Nipping the Cambrian ``explosion''
in the bud?
Simon Conway Morris
Summary
In recent years, two schools of thought have emerged
with regard to the Cambrian ``explosion''. One argues that
it was very quick, with phyla tumbling into existence in a
virtual geological instant. The other view has a more
relaxed temporal perspective. It looks to slow aeons of
cryptic metazoan history, which led to a final break-
through in the Cambrian, not in evolution but of
fossilization potential. Yet both views have serious
difficulties. Now, in a recent issue of
Biological Reviews,
Graham Budd and So
È
ren Jensen
(1)
argue for a third way.
In an intriguing blend of functional morphology, the fossil
record and cladistic thinking, they suggest that the
assembly of metazoan bodyplans took place in a surpris-
ingly straightforward manner. BioEssays 22:1053±
1056, 2000. ß 2000 John Wiley & Sons, Inc.
Introduction
The other day I was walking past an immense building from
which emanated uproar. The noise was terrific. A door swung
open and, looking in, I saw to my surprise that nearly everyone
was dressed in white. But, strange to say there was not one
pulpit but two. The crowd surged back and forth, spotless
laboratory coats streaming in the rush. From one pulpit the
proclamation rang out: ``The Cambrian `explosion' is real!!!
Hundreds of phyla evolved, almost instantaneously. Listen,
neo-darwinism is in terminal crisis, we must summon forth new
mechanisms of macroevolution''. From the other pulpit,
however, I heard the following: ``No, the Cambrian `explosion'
is a mirage, a mere artefact! For aeons tiny animalcules
slithered through the slime, avoiding fossilization, hoarding
their Hox genes, swaying to the sonorous tick of molecular
horology.''
This pastiche has a serious scientific point. Is the Cambrian
``explosion'' a real event, and if so did the welter of metazoan
bodyplans pour forth in a geologically brief episode of relative
genomic chaos and ecological mayhem? Alternatively, is this
evolutionary event no more than an artefact? In this second
scenario, it is still an ``explosion'' but one of fossils. In this view,
the Cambrian explosion was the result of a breakthrough in
fossilization potential, linked to skeletonization and increasing
body size, which in reality concealed an immensely long
history of cryptic diversity extending deep into the Precam-
brian. Opinions are, indeed, deeply polarized. Now, in an
exciting paper Graham Budd and So
È
ren Jensen
(1)
provide a
series of provocative and timely insights that neatly seem to
defuse the Cambrian ``explosion'', but in a way that is quite
different to the many strident claims issuing from either of
those two metaphorical pulpits.
What is a phylum?
The principal aim of Budd and Jensen
(1)
is to undermine the
venerable notion that all the phyla appeared suddenly at the
base of the Cambrian. This literalist view of the Cambrian
``explosion'' is fast becoming embedded in the textbooks, and it
is surely of more than passing interest
(2)
that this argument has
also started to attract the attention of the creationists.
(3)
The
assault on this position by Budd and Jensen starts with a
careful look at the twin concepts of bodyplan and phyla. As is
widely appreciated, the popularity of these terms is only
matched by a notorious imprecision in their meanings. Yet, a
proper understanding of what we want to mean by phylum and
bodyplan has important implications for the way we view
Cambrian evolution. First, we need to know the plans, if not
rules, of assembly,
(4)
yet all too often the origin of phyla has
attracted a sort of macroevolutionary mysticism.
Properly understood, the construction of a bodyplan offers
evolutionary insights into the roles of preadaptation and co-
option of gene function, not to mention the identification of
functional constraints. Moreover, at the early stages of diver-
gence, phyla per se are not going to be recognizable. Far from
being a problem, this actually offers rich opportunities to
palaeontologists both to contribute to the historical documen-
tation of bodyplan assembly, and also to constrain, if not
resolve, conflicting models of metazoan phylogeny.
(5)
Yet, as
others have pointed out this remains a substantial challenge
because the closely related taxa that will eventually diverge
into different phyla may well look very similar initially.
(4,6)
For
this discussion, Budd and Jensen
(1)
develop concepts, which
have long been familiar to adherents of cladistic methodology,
of the stem and crown groups (Fig. 1). In doing so they aim to
define operationally useful concepts of the bodyplan and a
tractable definition of the phylum.
Central to this definition is the assumption of an orderly
acquisition of derived characters, which in their totality will
define the phylum. This procedure is then applied to the
Cambrian fossil record, our understanding of which continues
BioEssays 22:1053±1056, ß 2000 John Wiley & Sons, Inc. BioEssays 22.12 1053
Department of Earth Sciences, University of Cambridge, Cambridge
CB2 3EQ, U.K. E-mail: sc113@esc.cam.ac.uk
What the papers say