Late Tertiary hominoids and human origins

Abstract

There are two principal hypotheses about the time of the emergence of the Hominidae, the family that includes man: first, the hominid lineage emerged before 16 Myr ago1–4; or second, living apes and man are so similar genetically that they must have diverged later5, perhaps as late as 5 Myr ago6,7. The first hypothesis is supported primarily by fossil evidence, whereas the second is championed by advocates of the molecular clock6,7. Hominid fossils are abundant back to 3.7 Myr (Australopithecus afarenesis)8 but are rare before that. Ramapithecus (16–8 Myr) was once classified as a member of Hominidae, but recent discoveries and interpretations show that it should be placed in its own family, Ramapithecidae9,10. Two fossil specimens classified as members of Hominidae predate A. afarensis, however: a lower molar from Lukeino in central Kenya dating to 6.5 Myr (KNM-LN 335)11 and a mandibular fragment from Lothagam Hill in north Kenya dated at 5–5.5 Myr (KNM-LT 329)12. Does their morphology justify classification as the first members of Hominidae? Their geological age is relatively secure. The Lukeino Formation which is up to 130m thick is bracketed by K/A dates of 7–5.4 Myr11. The deposits from which the Lothagam Hill mandible derives are disconformably overlain by an intrusive basalt sill with a K/A date of 3.7 Myr and are associated with mammalian fauna that yields a biostratigraphical age of 5–5.5 Myr12,13. We report here a morphological study which indicates that the Lukeino molar has cusp proportions similar to the modern chimpanzee and the Lothagam Hill mandible is morphologically intermediate between modern pongids and A. afarensis.