Evolution__3rd_Edition

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Multicellular life originated over
1 billion years ago
A good fossil record starts in the
Cambrian
on 3.5 billion-year-old possible fossils describes multicelled filaments of this sort. But
multicellular life, in the sense of life with work development and cell differentiation,
evolved much later. With only minor exceptions, all life forms with cell differentiation
are eukaryotic. The molecular clock suggests that multicellular life originated about
1.5 billion years ago. This is somewhat, but not much, before the oldest multicellular
fossils. Currently the earliest such fossils are algae from about 1.2 billion years ago
(Butterfield 2000).
The earliest definite fossils of multicellular animals (Metazoa) come from the
Ediacaran deposits in Australia. These, and similar deposits elsewhere in the world,
date to the period from 670 to 550 million years ago. The Ediacaran fossils are of softbodied
aquatic animals such as jellyfish and worms (Figure 18.4). Well preserved fossils
of both multicellular aquatic animals and aquatic plants are also found in China from
this time (Xiao et al. 1998). The Ediacaran fossils decline in abundance about 550 million
years ago. The decline has been attributed to a mass extinction, but likely reflects
changes in the conditions of fossil preservation; Ediacara-type fossils continue to exist
in the Cambrian (Jensen et al. 1998). However, for the main fossil record of animal life,
we need to move forward from the Precambrian.
18.4 The Cambrian explosion
Figure 18.4 (opposite)
Some Ediacaran fossil animals. (a) Charniodiscus arboreus,
an attached cnidarian; (b) Cyclomedusa radiata, a jellyfish;
(c) Spriggina, a worm (its name honors R.C. Sprigg, Assistant
Government Geologist of South Australia, who discovered
CHAPTER 18 / The History of Life 535
The fossil record of multicellular plants and animals does not really take off until the
Cambrian, which began about 540 million years ago. Indeed the main time periods of
the fossils record begin with the Cambrian (Figure 18.1). Until the 1940s, no pre-
Cambrian fossils were known and in Darwin’s time it was assumed that they did not
exist. Even though we now know they do, the picture is one of sudden proliferation,
rather than a sudden beginning, of fossil life a little over 500 million years ago.
Figure 18.5 illustrates the Cambrian explosion. It shows, for all nine animal phyla
for which we have a fossil record, when the earliest fossils date from. The majority of
them date back to the early Cambrian, or some time near it. A superficial reading of the
evidence could be dramatized as follows. Life has been evolving for about 4,000 million
years and is today grouped into a series of major phyla a chordates, mollusks, arthropods,
and so on. We might expect that these phyla originated at a relatively steady rate,
yet it appears that they almost all arose within less than 40 million years of each other
(Figure 18.5), or within a period of less than 1% of the history of life.
However, the molecular clock suggests a radically different view. If we measure the
molecular distance between the major animal groups, and calibrate the clock, we find
that the major groups diverged from a common ancestor more like 1,200 million years
ago. Several molecular studies have been made, of which Wray et al. (1996) has been
particularly influential. They inferred that the Metazoan Bilateria share a common
the Ediacaran fauna in 1946); (d) Dickinsonia costata,
another worm; and (e) Tribrachidium heraldicum, a possible
proechinoderm. (a and b) reprinted, by permission of the
publisher, from Glaessner & Wade (1996) and Wade (1972);
(c–e) courtesy of M.F. Glaessner.