CRADLE OF HUMANKIND, South Africa — Nine-year-old Matthew Berger dashed after his dog, Tau, into the high grass here one sunny morning, tripped over a log and stumbled onto a major archeological discovery. Scientists announced yesterday that he had found the bones of a new hominid species that lived almost 2 million years ago during the fateful, still mysterious period spanning the emergence of the human family.

“Dad, I found a fossil!’’ Matthew said he cried out to his father, Lee R. Berger, an American paleoanthropologist who had been searching for hominid bones just a hill-and-a-half away for almost two decades. Fossil hunters have profitably scoured these rolling grasslands north of Johannesburg since the 1930s.

Matthew held in his hands the ancient remains of a 4-foot-2 boy who had been just a few years older than Matthew. Berger, with the Institute for Human Evolution at the University of the Witwatersrand in Johannesburg, and his fellow researchers have since found much more of the boy’s skeleton, including his extraordinarily well-preserved skull, and three other individuals. South Africa’s children will compete to name the boy.

In a report being published today in the journal Science, Berger, 44, and a team of scientists said the fossils from the boy and a woman were a surprising mixture of primitive and advanced anatomy and thus qualified as a new species of hominid, the ancestors and other close relatives of humans. It has been named Australopithecus sediba.

The species sediba, which means fountain or wellspring in Sotho, strode upright on long legs, with human-shaped hips and pelvis, but still climbed through trees on apelike arms. It had the small teeth and more modern face of Homo, the genus that includes modern humans, but the relatively primitive feet and “tiny brain’’ of Australopithecus, Berger said.

Geologists estimated that the individuals lived 1.78 to 1.95 million years ago, probably closer to the older date, when australopithecines and early species of Homo were contemporaries.

Scientists not involved in the research are debating whether the bones belong to the Homo or Australopithecus genus, but most agree that the discovery is a major advance in the early fossil history of hominids.

On Aug. 15, 2008, when Matthew called his father to look at the bones he had found, Berger began cursing wildly as he neared his son. The boy mistook his father’s profanity for anger. But from 15 feet, Berger, who had done his doctoral work on hominid shoulder bones, among them the clavicle, was astounded to see that his son had in his hands a clavicle with the unmistakable shape of a hominid.

“I couldn’t believe it,’’ Berger giddily recalled. “I took the rock, and I turned it’’ and “sticking out of the back of the rock was a mandible with a tooth, a canine, sticking out. And I almost died,’’ he said.

In March 2009 he found the remarkably intact cranium of the sediba boy whose clavicle Matthew had picked up. Donald C. Johanson, who found the famed 3.2-million-year-old Lucy skeleton in Ethiopia in 1973, described the skull as “a fabulous specimen.’’

The first downpours of the rainy season could have swept the bodies into a pool of water rich with lime and sand — the ingredients of cement — that essentially froze them in place. Berger called the sediba fossils “a time machine’’ into evolutionary processes.

Researchers think the split between apes and the hominid lineage occurred around 7 million years ago in Africa. The sparse fossil record shows early hominid species already walking upright, but still relatively apelike.

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