Hominins from the Upper Laetolil and Upper Ndolanya Beds, Laetoli

Abstract

Renewed investigations at Laetoli in northern Tanzania have led to the recovery of a number of new fossil hominins. A lower canine and a mandibular fragment from the Upper Laetolil Beds (3.63–3.85 Ma) are referred to Australopithecus afarensis, and an edentulous maxilla and a proximal tibia from the Upper Ndolanya Beds (2.66 Ma) are attributed to Paranthropus aethiopicus and Hominini gen. et sp. indet., respectively. Additional hominin specimens from earlier collections are described here for the first time, including three specimens of A. afarensis, probably from the Upper Laetolil Beds, and a possible cranial fragment of an infant from the Upper Ndolanya Beds. The chronology and provenance of the Laetoli hominins are reconsidered. The species afarensis is provisionally retained in Australopithecus to reflect its anatomical and paleobiological similarities to the other species of Australopithecus sensu lato, but a reasonable case could be made on phylogenetic grounds to transfer it to Praeanthropus. It has been argued that the Laetoli sample of A. afarensis is morphologically and temporally intermediate between A. anamensis and the Hadar sample of A. afarensis, and that A. anamensis and A. afarensis represent a single anagenetically evolving lineage. However, the new specimens from the Upper Laetolil Beds help to close the gap between the Laetoli and Hadar samples, and a critical assessment of the morphological variation in the two samples indicates that there are few consistent differences separating them. Rather than being intermediate in morphology, the Laetoli sample appears to represents an earlier population of A. afarensis, with almost the full complement of derived features that characterizes the Hadar sample, but still retaining a few primitive traits. The morphological features that distinguish A. anamensis from A. afarensis are much more extensive, and these provide adequate justification for the recognition of a species distinction. The evidence best fits an evolutionary model involving a cladogenetic event rather than a simple anagenetic transformation of a single unbranched anamensis-afarensis lineage through time. The Paranthopus aethiopicus specimen from the Upper Ndolanya Beds represents the ­oldest securely dated specimen definitively attributable to this taxon and the first definitive record outside of the Turkana Basin. The Paranthropus clade probably immigrated into eastern Africa before 2.7 Ma, and became widely distributed throughout the region soon thereafter. The timing and biogeographic patterning of the occurrence of Paranthropus and Homo suggest that their respective dispersals into eastern Africa were not coincident or synchronous. Homo appeared somewhat later than Paranthropus across most of eastern Africa, except in the Awash region of Ethiopia where Homo makes its first appearance in the absence of Paranthropus. These differences in the timing and distribution suggest that Paranthropus and Homo may have had different biogeographic histories, and that their ancestral species may have had different ecological requirements at the time of their initial influx into eastern Africa.