Authors such as Christopher Ehret have built upon the little evidence available to develop scenarios of intensive usage of plants having built up first in North Africa, as a precursor to the development of true farming in the Fertile Crescent, but such suggestions are considered highly speculative until more North African archaeological evidence can be gathered. In fact, Weiss et al. have shown that the earliest known intensive usage of plants was in the Levant 23,000 years ago at the Ohalo II site.