THE HiSToRy oF LiFE 435
The fossil record
Some short parts of the fossil record in certain localities provide detailed evo-
lutionary histories, and some groups of organisms, such as abundant plank-
tonic protists with hard shells, have left an exceptionally good record. In some
respects, such as the temporal distribution of many higher taxa (e.g., phyla and
classes), the fossil record is adequate to provide a reasonably good portrait [9].
In some other respects, the fossil record is very incomplete [38]. Consequently,
the origins of many taxa have not been well documented. We know that the
fossil record is incomplete because continuing exploration constantly yields
new discoveries; for instance, most of the discoveries that have documented the
origin of birds from dinosaurs have been made in Chinese deposits in the last
20 years.
The incompleteness of the fossil record has several causes. First, many kinds of
organisms rarely become fossilized because they are delicate, or lack hard parts,
or occupy environments—such as humid forests—where decay is rapid. Second,
because sediments generally form in any given locality very episodically, they
typically contain only a small fraction of the species that inhabited the region
over time. Third, if fossils are to be found, the fossil-bearing sediments must
become solidified into rock; the rock must persist for millions of years without
being eroded, metamorphosed, or subducted; and the rock must then be exposed
and accessible to paleontologists. Finally, the evolutionary changes of interest may
not have occurred at the few localities that have strata from the right time; a spe-
cies that evolved new characteristics elsewhere may appear in a local record fully
transformed, after having migrated into the area. Paleontologists agree that the
approximately 250,000 described fossil species represent far fewer than 1 percent
of the species that lived in the past.Before Life Began
2The current universe came into existence about 14 billion years ago (14 Gya,
that is 14,000,000,000 years ago) through an explosion (the “big bang”) from
an infinitely dense point. Elementary particles formed hydrogen shortly after
the big bang, and hydrogen ultimately gave rise to the other chemical elements
through nuclear fusion in stars. The collapse of a cloud of dust and gas formed
our galaxy fewer than 10 Gya. Material expelled into interstellar space, espe-
cially during stellar explosions (supernovas), condensed into second- and third-
generation stars, of which the Sun is one. Our solar system was formed about 4.6
Gya, according to radiometric dating of meteorites and moon rocks. Earth is the
same age as those bodies, but because of geological processes such as subduc-
tion (see Figure 17.1), the oldest known rocks on Earth are younger, dating from
about 4 Gya.
Earth was probably formed by the collision and aggregation of many smaller
bodies, the impact of which produced enormous heat. Early Earth formed a solid
crust as it cooled, releasing gases that included water vapor but very little oxygen.
As Earth cooled, oceans of liquid water formed, probably by 4.5 Gya, and quickly
achieved the salinity of modern oceans. By 4 Gya there were probably many small
protocontinents, which gradually aggregated, by plate tectonics, to form large land
masses over the next billion years.(^2) This chapter differs from most of the others in this book by focusing on factual information
rather than general principles of evolution. It contains more information than you may wish to
memorize. You may consider it largely as a source of information, or you might simply enjoy
reading a sketch of one of the greatest stories of all time. Major events and important points that
a well-trained biologist should know are highlighted in italics.
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