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Heat, thirst, and, for the moment, dinosaurs were forgotten as the team members followed Hettwer back to the three dunes and began to gingerly survey their slopes. In just a few minutes they had counted dozens of human skeletons. Parts of skullcaps pushed up through the sand like upturned china bowls; jawbones clenched nearly full sets of teeth; a tiny hand, perhaps a child's, appeared to have floated up through the sand with all its finger bones intact. "It was as if the desert winds were pulling them from their final resting places," said Hettwer. Insinuated among the human bones was a profusion of clay potsherds, beads, and stone tools— finely worked arrowheads and axheads and well-worn grindstones. There were also hundreds of animal bones. In addition to antelope and giraffe, Sereno quickly recognized the remains of water-adapted creatures like crocodiles and hippos, then turtles, fish, and clams. "Everywhere you turned, there were bones belonging to animals that don't live in the desert," said Sereno. "I realized we were in the Green Sahara."

For much of the past 70,000 years, the Sahara has closely resembled the desert it is today. Some 12,000 years ago, however, a wobble in the Earth's axis and other factors caused Africa's seasonal monsoons to shift slightly north, bringing new rains to an area nearly the size of the contiguous United States. Lush watersheds stretched across the Sahara, from Egypt to Mauritania, drawing animal life and eventually people.

Archaeologists have inventoried the stone tools used by these early inhabitants and the patterns inscribed on their ceramics. They have also identified thousands of their rock engravings, which depict herds of ostriches, giraffes, and elephants. Some of the images suggest that along the way the people of the Green Sahara learned to domesticate cattle. But they remain veiled in mystery. Did they arrive here from the Mediterranean coast, central African jungles, or Nile Valley? Were they nomads, or did they stake out territories and build settlements? Did they trade with each other and intermarry, or did they wage war, or both? As the monsoons began to recede, how did they cope with a drying landscape? The only part of the story that then seems clear is that by some 3,500 years ago the desert had returned. The people vanished.

Seeking answers to such questions is normally the domain of anthropologists and archaeologists—not dinosaur hunters. But Sereno had become transfixed by the discovery. "There is something soul stirring about looking into the face of an ancient human skull and knowing this is my species," he said. Whenever he could steal a moment from his paleontological work, he pored through every scholarly publication he could find on the Green Saharans, tracked down the authors and badgered them with emails full of questions. Sometimes he would read all night before downing a cup of coffee and heading back to his lab. In 2003, during another dinosaur expedition in Niger, he took three days off to revisit the dunes and survey the site, counting at least 173 burials. To dig any deeper, however, would require more time, money, and expertise.

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