Figure 1. Lower Pleistocene hominin crania from the ‘Hanging Remnant’ unit of Member 1 of the Swartkrans Cave (South Africa). Specimen SK 847 (above) is commonly classified in Homo erectus and SK 48 (below) in Paranthropus robustus. According to the single species model, both would be assigned along with living humans to H. sapiens.

Figure 2. Genetic distances between humans, chimpanzees and the human–chimpanzee last common ancestor (HC-LCA). The distances between humans and the HC-LCA, and between chimpanzees and the HC-LCA, are corrected for a rate slow-down in the last 1 million years of the human lineage (see text).

Figure 3. Number of species in the direct lineage to modern humans estimated from genetic distances. Assuming a speciation model of cladogenesis, each time a new species arises within the direct lineage, bifurcation leads to the origin of a second (sister) species. Under this model, diversity within the hominin clade would be estimated at five species. (Note: white arrows represent the direct lineage to modern humans.)

Figure 4. Species lineages identified in the present study are approximately equivalent to five of the major genera used for the hominin clade.

Figure 5. Alternative scenario of species diversity in the hominin clade. Some of the assumptions and criteria used earlier have been relaxed. This estimate is close to reaching the maximum number of species that can be inferred using a genetic yardstick.