About 13,000 years ago, a colder and drier period known as the Younger Dryas caused the Natufian people to alter their lifestyle. A. As lakes and rivers dried up, Natufians abandoned their settlements in some areas and became mobile hunters, developing new, more efficient hunting weapons to improve their success in the hunt. B. Cereal yields dropped when the air became excessively concentrated with carbon dioxide, which impaired photosynthesis. C. In one Natufian village, an archaeologist found the oldest-known domesticated cereal grain, dating from more than 14,000 years ago. D. A major contribution of Natufian culture to early civilization was the invention of a new arrowhead called the Harif point. E. With wild grains no longer plentiful, the Natufians had to invent better cultivation techniques, resulting in new domesticated varieties. F. The climate change forced some Late Natufians to move away from their former woodland homes and inhabit the fertile lands left by the receding rivers and lakes.