Chapter Four

What caused the explosion of animal evolution in the Cambrian?

Why study the Cambrian explosion? The Cambrian explosion has been called one of the greatest unsolved mysteries in evolutionary biology: why do so many animal phyla (the highest taxonomic division in the animal kingdom) all appear in the fossil record during a single geological period? This burst of animal diversity has been interpreted by some as marking an unusually inventive period of evolution, with a greater rate of change in fundamental features of body plan than ever occurred before or since. Many hypotheses have been proposed to explain the apparent near-simultaneous appearance of many animal phyla in the fossil record, including environmental conditions (such as available oxygen) and ecological triggers (such as the rise of predation). In this chapter, we are going to examine only one hypothesis: that rapid change was facilitated by changes to a core genetic “toolkit” of body patterning genes. The universality, conservation and diversity of regulatory genes, such as those in the Hox cluster, and their association with body patterning in embryos, has led to suggestions that they have played a key role in the evolution of different animal body plans.

What are the main points?

• The Cambrian explosion presents two challenges to Darwinian gradualism: explaining evolution of novel features for which we have no direct evidence of intermediate forms, and explaining why animal phyla appear in a particular time period and not before or since.
• One proposed solution proposed to both these challenges is that body plans evolved not by a long series of gradual changes but by relatively few changes to developmental genes that allowed new distinct new forms to be generated without intermediates.
• To investigate this hypothesis, we need to know if such “macromutations” are possible, and if it is plausible that they could give rise to a successful lineage of animals with a novel body plan.

What techniques are covered?

• Evo-Devo:  the field of evolutionary development considers evolution of form in light of changes in gene regulation.
• Mutation and substitution: evolution is driven by heritable variation which must arise, rise in frequency and become a fixed part of persistent lineages.

What case studies will be included?

• Evo-Devo: Regulatory genes and the development of body plan

 

Image: Charles J Sharp, CC4.0