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 ago^1–4; or second, living apes and man are so similar genetically that they must have diverged later⁵, perhaps as late as 5 Myr ago^6,7. The first hypothesis is supported primarily by fossil evidence, whereas the second is championed by advocates of the molecular clock^6,7. 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, Ramapithecidae^9,10. 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 Myr^12,13. 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.