The first birds probably evolved as small specialized dinosaurs with
feathers,an event that occurred during the early to middle Mesozoic
era more than 150 million years ago (mya).Mammals evolved at least
225 mya from therapsid (mammallike) reptiles,and separation of the
dinosaur (archosaurian,or “ruling reptile”) lineage from the therapsid
lineage occurred during the Paleozoic era,at least 300mya.This means
that the suite of traits we share with birds has been stable during more
than 300 million years of the history of animal life.During the same 300
million years other traits evolved that differentiate us from humming
birds and ostriches.There was plenty of time for major divergences.
Within mammals,placentals appeared during the Cretaceous period
of the Mesozoic era,about 100mya.Primates may have diverged from
other species of placental mammals during the late Cretaceous,about
70 mya.Some early Cenzoic fossil primates resembled living tarsiers and
are grouped with them taxonomically.These are from the Lower Eocene
epoch,and are about fifty-five million years old;other primatelike
mammal fossils are about sixty million years old.And a few Cretaceous
primatelike mammal teeth are at least seventy million years old.One can
pick one’s date for the divergence of primates from other mammals,but
it happened a long time ago.An authoritative detailed discussion of
primate evolution,including quantitative analysis of the evolution of the
primate brain,is available in Martin (1990).
Gibbons,which call so impressively and musically,are related to
primate lesser ape fossils from which the human and great ape lines
diverged at least twenty mya,and we split from our great ape cousins
about five mya according to the current consensus.Whales and other
cetaceans diverged from other mammals early in the Cenozoic era,at
least fifty-five mya.The morphological diversity represented by these
groups illustrates the amount of evolutionary change that could occur
over these long intervals.
These dates must be evaluated with respect to the common and spe-
cific traits that we recognize in various living animals,including humans.
We know that many morphological traits,such as whether one has five
fingers or a hoof,or a wing or an arm,diverged relatively rapidly from
a basal condition.According to recently developed evidence (McKee,
Tobias,and Clarke 1996),it was only about three mya that the primate
foot with an opposable digit suitable for grasping and climbing evolved
into the human foot specialized for walking and running.This,by the way,
is evidence of the persistence of arboreal locomotion in some australo-
pithecine hominids.Fully terrestrial habits evolved within the australo-
pithecines and after hominids had separated from the great apes.The
conclusion to emphasize is that evolution can be relatively rapid when
environmental requirements drive it appropriately.180 Harry Jerison