.65 Ma.

The results of this study are in agreement with previous paleoenvironmental research, concluding that vegetation during the Early Pleistocene was likely more forested, with a significant expansion of grassland occurring after 1.27 Ma and ending prior to 1 Ma. It is likely that the increasingly arid and seasonal conditions generated by the MPT enabled the spread of tropical grasses and a great reduction in the presence of woody taxa in the So’a Basin. It is likely that savannah-like conditions predominated in the So’a Basin by the time of hominin arrival. These conditions remain relatively stable with no significant vegetation shifts recorded in the phytolith assemblages following 1 Ma. After this period savannah with a nearby forests and wetland component persisted through the basin.

Overall, this study finds phytoliths to be very well preserved in the So’a Basin. There is significant potential for their application in archaeological and paleontological research in future.