Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record.
We tested the hypothesis that early Pliocene Australopithecus anamensis was ancestral to A. afarensis by conducting a phylogenetic analysis of four temporally successive fossil samples assigned to these species (from earliest to latest: Kanapoi, Allia Bay, Laetoli, Hadar) using polarized character-state data from 20 morphological characters of the dentition and jaws. If the hypothesis that A. anamensis is ancestral to A. afarensis is true, then character-state changes between the temporally ordered site-samples should be congruent with hypothesized polarity transformations based on outgroup (African great ape) conditions. The most parsimonious reconstruction of character-state evolution suggests that each of the hominin OTUs shares apomorphies only with geologically younger OTUs, as predicted by the hypothesis of ancestry (tree length=31; Consistency Index=0.903). This concordance of stratigraphic and character-state data supports the idea that the A. anamensis and A. afarensis samples represent parts of an anagenetically evolving lineage, or evolutionary species. Each site-sample appears to capture a different point along this evolutionary trajectory. We discuss the implications of this conclusion for the taxonomy and adaptive evolution of these early-middle Pliocene hominins.
Kimbel
William H
WH
Institute of Human Origins, School of Human Evolution and Social Change, Arizona State University, Tempe, 80287-4101, USA. wkimbel.iho@asu.edu
Lockwood
Charles A
CA
Ward
Carol V
CV
Leakey
Meave G
MG
Rak
Yoel
Y
Johanson
Donald C
DC
D000818:Animals; D005075:Biological Evolution; D003817:Dentition; D005580:Fossils; D015186:Hominidae / Q000033:anatomy & histology*; D007568:Jaw / Q000033:anatomy & histology*