The Cradle occupies 47,000 hectares of grassland and bush about 50 kilometers northwest of Johannesburg. Beneath the surface lies a complex system of limestone caves formed in ancient dolomite rock, where natural shafts and fissures have been capturing animal and hominin remains for millions of years. The fossils are typically encased in breccia -- a cement-like mixture of limestone and sediment -- which both preserves them and makes extraction painstakingly slow. Hominin remains dating as far back as 3.5 million years have been recovered from these caves, making the Cradle the largest known concentration of human ancestral fossils anywhere in the world. The site was inscribed as a UNESCO World Heritage Site under the name Fossil Hominid Sites of South Africa, recognizing not just Sterkfontein but a constellation of caves including Swartkrans, Kromdraai, and Bolt's Farm.

Robert Broom began finding hominin fossils at Sterkfontein in 1935, building on Raymond Dart's 1924 discovery of the juvenile Australopithecus africanus skull -- the "Taung Child" -- found 300 kilometers to the southwest. In 1938, a schoolboy named Gert Terrblanche brought Dart skull fragments from nearby Kromdraai that turned out to be Paranthropus robustus, a robust-jawed cousin of our lineage. At Swartkrans, C. K. Brain spent three decades recovering the second-largest sample of hominin remains from the Cradle, including evidence of the oldest controlled use of fire by Homo erectus, dated to more than one million years ago. In 1966, Phillip Tobias began excavations at Sterkfontein that became the longest continuously running fossil dig in the world. By 1997, Ron Clarke had discovered "Little Foot" -- a near-complete Australopithecus skeleton initially dated to approximately 3.3 million years ago.

In October 2013, cavers Rick Hunter and Steven Tucker squeezed through a narrow chute in the Rising Star Cave system, near Swartkrans, and found a chamber littered with hominin bones. Paleoanthropologist Lee Berger of the University of the Witwatersrand organized an expedition that November with National Geographic. Because the passage to the chamber was only about 18 centimeters wide in places, Berger assembled a team of six women scientists -- K. Lindsay Eaves, Marina Elliott, Elen Feuerriegel, Alia Gurtov, Hannah Morris, and Becca Peixotto -- chosen for their paleontological expertise, caving skills, and small frames. In three weeks they recovered more than 1,200 fossil specimens. In September 2015, Berger announced that the bones belonged to a new species: Homo naledi. The Dinaledi Chamber -- "chamber of stars" in Sesotho -- contained more than 1,500 bone specimens from at least 15 individuals, the most extensive single-species discovery ever found in Africa.

Homo naledi complicated the story of human evolution in ways that researchers are still absorbing. The species had a brain roughly a third the size of a modern human's, yet appears to have deliberately deposited its dead in the remote Dinaledi Chamber -- a behavior previously thought exclusive to much larger-brained Homo species. Intentional body disposal implies a relationship with death, a sense of ritual or at least of purpose, that challenges assumptions about what cognitive capacity is needed for such behavior. The Rising Star system also yielded fossils from a second site, designated UW-102, though the relationship between the two deposits remains uncertain. Sterkfontein alone had produced more than a third of all early hominin fossils found worldwide prior to 2010. The Cradle keeps rewriting the textbooks.

The Maropeng Visitors Centre, opened by President Thabo Mbeki in December 2005, sits at the surface of this deep-time archive. Visitors can see fossils, examine stone tools, and tour the Sterkfontein Caves where Mrs. Ples and Little Foot were found. A moveable steel structure called the Beetle has been placed over the Malapa Fossil Site, where Berger discovered Australopithecus sediba in 2008 -- two partial skeletons of hominins who lived between 1.78 and 1.95 million years ago. Excavation at Malapa has been on hold since 2009, when the remains of four individuals were carefully removed. The Cradle is not a finished archaeological project. It is an ongoing conversation between the present and a past that stretches back three and a half million years, recorded in bone and stone beneath an unremarkable stretch of South African grassland.