CAMBRIAN EXPLOSION / ORIGIN OF THE
"When Charles Darwin wrote "The Origin of Species " in
1859, the sudden appearance of animal fossils at the beginning of the Cambrian
was of particular concern to him. It was at odds with his view that the
diversification of life on earth through natural selection had required
a long period of time. Darwin's theory predicted that the major groups of
animals should gradually diverge during evolution. He knew that the sudden
appearance of fossils would be used by his opponents as a powerful argument
against his theories of descent with modification and natural selection.
Consequently, he argued that a long period of time, unrepresented in the
fossil record, must have preceded the Cambrian to allow the various major
groups of animals to diverge. At that time the strata that we now regard
as Cambrian were subsumed within the concept of the Silurian, so Darwin
'I cannot doubt that all the Silurian trilobites have descended from
some one crustacean, which must have lived long before the Silurian age....Consequently,
if my theory be true, it is indisputable that before the lowest Silurian
strata was deposited, long periods elapsed, as long as, or probably longer
than, the whole interval from the Silurian to the present day.....The case
must at present remain inexplicable; and may be truely urged as a valid
argument against the views here entertained'
The Origin of Species, 1859, pp. 313 - 314
- Derek E.G. Briggs, Douglas H. Erwin, & Frederick J. Collier
"The Fossils of the Burgess Shale," 1994, Smithsonian Institution,
"On the sudden appearance of groups of Allied Species in the
lowest known fossiliferous strata"
Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as,
or probably far longer than, the whole interval from the Silurian age to
the present day; and that during these vast, yet quite unknown periods of
time, the world swarmed with living creatures. To the question why we do
not find records of these vast primordial periods, I can give no satisfactory
- Darwin, Charles
On the Origin of Species, 1st edition
Harvard Univ. Press, facsimile reprint, 1964, p. 307
Note: In Darwin's time, the "Silurian" was the name given
the oldest known fossil-bearing strata. "Cambrian" does not occur
as an index entry in this edition of the Origin.
Most families, orders, classes, and phyla appear rather suddenly in the
fossil record, often without anatomically intermediate forms smoothly interlinking
evolutionarily derived descendant taxa with their presumed ancestors.
- Eldredge, N., 1989
Macro-Evolutionary Dynamics: Species, Niches, and Adaptive Peaks
McGraw-Hill Publishing Company, New York, p. 22
The record jumps, and all the evidence shows that the
record is real: the gaps we see reflect real events in life's history --
not the artifact of a poor fossil record.
- Eldredge, N. and Tattersall, I. (1982)
The Myths of Human Evolution
Columbia University Press, p. 59
The fossil record suggests that the major pulse of diversification
of phyla occurs before that of classes, classes before that of orders, and
orders before families. This is not to say that each higher taxon originated
before species (each phylum, class, or order contained at least one species,
genus, family, etc. upon appearance), but the higher taxa do not seem to
have diverged through an accumulation of lower taxa (Erwin, Valentine, and
- Erwin, D., Valentine, J., and Sepkoski, J. (1988)
"A Comparative Study of Diversification Events"
Evolution, vol. 41, p. 1183
Described recently as "the most important evolutionary
event during the entire history of the Metazoa," the Cambrian explosion
established virtually all the major animal body forms -- Bauplane or phyla
-- that would exist thereafter, including many that were 'weeded out' and
became extinct. Compared with the 30 or so extant phyla, some people estimate
that the Cambrian explosion may have generated as many as 100. The evolutionary
innovation of the Precambrian/Cambrian boundary had clearly been extremely
broad: "unprecedented and unsurpassed," as James Valentine of
the University of California, Santa Barbara, recently put it (Lewin, 1988).
Lewin then asked the all important question:
"Why, in subsequent periods of great evolutionary activity when countless
species, genera, and families arose, have there been no new animal body
plans produced, no new phyla?"
- Lewin, R. (1988)
Science, vol. 241, 15 July, p. 291
The Meanings of Diversity and Disparity
"I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about "diversity" as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of "diversity" as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do -- even though each assemblage contains just three species.
The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life. How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species? -- for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16). ...
Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have siggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense -- number of species. The second sense--- difference in body plansshould then be called disparity. Using this terminology, we may acknowledge a central and surprising fact of life's history -- marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."
- Stephen Jay Gould (1989)
Wonderful Life: The Burgess Shale and the Nature of History
W. W. Norton & Company, New York, London, p. 49
[G]aps between higher taxonomic levels are general and
- Raff, R. A. and Kaufman, T. C., 1991
Embryos, Genes, and Evolution: The Developmental-Genetic Basis of Evolutionary
Indiana University Press, p. 35
Evidence of gradualism between phyla, classes and even
orders is either non-existent or is much disputed. Certainly, no pervasive
pattern of gradualism exists. George Gaylord Simpson acknowledged this decades
ago as he described the situation in these terms:
"This is true of all thirty-two orders of mammals...The earliest and
most primitive known members of every order already have the basic ordinal
characters, and in no case is an approximately continuous sequence from
one order to another known. In most cases the break is so sharp and the
gap so large that the origin of the order is speculative and much disputed...
