SUMMARy
■■Evidence from living organisms indicates that all
living things are descended from a single com-
mon ancestor. Some progress has been made in
understanding the origin of life, but a great deal
remains unknown.
■■The first fossil evidence of life dates from about
3.5 Gya, about 1 Gy after the formation of Earth.
The earliest life forms of which we have evidence
were prokaryotes.
■■Eukaryotes evolved about 1.8 Gya. Their mito-
chondria and chloroplasts evolved from endo-
symbiotic bacteria.
■■Although stem lineages of some modern phyla
evolved long before the Cambrian period, the
fossil record displays an explosive diversifica-
tion of the animal phyla near the beginning of
the Cambrian, about 541 Mya. The causes of
this rapid diversification are debated, but may
include a combination of genetic and ecological
events. Jawless, limbless vertebrates evolved by
the late Cambrian.
■■Terrestrial plant and arthropod fossils are found
first in the Silurian, and insects in the Devonian.
Vertebrates (fishes) with jaws and limbs (fins)
evolved in the Silurian, and tetrapods evolved in
the late Devonian from lobe-finned fishes.
■■The most devastating mass extinction of all time
occurred at the end of the Permian (about 252
Mya). it profoundly altered the taxonomic com-
position of Earth’s biotas.
■■Seed plants and amniotes became diverse and
ecologically dominant during the Mesozoic era(252–66 Mya). Early mammaliaforms evolved in
the Triassic, and archosaurs, including especially
the dinosaurs, dominated Jurassic and Creta-
ceous landscapes. Flying dinosaurs, the anteced-
ents of birds, evolved in the Jurassic, and gave
rise to some lineages of modern birds in the late
Cretaceous. Flowering plants and plant-associ-
ated insects diversified greatly from the middle
of the Cretaceous onward. A mass extinction (the
K/Pg or K/T extinction) at the end of the Meso-
zoic included the extinction of the last nonavian
dinosaurs.
■■The climate became drier during the Cenozoic
era, favoring the development of grasslands and
the evolution of herbaceous plants and grass-
land-adapted animals.
■■Most orders of placental mammals originated
in the late Cretaceous, but underwent adaptive
radiation in the early Paleogene. Many groups
of mammals were once more diverse than they
are now, and some are extinct. A few groups,
such as rodents and artiodactyls, maintained high
diversit y.
■■A series of glacial and interglacial episodes oc-
curred during the Pleistocene (the last 2.6 My),
during which some extinctions occurred and the
distributions of species were greatly altered.
■■Humans have caused species extinctions since
the spread of agriculture or earlier. Human
population growth and technology have had an
accelerating impact on biological diversity, and
have initiated another major extinction.TERMS AND CoNCEPTS
Cambrian explosion
endosymbiont
GondwanaLaurasia
mass extinction
megafaunal
extinctionPangaea
radiometric dating
refugiastrataSUGGESTioNS FoR FURTHER READiNG
S. M. Stanley, Earth and Life through Time, sec-
ond edition (W. H. Freeman, New york, 1993),
is a comprehensive introduction to historical
geology and the fossil record. in the fourth
edition of Life of the Past (Prentice-Hall, Upper
Saddle River, NJ, 1999), W. i. Ausich and N. G.
Lane provide a well-illustrated introduction to
the theme of this chapter. A good overview of
the origin of life is by A. Lazcano in Evolution
Since Darwin: The First 150 Years, M. A. Bell et
al. (eds.) (Sinauer Associates, Sunderland, MA,
2010), pp. 353–375. The Cambrian explosionis the subject of a comprehensive article by
D. E. Erwin et al., “The Cambrian conundrum:
Early divergence and later ecological success
in the early history of animals” (Science 334:
1091–1097, 2011). J. Maynard Smith and E.
Szathmáry’s The Major Transitions in Evolution
(W. H. Freeman, San Francisco, 1995) is an in-
terpretation by leading evolutionary theoreti-
cians of major events, ranging from the origin
of life to the origins of societies and languag-
es. The history of ecological diversification of
animals is the subject of “Paleoecologic mega-17_EVOL4E_CH17.indd 466 3/22/17 1:37 PM