Does Lots of Sediment in the Ocean Solve the "Mystery" of the Cambrian Explosion?
A recent Science Daily article admits that the cause of the Cambrian explosion has been a "mystery":
The results of this Cambrian explosion are well documented in the fossil record, but its cause -- why and when it happened, and perhaps why nothing similar has happened since -- has been a mystery.But according to the article, the "mystery" is now basically solved. The solution? It's simple: There was lots of sediment in the ocean. The Nature paper highlighted by Science Daily puts it in more technical language: "anomalous patterns of chemical sedimentation" from 540-480 million years ago "are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust," and "a protracted period of widespread continental denudation." According to the paper, this may have "affected seawater chemistry during a time of profound expansion of shallow marine habitats" which "may have been an environmental trigger for the evolution of biomineralization and the 'Cambrian explosion' of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals."
In other words, lots of chemicals from continental erosion in the water caused the Cambrian explosion.
Now, a skeptic might reply that we don't observe sediment in the ocean abruptly causing increases in the information content of genomes, much less do we have any experience with sediment creating new body plans. That sounds, in fact, like quite reasonable grounds for skepticism.
But this paper has no interest in explaining the origin of new information required to build the complex body plans that appear abruptly in the Cambrian explosion. It really isn't explaining the Cambrian explosion at all. Rather, it's trying to explain away the Cambrian explosion.
The premise behind the paper's argument is that somehow the increased chemical sedimentation allowed early animals to build hard parts, like shells or skeletons, which fossilize more easily. As the ScienceDaily article states, "Our hypothesis is that biomineralization evolved as a biogeochemical response to an increased influx of continental weathering products."
The argument is that the Cambrian explosion really doesn't represent the abrupt evolution of new animals, but rather just the ability to produce hard parts -- biomineralization. In their view, the Cambrian explosion simply represents the appearance of hard parts, which were finally able to leave fossils. Soft-bodied animals might have been common prior to the Cambrian, but they weren't fossilized.
Called the "artifact" hypothesis, this view suggests that the Cambrian explosion doesn't represent the abrupt evolution of animal body plans, just the appearance of parts that could leave fossils. As the paper states, the Cambrian strata "preserve the first skeletonized crown-group animals, a fact that some palaeontologists have interpreted as evidence for stratigraphic bias and an incomplete record of early animal evolution." The whole point is to make the Cambrian explosion less explosive.
There's a problem with that view, however: soft-bodied fossils are common throughout the strata representing the Cambrian explosion; the Cambrian explosion does not merely represent the abrupt appearance of hard-parts. A 2010 paper in Nature cites the "exquisite preservation of soft-bodied animals in Burgess Shale-type deposits." Likewise, Simon Conway Morris explains:
Soft-bodied marine faunas from the Lower and Middle Cambrian, exemplified by the Burgess Shale of British Columbia, are a key component in understanding the major adaptive radiations at the beginning of the Phanerozoic ('Cambrian explosion')."According to Conway Morris, when we look at the Burgess Shale specimens of the Cambrian explosion, some "95 per cent are either soft-bodied or have thin skeletons." (Crucible of Creation, p. 140) Thus, in the Darwin's Dilemma documentary he states:
(Simon Conway Morris, "Burgess Shale Faunas and the Cambrian Explosion," Science, Vol. 246 (4928):339-346 (October 20, 1989).
I think the Cambrian fossil record is surprisingly complete. I think it may be more complete than we realize. The reason for that is, for instance, if you look at the stratigraphy of the world, if I go and collect Cambrian rocks in Wales and find certain fossils, if I then go to China, I don't find the same species but I find the same sorts of fossils. If I go into Carboniferous rocks, I go to Canada, they are the same as what I find in this country. So there is a clear set of faunas and floras that take us through geological time. The overall framework is falling into position.Thus, the Cambrian explosion appears to have been a worldwide event, which involved the geologically abrupt appearance of both hard and soft-bodied organisms. It was a real event, not one that is simply an artifact of an imperfect fossil record.
Citing increased chemical weathering around the time of the Cambrian explosion doesn't explain the abrupt appearance of new genes and other genetic information needed to generate new body plans. If they expect us to believe that sedimentation rates explain the sudden origin of new body plans, then it would seem that the Cambrian explosion is still a "mystery."
Photo credit: Jordan Oram.