This regular absence of transitional forms is not confined to mammals, but
is an almost universal phenomenon, as has long been noted by paleontologists.
It is true of almost all classes of animals, both vertebrate and invertebrate...it
is true of the classes, and of the major animal phyla, and it is apparently
also true of analogous categories of plants.
- Simpson, G. G. (1944)
Tempo and Mode in Evolution
Columbia University Press, New York, p. 105, 107
[T]he fossil record itself provided no documentation of continuity
-- of gradual transitions from one kind of animal or plant to another of
quite different form.
- Stanley, S. M., 1981
The New Evolutionary Timetable: Fossils, Genes, and the Origin of Species
Basic Books, Inc., Publishers, N.Y., p. 40
The gaps in the fossil record are real, however. The absence
of a record of any important branching is quite phenomenal. Species are
usually static, or nearly so, for long periods, species seldom and genera
never show evolution into new species or genera but replacement of one by
another, and change is more or less abrupt.
- Wesson, R., 1991
Beyond Natural Selection
MIT Press, Cambridge, MA, p. 45
[T]he origin of no innovation of large evolutionary significance
- Wesson, R., 1991
- Beyond Natural Selection
- MIT Press, Cambridge, MA, p. 45
[L]arge evolutionary innovations are not well understood.
None has ever been observed, and we have no idea whether any may be in progress.
There is no good fossil record of any.
- Wesson, R., 1991
Beyond Natural Selection
MIT Press, Cambridge, MA, p. 206
Taxa recognized as orders during the (Precambrian-Cambrian)
transition chiefly appear without connection to an ancestral clade via a
fossil intermediate. This situation is in fact true of most invertebrate
orders during the remaining Phanerozoic as well. There are no chains of
taxa leading gradually from an ancestral condition to the new ordinal body
type. Orders thus appear as rather distinctive subdivisions of classes rather
than as being segments in some sort of morphological continuum.
- Valentine, J.W., Awramik, S.M., Signor, P.W., and Sadler, P.M. (1991)
"The Biological Explosion at the Precambrian-Cambrian Boundary"
Evolutionary Biology, Vol. 25, Max K. Hecht, editor, Plenum Press,
New York and London, p.284
Valentine and Erwin review hypotheses as to the mode
of origin of animal body plans for consistency with the fossil evidence.
They conclude that both Darwinian gradualism and punctuated equilibrium
are inadequate to account for the appearance of invertebrate body plans
and their major modifications:
"The models we consider are of three sorts: those that extrapolate
processes of speciation to account for higher taxa via divergence, those
that invoke selection among species, and those that emphasize that many
higher taxa originated as novel lineages in their own right, not only as
a consequence of species-level processes. It is in this latter class of
model that we believe the record favors." (Valentine and Erwin, 1985,
If large populations have gradually evolved there should be unmistakable
evidence in the fossil record, yet it is simply not found.
"... many of the large populations should have been preserved, yet
we simply do not find them. Small populations are called for, then, but
there are difficulties here also. The populations must remain small (and
undetected) and evolve steadily and consistently toward the body plan that
comprises the basis of a new phylum (or class). This is asking a lot. Deleterious
mutations would tend to accumulate in small populations to form genetic
loads that selection might not be able to handle. Stable intermediate adaptive
modes cannot be invoked as a regular feature, since we are then again faced
with the problem of just where their remains are. We might imagine vast
arrays of such small populations fanning continually and incessantly into
adaptive space. Vast arrays should have produced at least some fossil remains
also. Perhaps an even greater difficulty is the requirement that these arrays
of lineages change along a rather straight and true course --- morphological
side trips or detours of any frequency should lengthen the time of origin
of higher taxa beyond what appears to be available. Why should an opportunistic,
tinkering process set on such a course and hold it for so long successfully
among so many lineages?
We conclude that the extrapolation of microevolutionary rates to explain
the origin of new body plans is possible, but does not accord with the primary
evidence." (Valentine and Erwin, 1985, pp. 95, 96)
The model of punctuated equilibrium or species selection attempts to
account for the lack of evidence by relying primarily on the evolution of
small isolated populations which would have a diminished chance of leaving
a fossil record. This scenario has its difficulties, however, as Valentine
and Erwin point out:
"The required rapidity of the change implies either a few large steps
or many and exceedingly rapid smaller ones. Large steps are tantamount to
saltations and raise the problems of fitness barriers; small steps must
be numerous and entail the problems discussed under microevolution. The
periods of stasis raise the possibility that the lineage would enter the
fossil record, and we reiterate that we can identify none of the postulated
intermediate forms. Finally, the large numbers of species that must be generated
so as to form a pool from which the successful lineage is selected are nowhere
to be found. We conclude that the probability that species selection is
a general solution to the origin of higher taxa is not great, and that neither
of the contending theories of evolutionary change at the species level,
phyletic gradualism or punctuated equilibrium, seem applicable to the origin
of new body plans." (Valentine and Erwin, 1985, p. 96)
- Valentine, J., and Erwin, D. (1985)
"Interpreting Great Developmental Experiments: The Fossil Record"
Development as an Evolutionary Process
Rudolf A. Raff and Elizabeth C. Raff, Editors
Alan R. Liss, Inc., New York, pp. 71, 95, 96