Evolution__3rd_Edition

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. . . and cynodonts
but their temporal fenestrae are generally larger and more mammal-like than pelycosaurs,
their teeth in some cases show more serial differentiation, and later forms had
evolved a secondary palate. A secondary palate enables an animal to eat and breathe
at the same time and is a sign of a more active, perhaps warm-blooded, way of life
(Section 10.7.5, p. 284).
One subgroup of therapsids, the cynodonts, are of particular importance in tracing
the origin of mammals, and they make up the third phase of mammal-like reptilian
evolution. The jaws of cynodonts resemble modern mammal jaws more closely and
their teeth are multicusped and differentiated down the jaw. Some cynodonts show a
particularly interesting intermediate stage in jaw evolution. Recall that the reptilian jaw
articulates in a different place from the mammalian jaw, a change associated with the
evolution both of more precise chewing and of hearing in mammals. Some cynodonts
seem to have had a double jaw articulation; their jaws articulated in both the mammalian
and the reptilian positions. This suggests one way in which evolution can proceed
from one structure to another without a non-functional intermediate stage: the structure
evolved from state A to state A + B, then A was lost, giving state B alone. The jaw was
a functional structure throughout. The cynodonts complete the story of the mammallike
reptiles, because it was from a line of cynodonts that the ancestors of the modern
mammals evolved. The identity of the exact cynodont line from which modern mammals
descended is uncertain, but Probainognathus (Figure 18.10) is close to it.
A further series of fossils connect the last mammal-like reptiles with modern mammals.
Living mammals are divided into three groups: Prototheria (including the
echidna), Metatheria, and Eutheria. Metatheria and Eutheria are also known as marsupials
and placentals, respectively. The earliest known eutherian fossils are from
the Yixian formation in China and date to the early Cretaceous (Ji et al. 2002). The
three main modern mammal groups probably diverged in the Jurassic. The Eutheria in
turn diverged into several main orders (that is, groups such as the primates, carnivores,
proboscideans, and rodents). The timing of this divergence is controversial (Section
23.7.3, p. 671), but the primates probably originated in the Cretaceous even though
their fossil record only takes off in the Tertiary. The next event we look at here is the
origin of humans within the primate order.
18.7 Human evolution
CHAPTER 18 / The History of Life 545
18.7.1 Four main classes of change occurred during hominin evolution
Humans are primates, and our ancestors from about 60 (or more) million years ago to
about 5–10 million years ago were tree-dwelling primates. Some of the trends we see in
human evolution began in these ancestors. Primates have, compared with other mammals,
relatively flat faces and large brains. Their flat faces provide their two eyes with a
large overlap in their visual fields, giving good stereoscopic vision. Stereoscopic vision
improves perception of depth, and is advantageous in leaping between branches.
Arboreal primates also have their thumbs and big toes (hallux) relatively separate
from their other four digits. This allows them to grip branches. All primates have relat-