How old are Animals?
M. de L. Brooke.
Trends in Ecology & Evolution, 1999, 14(6), 211.
Brooke's short essay is a reaction to molecular dating relating to the
divergence of phyla published earlier this year by Wang, D.Y-C.,
Kumar, S. and Hedges, S.B. "Divergence time estimates for the early
history of animal phyla and the origin of plants, animals and fungi".
Proc. R. Soc. London Ser. B. 1999, 266, 163-171.
Numerous previous studies have been made, but this new study uses more
genes (75 altogether) and takes into account the possibility that
different sites can differ in their rate of change and that there can
be rate differences between lineages. The conclusions are quite
startling: "·the nematodes diverged from the arthropod-chordate
lineage [· by·] about 1180 Mya, and that the three-way split of
plants, animals and fungi occurred about 1580 Mya. By inference,
basal animal pgyla, such as sponges and comb jellies, originated
between about 1200 and 1500 Mya."
A considerable part of Brooke's commentary is devoted to expressions
of surprise. "Such figures would raise fewer eyebrows if they did not
contrast so spectacularly with the picture of the fossil record." He
is referring to the Cambrian explosion which he dates at about 550
Mya. "Two broad questions arise. The first obviously concerns the
reliability of the molecular data. If those data can be believed,
then the second pertinent question is why should there be such an
enormous gap between the origin of phyla and their appearance in the
fossil record." "If the main groups of animals were up and wriggling
some 400 million years before they appear in the fossil record, it is
a major puzzle why they have not been found in the rocks."
If true, the basic thesis of Stephen Jay Gould in "Wonderful Life"
must be thrown out. "· the notion that the Cambrian explosion of
animal life corresponded to the origin of the major groups of animals
is not tenable if the phyla had been in business for several hundred
million years before appearing in the fossil record".
Brooke's tentative response to the puzzle is to suggest that the
earliest representatives of the different phyla were all very small,
even microscopic. They remained small and soft-bodied until the
Cambrian, when they evolved hard parts and rapidly increased in size.
However, Brooke is not really convinced by this proposal. He
concludes: "· while the molecular data do increasingly push metazoan
divergence back into the Pre-Cambrian, many people would feel more
comfortable if just a few more robustly dated fossils were available
to prop up the history read from the genes."
I wonder if anyone else sees these research findings through Kuhnian
eyes? When do the anomalies trigger the response in us that these
data are not coherent and that the consensus picture of the origin of
phyla is in urgent need of a revolution?
David J. Tyler.