taken from here Seems like a good intro
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Genes or Skeletons?
Charles Darwin (1859, p. 308) recognised that the sudden appearance of a diverse and highly derived fossil fauna in the Cambrian posed a problem for his theory of natural selection, "and may be truly urged as a valid argument against the views here entertained." Two obvious possibilities are that animal life did, indeed, evolve very abruptly about that time, or, alternatively, had existed long before, but that, for whatever reason, we have failed to find fossil evidence: the "late-" and "early-arrival" models, respectively.
The two patterns – Cambrian Explosion versus a very long (though obscure) metazoan history – are not necessarily incompatible, because they are based upon different criteria. The genetic evidence documents lines of descent and inheritance, irrespective of morphology, whereas the fossil record documents only the external expression of the genes.
Late-Arrival Models
The ‘late arrival’ model embodies a literal interpretation of the fossil record: that the evolution of the main animal groups took place both late in Earth’s history, and as we now know, extremely rapidly.
What kind of mechanisms could have prompted such an accelerated pace of evolution? Various proposals have been advanced over the years, including:
A response to the evolution of Hox genes (Erwin et al. 1997).
A rise in macroscopic predation perhaps leading to an ‘arms-race’ style of evolution (Conway Morris 2000). Note, however, that the specific suggestion by Parker (2003) that such an arms race could have been fuelled by the acquisition of high-resolution vision in one or more groups, seems highly implausible (see the review by Conway Morris for a detailed critique).
Finally, perhaps we can look to the lifting of some external constraint such as insufficient atmospheric oxygen (Runnegar 1982; Brasier 1998, p. 548; Adouette et al. 2000, p. 4455).
Nowhere is the late-arrival model more eloquently discussed than in Stephen Gould’s Wonderful Life (Gould 1989), though nearly everyone would now agree that, in this book, Gould overstated his case by some orders of magnitude.
(Read more.)
Early-Arrival Models
Darwin himself preferred the early-arrival explanation, noting that "before the lowest Silurian [the Cambrian system had not yet been recognised] 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" (Darwin 1859, p. 307).
Today we might regard Darwin’s views as wonderfully prescient, for numerous Precambrian fossils have now indeed been collected, and modern techniques – impossible in Darwin’s day – such as ‘molecular clock’ studies, strongly indicate metazoan evolutionary events having occurred deep within the Precambrian.
There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed in the Vendian, and possibly early in the Vendian. Although some traces are simple, rather featureless, winding trails, "others display transverse rugae and contain pellets that can be interpreted as of fecal origin. The bilaterian nature of these traces is not in dispute. Furthermore, such traces must have been made by worms, some of which had lengths measured in centimetres, with through guts, which were capable of displacing sediment during some form of peristaltic locomotion, implying a system of body wall muscles antagonized by a hydrostatic skeleton. Such worms are more complex than flatworms, which cannot create such trails and do not leave fecal strings" (Valentine 1995, p. 90).
"The lack of any evidence of horizontal burrowing in rocks older than about 575 Mya and of vertical burrowing in rocks older than 543 Mya is a strong argument that there existed no animals about 1 cm or longer that were capable of disturbing sedimentary layers before this time. When they do appear, these bilaterian traces indicate the presence of animals that had AP [anterior-posterior] differentiation, but there is no evidence of limbs" (Erwin & Davidson 2002, p. 3023, but note these authors were apparently unaware of Arenicolites).
Sets of paired hypichnial ridges strongly hint at an arthropod s.l. presence. [Where and when? Reference?]
An entirely independent line of study is reported in Lieberman 2003. Here, the results of a phylogenetic biogeographic analysis of Early Cambrian olenellid trilobites are calibrated by a tectonic event, the breakup of Pannotia at 600 to 550 Ma, providing evidence that "trilobites likely had evolved and begun to diversify minimally by between 550 to 600 Ma" (Lieberman 2003, p. 231).
In preserving evidence of bilaterians, the Vendian record provides constraints on the protostome-deuterostome (P-D) divergence. If Kimberella is indeed a mollusc, as suggested by Fedonkin & Waggoner 1997, or certain trace fossils recorded from the Ediacara Hills and Zimnie Gory are correctly interpreted as radula scratches, we have evidence for derived protostomes at 555 Ma. Similarly, if Arkarua adami (from the Pound Subgroup, South Australia; Gehling 1987) is correctly interpreted as an echinoderm, we have evidence for a derived deuterostome of similar age. In either case, it follows that the protostome-deuterostome split must have occurred well before 555 Ma, which is consistent with most ‘molecular clock’ studies.
The latter, however, mostly favour a P-D split far deeper in the Precambrian: some as early as 1,200 Ma (table 1). If correct, then the Cambrian explosion is clearly an artefact.
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