Evolution__3rd_Edition

..
Present
~540 Myr
~1,200 Myr
Echinoderms
Chordates
Annelids
Several hypotheses exist to explain
the Cambrian explosion
Mollusks
Arthropods
Invisible ancestors
Fossils deposited
Origin of hard parts
CHAPTER 18 / The History of Life 537
Figure 18.6
Reconciling fossil and molecular evidence on the timing of
animal evolution. The main animal groups could have diverged
around 1,200 million years ago, as the molecular evidence
suggests, but been invisible in the fossil record until the origin
of hard parts around 540 million years ago. The fossil invisibility
of animals between 1,200 and 540 million years ago could be due
to the animals being rare and soft. The time of proliferation in the
fossil record and the time or origin of a group would then not be
the same.
were too rare or fragile, or in the wrong place, to leave fossils (Figure 18.6). In Cooper &
Fortey’s (1998) image, a “phylogenetic fuse” preceded the Cambrian explosion.
Why should there have been a long period a 500 million years or more a when
ancestors of the modern animal phyla existed but no fossils were deposited? The
Cambrian explosion is a fossil event, and probably marks the time of origin of hard
parts. Animals with hard skeletons or shells leave fossils much more often than animals
with only soft parts. But if hard parts originated about 540 million years ago, that raises
the question of why hard parts suddenly became advantageous in so many groups at
about the same time.
Predators in general is one hypothesis and visually hunting predators in particular is
a second hypothesis. After predators evolved, hard parts became advantageous for
defensive reasons. Another factor is that oxygen levels may have increased towards the
end of the Precambrian. This may have been caused by an increase in plant a that is,
phytoplankton a productivity (Knoll & Carroll 1999). The higher plant production
(if it occurred then) would have supported a greater mass and diversity of animals.
The greater quantity of potential prey could have created an opportunity leading to the
evolution of predators.
The “snowball Earth” hypothesis suggests a further environmental factor that could
have been at work. For at least some of the period before the Cambrian, the Earth may
have been almost completely covered with ice and glaciers. Life would have been rare,
confined to areas near hot springs or ocean vents, or small localities where enough
ice had melted to allow sunlight through and photosynthesis to occur. This would
help explain the paucity of fossils before the Cambrian. Moreover, the ancestors of
arthropods, mollusks, and chordates would then have been tiny creatures, small
enough to have been supported by the very limited ecological productivity.
The Cambrian explosion is the subject of intense research at present. Biologists
and paleontologists are studying just how suddenly the fossil event was: maybe it was
less explosive than Figure 18.5 shows. Others are studying the molecular evidence,
with further molecules and new calibration procedures. If, as seems likely, some
major evolutionary event did occur around 540 million years ago, the big question
is what caused it? The hypotheses at present are looking at external environmental