One evening in September, 2013, two amateur cavers, Steven Tucker and Rick Hunter, drove into a swath of semi-wilderness an hour northwest of Johannesburg and parked at the foot of a stony slope. Wearing jumpsuits and helmets with headlamps, they ducked into the mouth of a cave, descending into a maze of jagged limestone. After worming through a series of narrow passages, they climbed a rise of rock and squeezed through one last fissure, reaching what appeared to be the end of the path. But there was a hole in the floor: a “chimney” chute leading downward.

Caving is a form of improvisation: you say yes to whatever door the earth opens. The vertical crevice measured barely seven inches wide, but Tucker, a human reed, was able to squirm down it. Forty feet below, he dropped into a chamber the size of a walk-in closet. He walked a little farther. The ceiling was spiked with stalactites. On the floor, everywhere, was bone.

The cavers hadn’t been searching for fossils that day, but they knew someone who would be very eager to see them: a paleoanthropologist named Lee Berger. Fossils of hominins—ancestral humans and their relatives—have been discovered in South Africa since the nineteenth century, when prospectors started blasting for lime, which is used in refining gold. The area surrounding this cave is known as the Cradle of Humankind, because skeletal remains of our early ancestors have been found there. But Berger was the first paleoanthropologist to systematically search underground. He was paying a former student, Pedro Boshoff, an ex-diamond prospector who rode motorcycles and wore a skull-emblazoned do-rag, to scout for him. Boshoff couldn’t fit through some openings, so he had asked local cavers—among them Tucker and Hunter—to keep an eye out for bone.

Soon after Tucker and Hunter made their discovery, they returned to the chamber and photographed the remains. When Boshoff saw the images, he and Tucker rushed them to Berger’s house, even though it was late at night. One scrap stood out: a partial jawbone, still wearing its teeth. Berger brought out a round of drinks.

Berger, who presents himself as equal parts explorer and scientist, grew up near Savannah, Georgia, and earned his Ph.D. at the University of the Witwatersrand, or “Wits,” in Johannesburg. He’s now a research professor there. He hopes to surpass the groundbreaking finds of East Africa, including the iconic australopithecine, Lucy, a 3.2-million-year-old fossil discovered by the paleoanthropologist Donald Johanson, in Ethiopia, in 1974. For many years, Berger found so little that he considered abandoning exploration. Then, in August, 2008, his son, Matthew—a nine-year-old who sometimes joined his forays into the Cradle—came across a loose rock in an old limestone mine. Embedded in the rock was a clavicle and a jaw fragment. An excavation led by Berger revealed a profusion of bones nearby, including the partial skeleton of an adolescent boy and one of a woman of about thirty, both nearly two million years old. Berger named the site Malapa, a word that in the Sesotho language means “homestead.”

All early human remains are scientifically valuable, but those dated in the vicinity of two million years old are especially prized, because they fall near a key point in the fossil record: the origin of Homo. There isn’t a paleoanthropologist alive who wouldn’t like to clarify what happened in the million-year evidentiary gap between the small-brained, long-armed australopithecines and our own, big-brained genus. The Malapa fossils showed an odd mixture of primitive and modern traits. In a series of papers published in Science between 2010 and 2013, Berger and more than a dozen co-authors described a new species: Australopithecus sediba.

Berger aggressively promotes his scientific papers. He called a press conference at the Cradle of Humankind’s visitor center to announce the discovery of sediba, which means “spring.” He later told Science, “We’re not saying this is the direct ancestor, but, if you start weighing this all, it will end up as the most probable ancestor.”

Paleoanthropologists were excited by the Malapa discovery, but many were skeptical about Berger’s bold evolutionary claims. To some, he had long seemed more interested in fame than in careful science, and his press conference struck them as theatrical and unscholarly. Yet any scientist who wanted to vet his sediba research could do so: Berger shared his data and declared the fossils available for outside study, something that paleoanthropologists traditionally had not done. Ian Tattersall, a paleoanthropologist at the American Museum of Natural History, has said that the field often resembles “a swamp of ego, paranoia, possessiveness, and intellectual mercantilism.”

Berger donated replicas of the Malapa bones to museums and schools, and started attending conferences with a sediba cast, allowing anyone to inspect it. Jeremy DeSilva, a Dartmouth paleoanthropologist who collaborates with Berger, recalls that when he visited Wits in 2009 Berger offered to open the fossil vault. “A lot of people in our business are petrified to be wrong,” DeSilva told me. “You have to be willing to be wrong. What Lee is doing takes that to another level.”

As specialists debated whether the Malapa fossils truly represented a new species, sediba became a cultural icon. A female hand was bronzed, so that South African politicians could present it to foreign dignitaries. Gift shops sold sediba earrings. Berger arranged for a tourist platform and an open-air laboratory to be built at Malapa. (It opens this summer.) Then he returned to his explorations.

South Africa’s cave openings can be hard to spot, but many have wild olive and white stinkwood trees growing near them. Berger used Google Earth to find these natural markers. Some of the emerald clusters that appeared on his computer screen might as well have been flashing arrows. The cave where Tucker and Hunter had found the chamber of bones was well known to spelunkers, but satellite images led Berger to locate an entire underground network that had not been combed for fossils. When he drove me into the Cradle, last December, he pointed out what looked like solid earth and said, “That’s a cave. And that’s a cave.” In his public appearances, Berger often shows a photograph of the golden high veldt and tells audiences, “When I look at that, I see Swiss cheese.”

In the century and a half during which scientists have been formally studying humankind’s earliest ancestry, they’ve found fossil remains of only about six thousand individuals. Most have been fragments and isolated finds. Donald Johanson, who is now seventy-two, has said that before he found Lucy all of the hominid fossils older than three million years could “fit in the palm of your hand.” The skull is the anatomical key to identifying a species and deducing how its face looked, and how it thought and ate. But the merest scraps of a hominid—a rib, a toe bone—are so rare that they are deeply coveted.

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Paleoanthropologists have pieced together fossil evidence showing that the ancestry of humankind and our relatives begins about six million years ago, moving from Sahelanthropus to Ardipithecus, and from Australopithecus to Homo, of which Homo sapiens is the last surviving species. The time line remains somewhat contested and fluid: new discoveries and interpretations have overturned old theories. Gone is the early metaphor of human evolution as a straightforward family tree. As more fossils surfaced and better research tools allowed for nuanced comparisons, the tree became a bush with many branches, depicting diverse species that overlapped in time. Genetic analysis revealed that some of our ancient relatives were surprisingly intimate with one another, encoding traces of their hookups in our DNA.