MADISON, WISCONSIN--When the three wise men came to Bethlehem bearing gifts, the Bible story goes, one brought frankincense, which historians say was grown in the highlands of southern Arabia. But today, the region is a sandy desert with few frankincense trees. Now, by sifting through ancient piles of animal dung, researchers have found independent evidence that the region was moist enough to support agriculture, providing a more detailed history of the region's shifting climate.
To infer past climates, paleoecologist Ken Cole of the U.S. Geological Survey's Colorado Plateau Field Station in Flagstaff, Arizona, and his colleagues have spent 25 years sifting through ancient dung piles, or middens, of packrats. Pollen and seeds preserved in these middens indicate the climate by analogy with modern plants. Such work has shown that the southwestern United States became drier about 10,000 years ago, after the last ice age ended. But archaeological and historical evidence suggested that ancient Arabia and nearby east Africa stayed moist for thousands of years longer.
Working near an archaeological site in southern Yemen, Cole and his colleagues, with the help of a Bedouin guide, identified 25 middens that had been deposited in desert caves by a groundhog-sized animal called a hyrax. By picking through the seeds, twigs, leaves, and plant spines, they spotted more than 10 plant species. Together with new radiometric dates and the existing archaeological evidence, the results, presented here on 6 August at the annual meeting of the Ecological Society of America, suggest the following timeline: A moist climate from 9000 to 6000 years ago supported early farmers. They left when the area dried out about 5500 years ago, then came back after the rains returned about 2500 years ago and began growing crops, including frankincense. Since then, along with a drier climate, overgrazing by goats and camels has turned the land into a desert.
The discovery of a wetter climate a mere few thousand years ago represents "a classic surprise in paleoecology," says paleoecologist Sara Hotchkiss of the University of Wisconsin, Madison. Cole says that studying more hyrax middens could help confirm archaeological evidence of a moist climate in the region between 9000 and 6000 years ago, at the very dawn of agriculture.