Evolution__3rd_Edition

..
. . . and sexual dimorphism all
occurred in human evolution
The fossil “Lucy” was bipedal
Fossils may be relatives of our
ancestors, not ancestors
CHAPTER 18 / The History of Life 547
in gorillas and orang-utans, and about 1.35 times as much in chimpanzees. In
humans sexual dimorphism is reduced; males on average weight about 1.2 times as
much as females. Sexual dimorphism may have reduced in our ancestors when we
evolved reproductive pair bonds a prolonged pairing is found in most human societies
but not in other great apes. The cultural state of a society can be followed in
tools and other artifacts associated with fossils. The main innovation underlying
modern human culture is language. The origin of language is hard to study, with
very indirect clues coming from jaw and throat anatomy and from the symbolic
richness of artifacts associated with fossils.
18.7.2 Fossil records show something of our ancestors for the past
4 million years
We saw in Section 15.13 (p. 460) how the hominin lineage probably originated about
5 million years ago. 1 Currently, the earliest fossils that are generally accepted to be
members of the hominin lineage are about 4.4 million years old and are classified in
two species, Australopithecus anamensis and A. afarensis. Earlier fossils are known that
may be hominins a but the fossils are fragmentary and could as easily be more closely
related to other apes. A. afarensis is much the best known early australopithecine,
because it includes the exceptionally complete specimen known as “Lucy.” Lucy’s
skeleton, together with trace fossils of footprints, tell us that A. afarensis was bipedal.
However, in other respects A. afarensis retained the ancestral conditions. Its brain size,
relative to body size, was similar to that of a chimpanzee, and its jaws retain the ancestral
shape. Australopithecines generally had evolved closer to the modern human condition
in their method of locomotion than in their jaws and brains. Australopithecines
are sometimes informally described as being like humans below the neck and like apes
above the neck. Also, several dimorphism had not reduced in A. afarensis: males
weighed about 1.5 times as much as females.
Species such as A. afarensis may have been direct ancestors of modern humans.
Alternatively, they may have been relatives of our ancestors but not on the line leading
to us. The fossil record is too incomplete for us to know which is true. In a figure such as
Figure 18.12, the main fossil species are drawn as if some were ancestral to others, but it
is more accurate to treat the figure as a simplification. For many points, we do not need
to know whether one species is a direct ancestor of ours or not; for instance, we can
conclude from the fossil evidence that the first main event in human evolution was
bipedality. This is likely to be true whether A. afarensis is our ancestor or a relative of
our ancestors.
1 The “hominin” lineage is the one containing species more closely related to us than any other living species.
Some use the word hominid a it depends on whether humans are a family (Hominidae) or subfamily
(Homininae). The molecular evidence about our ape relations suggests to many that we should have a subfamily.
Hence a recent trend toward the use of the term hominin, rather than the longer established hominid.