Evolution__3rd_Edition

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Summary
1 Fossils are formed when the remains of an organism
are preserved in the sediment deposited at the bottom
of the water column; the sediment may then form
a sedimentary rock by compaction over time. If that
sedimentary rock is later exposed at the surface of
the Earth, the fossils can be removed from it.
2 The history of the Earth is divided into a series of
time stages. Most fossils are from organisms that lived
in the past 600 million years. The 600 million-year
period is divided into three eras (Paleozoic, Mesozoic,
and Cenozoic); the eras in turn are divided into successively
into periods and epochs.
3 Rock ages can be measured absolutely using their
radioisotopic composition, and relatively by correlating
their fossil content with other rocks elsewhere.
Magnetic time zones also provide useful chronological
evidence.
4 Life probably originated about 4 billion years ago.
The oldest, chemical, evidence of life is from 3.8 billion
years ago. The oldest (currently controversial) body
fossils, in the form of cells, are 3.5 billion years old.
5 Eukaryotic cells evolved around 2 billion years
ago in a series of events including symbiosis. The
atmospheric oxygen concentration increased at a
similar time.
6 Almost all multicellular life forms with cell differentiation
are eukaryotic. Eukaryotic life forms with more
than one cell type probably evolved about 1.5 years
ago; the oldest fossils are algae from about 1.2 billion
years ago.
7 Animal life underwent an apparently explosive
radiation in the fossil record in the Cambrian, about
540 million years ago. Molecular evidence suggests
CHAPTER 18 / The History of Life 553
that the common ancestor of the major animal groups
lived about 1,200 million years ago. The fossil and
molecular dates contradict each other on some interpretations,
but can be reconciled.
8 The earliest fossil evidence of terrestrial plant life
consists of spores from about 475, or even over 500,
million years ago. Fossils from 420 million years ago
show branching plant structures that lack roots and
leaves. Leaves appear about 40 million years later, and
their evolution may have followed a photosynthetically
caused reduction in atmospheric carbon dioxide.
9 Terrestrial vertebrates evolved from fish in the
Devonian. The tetrapod limb probably first evolved
for paddling under water, not for walking on land.
10 The evolution of mammals from reptiles is an
example of adaptive evolution and the fossil record
reveals that it proceeded in a series of stages, through
various groups of mammal-like reptiles.
11 Human evolution can be studied in the fossil
record for the past 4 million years. The main observable
changes are in bipedality, a reduction of jaws and
teeth, and an increase in brain size. Other important
changes were in culture and social behavior, including
language, which are less easily studied in fossils.
12 Large-scale evolution, or macroevolution, may be
caused by small-scale evolution, or microevolution,
extended over a long period. The origin of mammals
from reptiles is a possible example.
13 In other cases, macroevolution may not simply be
due to microevolution over an extended period. For
instance, the species in which evolutionary breakthroughs
occur may be a non-random sample of all the
species existing at the time.