Paleoenvironment of the Lowest Hominid-bearing Bed in the Sangiran Area, Indonesia

The first hominid migration out of Africa and dispersion to the eastern Asia are one of the main anthropological issues. However, we do not have consensus on their ages and pathway. The Sangiran area, Indonesia, has yielded more than 80 Homo erectus fossils. The mandibular bone, Pithecanthropus B (Sangiran 1b), has been found around the T11 volcanic ash layer in the upper part of the Sangiran Formation (the Lower Pleistocene). This specimen is one of the oldest Sangiran hominid fossils. Magnetostratigraphy shows that the T11 lies around the lower boundary of the Jaramillo subchronozone (1.07 Ma). We carried out pollen analysis of the sequence across T11 to reveal paleoenvironment when the possible first hominid arrival occurred in the Sangiran area, and discuss the causes of the emigration and settlement. In the layer from 1.0 m below to 0.5 m above T11, herbaceous taxa including Gramineae (> 50%) is dominant. They are accompanied by the aquatic taxa Typha and freshwater algae Pediastrum. The abundance of Gramineae shows dry environment. This sequence is also characterized by occurrences of Casuarinaga and Lagerstroemia, Salix, Corylus and Myrica, all living in dry lowlands. These features show that there was an open forest near freshwater. Around 0.5 m above T11, a big change in environment occurred. In the sequence above it, Gramineae (30-50%) decreased and Casuarinaga almost disappeared. Additionally, an increase in spores (10-40%) preferring a wet land shows an occurrence of wetting. At the same time, Manilkara (20-50%) increased sharply and became a main component of vegetation. The percentages of Psidium and Musa rose slightly. These taxa consist of many edible fruits. The lowland taxa like Lagerstroemia or Salix disappeared, which might show an occurrence of cooling. These environmental changes are consistent with the previous study based on diatom analysis. The result shows that around T11 there was a transgression, which may be correlated with marine oxygen isotope stage (MIS) 31. In summary, the early Indonesian hominids possibly migrated before the warm period of MIS 31. In a short time, the climate in this region became humid and they had plenty of edible fruits, which may have caused the settlement of the Sangiran hominids.