Originally posted by lee_merrill
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While it is true that most of the phyla that are around today arose during the Cambrian (some like sponges, annelids and cnidarians arose earlier in the Precambrian[1]) they are not anything like what we see today. Back in the Cambrian there were no amphibians, reptiles, birds, or mammals (much less humans). Simply put there were hardly any chordates at all, to say nothing of vertebrates. As far as fish go, they were limited to primitive, invertebrate jawless creatures that one can classify as "fish" like Haikouichthys, but no fully developed modern fish. Fish like we commonly find today are nowhere to be found in Cambrian deposits.
And there were few if any terrestrial insects (which represent well over 50% of all animal life currently existing on the planet).
Flora-wise there were not only any flowering plants (angiosperms) there weren't even any gymnosperms from which they arose from. There wasn't much in the way of any sort of terrestrial plants at all (vascular plants first arose during the Silurian long after end of the Cambrian).
As Graham Budd and Sören Jensen noted in their 2000 paper, A critical reappraisal of the fossil record of the bilaterian phyla, Cambrian life was still unlike almost anything that we observe today. While a number of phyla appear to have diverged in the Early Cambrian or earlier, most of the phylum-level body plans first appear in the fossil record much later on.
This flies in the face of the oft repeated claim made by creationists that, as Jonathan Wells (Senior Fellow at the Discovery Institute) puts it, "Most animal forms appear in the form they currently have in the present." Nope. Not even remotely close.
Secondly, the type of photosynthesis that first arose was likely very different than what we think of now. For instance, from a decade and a half ago, but I don't think it is obsolete, the abstract from Photosynthesis in the Archean Era
Abstract
The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga.
The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga.
Still, to be honest, I really don't know how long it all would take to evolve, but given the above considerations, the amount of time that may have been available from the origin of life to some type of photosynthesis doesn't seem unreasonable. Look at for instance what conservative estimates are for how long it would take eyes to have evolved[2]
1. And at least one phyla arose after the Cambrian. Bryozoa, for instance, is not known before the early Ordovician.
2. Back in 1994 Dan-Erik Nilsson and Susanne Pelger worked out a mathematical model on the time needed for a patch of light sensitive cells or photoreceptors covered by a layer of transparent tissue to evolve into a lensed eye resembling those commonly seen in many fish was reached. They found that it would take roughly 364,000 generations -- which equates to less than half a million years.
It took roughly 400 steps for the photoreceptor layer and pigment layer to form a retinal pit which continued to deepen until after approximately 1000 steps until it formed into a pin-hole camera eye. After this the lens shape continued evolving and the iris flattened allowing better focusing thereby providing improved optical properties.
In the end they found that the complete evolution of an eye like those found in a vertebrate or octopus took less than 2000 steps.
Moreover, Nilsson and Pelger took great pains to deliberately choose very conservative (low), pessimistic assumptions in their calculations so in reality it probably would have taken much less time to take place. For instance, they assumed that for every 101 organisms that got a certain mutation which provided them improved vision that 100 without this improvement also survived. This assumes that you are essentially as well off without the improvement in vision as you are with it which, in the real word, is extremely unlikely.
For those actually interested the paper can be found here: A pessimistic estimate of the time required for an eye to evolve
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