Discredited hypotheses for the Cambrian explosion

As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some postulated causes for the Cambrian explosion look improbable. Some examples are the evolution of herbivory, vast changes in plate tectonic rates or orbital motion, or different evolutionary mechanisms in force.

Developmental explanations

Regulatory genes

Hox genes regulate the operation of other genes by switching them on or off in various parts of the body, for example “make an eye here” or “make a leg there”. Very similar Hox genes are found in all animals from Cnidaria (e.g. jellyfish) to humans, although mammals have 4 sets of Hox genes while Cnidaria have only one.^[1] Hox genes in different animal groups are so similar that, for example, one can transplant a human “make an eye” Hox gene into a fruitfly embryo and it still causes an eye to form – but it’s a fruitfly eye, because the genes that the transplanted Hox gene activates are fruitfly genes.^[2]

The fact that all animals have such similar Hox genes strongly suggests that the last common ancestor of all bilaterians had similar Hox genes. This led to speculation that the evolution of Hox genes may have paved the way for the development of complex body forms.

This is where molecular dating can be useful. Remember that we can get rough estimates of the age of lineages, and indeed genes, by assuming that mutations accumulate at a constant rate in the genome. While the dates produced by this technique are often imprecise, and vary from method to method, the technique can provide an indication of the relative age of appearances.

Molecular clocks detect the radiation of three major clades of bilaterians – lophotrochozoans, deuterostomes and ecdysozoans – during the early Cambrian. The same study also found that Hox genes diversified long before these groups did – meaning that they cannot have been the innovation that caused the explosion.^[3]

Developmental entrenchment

Several scientists suggest that, as organisms become more complex, changes to their basic developmental mechanisms are more likely to be disadvantageous. In this way, the basic body design becomes "stuck in a rut", because any mutation that affects such central mechanisms will probably prevent the organism from developing; mutations are only likely to be advantageous where they affect less fundamental parts of the organism.^[4] By analogy, it is difficult to make modifications to the foundations of a building without causing it to collapse, whereas alterations can easily be made to the upper floors. The taller the building grows, the more likely a modification to the foundations will be to cause it to collapse.

However, as our understanding of the developmental pathways of organisms has improved, even the original proponent of this idea has realised that it is not likely to be a major factor,^[5] and examples of major post-Cambrian changes at a fundamental level have been found.^[6]

Ecological explanations

The appearance of herbivorous organisms

Stanley (1973) suggested that the appearance about 700 million years ago of protists (single-celled eukaryotes) that "cropped" microbial mats greatly expanded food chains and thus allowed rapid diversification, which led to the Cambrian explosion.^[7] But it is now thought that "cropping" arose before 1 billion years ago, as stromatolites began to decline about 1.25 billion years ago. ^[8][9]

Environmental explanations

Methane release

Others have suggested that each short-term decrease in the ¹³C/¹²C ratio throughout the early Cambrian represents a methane “burp” which, by raising global temperatures, triggered an increase in diversity.^[10] But this hypothesis also fails to explain the increase in disparity.^[6]

Changes in orbital parameters

References

1. ^ Erwin,D.H., and Davidson, E.H. (July 1, 2002). "The last common bilaterian ancestor" (abstract). Development (The Company of Biologists Limited) 129 (13): 3021–3032. PMID 12070079. http://dev.biologists.org/cgi/content/abstract/129/13/3021. 
2. ^ "Understanding complexity". University of California, Berkeley. http://evolution.berkeley.edu/evolibrary/article/0_0_0/evodevo_05. 
3. ^ de Rosa; Grenier, JK; Andreeva, T; Cook, CE; Adoutte, A; Akam, M; Carroll, SB; Balavoine, G (1999). "Hox genes in brachiopods and priapulids and protostome evolution.". Nature 399 (6738): 772–6. Bibcode 1999Natur.399..772D. doi:10.1038/21631. PMID 10391241. 
4. ^ Valentine, J.W. (1986). "Fossil record of the origin of Baupläne and its implications". In Raup, D.M., and Jablonski, D.. Patterns and Processes in the History of Life. pp. 209–231. 
5. ^ Valentine, J.W. (April 1995). "Why No New Phyla after the Cambrian? Genome and Ecospace Hypotheses Revisited" (abstract). PALAIOS (SEPM Society for Sedimentary Geology) 10 (2): 190–194. doi:10.2307/3515182. JSTOR 3515182. 
6. ^ ^{a b} Marshall, C.R. (2006). "Explaining the Cambrian "Explosion" of Animals" (abstract). Annual Review of Earth Planetary Sciences 34 (1): 355–384. Bibcode 2006AREPS..34..355M. doi:10.1146/annurev.earth.33.031504.103001. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.33.031504.103001?journalCode=earth. 
7. ^ Stanley, S.M. (May 1973). "An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian". Proc Natl Acad Sci USA 70 (5): 1486–1489. Bibcode 1973PNAS...70.1486S. doi:10.1073/pnas.70.5.1486. PMC 433525. PMID 16592084. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=433525. 
8. ^ McNamara, K.J. (20 December 1996). "Dating the Origin of Animals". Science 274 (5295): 1993–1997. Bibcode 1996Sci...274.1993M. doi:10.1126/science.274.5295.1993f. 
9. ^ Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance" (abstract). Science 174 (4011): 825–827. Bibcode 1971Sci...174..825A. doi:10.1126/science.174.4011.825. PMID 17759393. http://www.sciencemag.org/cgi/content/abstract/174/4011/825. Retrieved 2007-12-01. 
10. ^ Kirschvink, J.L., and Raub, T.D. (January 2003). "A methane fuse for the Cambrian explosion: carbon cycles and true polar wander" (PDF). Comptes Rendus Geosciences 335 (1): 65–78. Bibcode 2003CRGeo.335...65K. doi:10.1016/S1631-0713(03)00011-7. http://www.gps.caltech.edu/~jkirschvink/pdfs/KirschvinkRaubComptesRendus.pdf.