(Click on crania to see fossils)
(Click on crania to see fossils)
deMenocal, P.B. African climate change and faunal evolution during the Pliocene-Pleistocene.
Earth and Plant. Sci. Lett, Frontiers, 6976, 1-22, 2004 (PDF).
One of the best websites on hominid evolution can be found
at
The Smithsonian Institutions's Hall
of Human Origins
...plus, their very cool QTVR (QuickTime Virtual Reality) images of selected crania
| A summary diagram of African hominid evolution over the last 4.5 Ma [original figure in deMenocal, 2004, ranges and phylogeny after Wood, 1992; Asfaw, 1999; Kimbel, 1995]. Throughout most of the early to mid-Pliocene African hominids were represented by smaller, sexually dimorphic, and less encephalized Australopithecus afarensis (between ca. 3.9-2.9 Ma, green). After ca. 3.0-2.5 Ma, this species became extinct and was replaced by two broad taxonomic groups: early Homo (blue) and the robust Australopithecines (red) (cf. Paranthropus). The first stone tools of any kind appeared near 2.6 Ma. Yellow horizontal bars indicate step-like shifts in African climate towards more aridi conditions [after deMenocal et al., 1995; deMenocal and Bloemendal, 1995; Cerling, 1992, Cerling et al., 1985; Dupont and LeRoy, 1995]. |
Hominin Evolution and African Climate
Hominin evolution over the last 5 Ma of the Pliocene-Pleistocene was punctuated by dramatic changes in species morphology, innovation, and diversity centered near 2.8 Ma, 1.7 Ma, and ca. 1 Ma [Kimbel, 1995; Vrba, 1995; Vrba et al., 1989; Wood, 1992]. Significant changes in the evolution of East African antelopes [Vrba, 1995; Vrba et al., 1989] and micromammals [Wesselman, 1985] are evidently synchronous with some of these hominin speciations, suggesting broad changes in African faunal compositions at specific times during the Pliocene-Pleistocene, although the amplitudes of these speciation events have been debated [Behrensmeyer et al., 1997].
The earliest hominin presently documented is the 4.4 Ma specimen
of Ardipithecus ramidus (Fig. 1) based on framentary but
well-dated fossil evidence from Aramis, Middle Awash in northern
Ethiopia [White et al., 1994]. The postcranial fossil evidence
suggests that this species was somewhat smaller than many specimens
of Australopithecus afarensis ("Lucy"). Dental
morphology suggests that A. ramidus is the most "apelike"
of hominin yet described and is probably close to the last common
ancenstor of apes and humans. Sedimentological evidence at the
site suggests a wooded habitat in this now hyperarid terraine
[WoldeGabriel et al., 1994]. Australopithecus anamensis
has been described at several sites near Lake Turkana, Kenya spanning
an interval between 4.2-3.9 Ma [Leakey et al., 1998]. Between
3.9-2.9 Ma, a single australopithecine species, A. afarensis,
is known [Kimbel, 1995; White, 1993]. Many specimens
of A. afarensis have now been described and they were evidently
a single, ecologically-diverse, highly sexually dimorphic, bipedal
Pliocene hominin whose known range encompassed Ethiopia to Tanzania
[White, 1993].
The most significant event in early hominid evolution occurred
near ca. 2.7-2.8 Ma when at least two separate lineages emerged
from this bipedal ancestral line (Fig. 1). Earliest members of
the "robust" australopithecine lineage first occur in
the fossil record near 2.8 Ma [Grine, 1986; Kimbel,
1995; Klein, 1988; Suwa et al., 1997a] and were
distinguished by uniquely large masticatory adaptations [Brain,
1981; Grine, 1988; Klein, 1988]. A second lineage,
represented by the earliest members of our genus Homo,
first occurred in East African sections near 2.5 Ma [Kimbel,
1995; Schrenk et al., 1993; Wood, 1992] (Fig. 1).
Earliest fossils of the Homo clade are characterized by
much larger absolute cranial volumes [Wood, 1992] than
any prior hominin species. The earliest known stone tools (the
first crude choppers and scrapers comprising the Oldowan complex)
are now well dated to near 2.5-2.6 Ma [deHeinzelin et al.,
1999]. The synchronous existence of two distinct hominid lineages
has been interpreted to reflect separate adaptations to a more
arid, varied environment [Grine, 1986; Klein, 1988].
Fossil African bovid and rodent assemblages indicate shifts toward
arid-adapted species near 2.7-2.8 Ma [Vrba, 1995; Vrba,
1985; Vrba et al., 1989].
By 1.6 Ma, Homo habilis became extinct and its immediate
successor, and our direct ancestor, H. erectus, first occurs
in the fossil record near 1.8 Ma [Kimbel, 1995]. H.
erectus may have migrated to southeast Asia as early as 1.8
Ma [Swisher et al., 1994]. Near 1.7 Ma, East African bovid
assemblages shift toward further absolute increases in the abundance
of arid-adapted species [Vrba, 1995; Vrba, 1985;
Vrba et al., 1989]. Enhanced East African aridity near
1.8-1.6 Ma is supported by soil carbonate stable isotopic evidence
for broadly expanded savannah vegetation in East Africa ([Cerling,
1992; Cerling and Hay, 1988; Cerling et al., 1994];
Fig. 1). Earliest occurrences of the more sophisticated Acheulean
tool kit (bifacial handaxes) occur near 1.4 Ma [Asfaw et al.,
1992]. The "robust" australopithecine lineage became
extinct near 1.4 Ma [Kimbel, 1995], although for taphonomic
reasons this datum may be considerably younger. By 1 Ma H.
erectus had broadly expanded its geographic range and occupied
sites in North Africa, Europe, and western Asia [Abbate,
1998; Harris, 1983]. The fossil record of African bovidae
documents a final phase of increased arid-adapted species composition
near 1 Ma [Vrba, 1995; Vrba, 1985; Vrba et al.,
1989].
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