Evolution__3rd_Edition

548 PART 5 / Macroevolution
Myr ago
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Homo erectus
(Asia)
Homo
sapiens
Homo erectus
Homo habilis
Australopithecus africanus
Australopithecus afarensis
Australopithecus anamensis
Australopithecines included gracile
and robust forms
Homo
neanderthalensis
Paranthropus
species
Figure 18.12
Hominin fossil species. The fossil record from East and South
Africa shows the evolution of modern humans in many stages
from ape-like ancestors. The picture is simplified: some fossil
species, or possible species, have been omitted; the phylogenetic
relations among the species that are shown is in most cases
uncertain; and the species shown as an ancestor–descendant
sequence up the center of the figure may well, in some cases,
not be accurate. The descendant species might be descended
from a close relative of the species shown as its ancestor. Many
of the species names are controversial, as discussed further in
the text for Homo erectus, H. sapiens, and H. neanderthalensis.
Paranthropus is often included in the genus Australopithecus.
Fossils of A. afarensis have been found from about 4 to 3 million years ago. That
brings us near to the time, about 3–2.5 million years ago, when the other fossil australopithecines
are distinguished: A. africanus and A. garhi. A. africanus is much the better
known and was the first australopithecine to be described, in 1924. It is somewhat more
human-like than Lucy, and has often been taken for the next stage in human evolution.
However, A. africanus fossils all come from South Africa, whereas Lucy and the
later fossils that are thought to be closest to the human line are all from East Africa a
particularly sites in Kenya and Ethiopia. Asfaw et al. (1999) described a new 2.5 millionyear-old
fossil from Ethiopia, and named it A. garhi. A. garhi may be closer to the
human line than the South African A. africanus.
All the australopithecines mentioned so far are “gracile,” with relatively light bones
and jaws. Around 2.5–2 million years ago in both East and South Africa, the australopithecines
diverged into gracile and robus forms. Two species, Paranthropus robustus
and P. boisei, arose with much more powerful jaws, skulls, and cheek bones, who were
able to eat tougher food than the gracile forms. Paleoanthropologists generally accept
that Homo is more closely related to the gracile than the robust australopithecines. The
robust species went extinct and have no descendants today.
By 2 or more million years ago, fossils classified as Homo start to be found. Homo
habilis may date back as far as 2.5 million years ago. These fossils are closer to the modern
human condition above as well as below the neck. H. habilis fossils are associated with
stone tools, and the brain is larger a perhaps around 600–750 cm 3 . Its jaws and teeth
are reduced, though the jaw shape remains somewhat prognathic as compared with
ours. Its sexual dimorphism was similar to modern humans, with males about 1.2 times
heavier than females on average. H. erectus was the first hominid to move out of Africa
and colonized Asia at least 1.5 million years ago, and Europe at an uncertain date.
In Europe, H. erectus evolved into the Neanderthals a fossil humans that are found in
Europe from about 200,000 to 40,000 years ago. (Some experts classify the African
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