Stone Age graveyard shows Sahara was once green

WASHINGTON, Aug 14 (Reuters) - A Stone Age graveyard on the shores of an ancient, dried-up lake in the Sahara is brimming with the skeletons of people, fish and crocodiles who thrived when the African desert was briefly green, researchers reported on Thursday.

The 10,000-year-old site in Niger, called Gobero after the Tuareg name for the area, was discovered in 2000 but the group has only now gathered enough information to make a full report, said University of Chicago paleontologist Paul Sereno.

The team stumbled onto the assortment of human and animal bones and artifacts while looking for dinosaur fossils.

"I realized we were in the green Sahara," Sereno, who discovered the site while working for National Geographic, said in a statement.

The site contains at least 200 graves that appear to have been left by two separate settlements 1,000 years apart.

Perhaps the most dramatic is a woman and two children, their arms entwined, laid to rest on a bed of flowers around 5,000 years ago.

The older group were tall, robust hunter-gathers known as Kiffians who apparently abandoned the area during a long drought that dried up the lake around 8,000 years ago, Sereno's team reports in the Public Library of Science journal PLoS ONE.

A second group settled in the area between 7,000 and 4,500 years ago, they said. These were Tenerians, smaller, shorter people who hunted, herded and fished.

Both left many artifacts, including tool kits, fishhooks, ceramics and jewelry, the researchers said.

"At first glance, it's hard to imagine two more biologically distinct groups of people burying their dead in the same place," said Chris Stojanowski, a bioarchaeologist from Arizona State University who has been working on the site.

The Sahara is the world's largest desert and has been for tens of thousands of years, but changes in the Earth's orbit 12,000 years ago brought monsoons further north for a while.

The team sampled tooth enamel from the skeletons, pollen, bones and examined soil and tools to date the site, artifacts and remains.

"The data from Gobero, when combined with existing sites in North Africa, indicate we are just beginning to understand the complex history of biosocial evolution in the face of severe climate fluctuation in the Sahara," the researchers wrote in their report.

Editing by Will Dunham and Cynthia Osterman