The Deep time versus the Late arrival hypotheses

How reliable is the fossil record as a guide to evolutionary events? Much attention has focused recently upon an apparent mismatch between the evidence

Arthropods

Nsmatoda

Priapulida

Mollusca

Annelida

Platyhelminthas

Brachiopoda

Chordata

Echinodermata

Cnidaria

Porifera

\l I I I ^Doushantuo embryos T ' > i poushantuo acritarch microflora ¡mm m trace fossils g +5

\l I I I ^Doushantuo embryos T ' > i poushantuo acritarch microflora ¡mm m trace fossils

Fig. 7.6 Changes in ocean chemistry as shown by 813C and 834S and 87Sr/86Sr, shown alongside evolutionary changes in the fossil record during the Late Proterozoic and Cambrian. P, phosphatized microfossil assemblages (see text); * snowball earth glaciations; E, suspected mass extinctions of acritarch phytoplankton (Redrawn after Brasier 2000.)

Fig. 7.6 Changes in ocean chemistry as shown by 813C and 834S and 87Sr/86Sr, shown alongside evolutionary changes in the fossil record during the Late Proterozoic and Cambrian. P, phosphatized microfossil assemblages (see text); * snowball earth glaciations; E, suspected mass extinctions of acritarch phytoplankton (Redrawn after Brasier 2000.)

provided by fossils and that provided by molecular clocks. The oldest unequivocal fossil evidence for metazoans is ~600 million years old but, according to some researchers, this is much younger than evidence suggested by the rRNA of living organisms. Assuming that gene sequences evolve with such regularity that differences can be used as 'molecular clocks', it can be argued that invertebrate lineages began to diverge about 1.2 Ga (Wray et al. 1996). This would mean that the earliest animals are missing from the fossil record because of a low fossilization potential. For example, they may have lived in the water column or as microscopic life in the sediment. The so-called Cambrian 'explosion' after 600 million years would then relate in large part to the acquisition of skeletons.

Molecular clocks are notoriously difficult to calibrate, however, and these 'deep time' estimates for the divergence of major animal phyla have been drastically scaled down to nearer 670 million years, bringing the figures more into line with the fossil record (Ayala & Rzhetsky

1998). A 'late arrival' model would also imply that the evolution of the animal phyla took place both late and rapidly, perhaps in response to the evolution of Hox genes (Erwin et al. 1997), or in response to the lifting of some external ecological constraints such as an increase in atmospheric oxygen (Schopf & Klein 1992). Thus while the Cambrian explosion could be viewed as the almost inevitable consequence of the evolution of sexuality and multicellularity between about 1.3 Ga and 600 Ma, its timing appears to have coincided with major geological changes at the Earth's surface.

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