The earliest settlers may have been Austroasiatic languages speakers from Southeast Asia, followed by Tibeto-Burmese from China and by 500 BCE Indo-Aryans speakers from Gangetic Plains. Due to the bio- and crop diversity of the region, archaeological researchers believe that early settlers of Northeast India had domesticated several important plants. Writers believe that the 100 BCE writings of Chinese explorer Zhang Qian indicate an early trade route via Northeast India. The "Periplus of the Erythraean Sea" mention a people called Sêsatai in the region, who produced malabathron, so prized in the old world. Ptolemy's Geographia (2nd century CE) calls the region "Kirrhadia" after the Kirata population.