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Burst of life in the Cambrian explosion explained by increased evolutionary rates

The Cambrian explosion saw an incredible burst of innovation in the body plans and habits of animals. In the first rigorous estimates of early arthropod evolutionary rates, research suggests that this “big bang” is compatible with Darwin’s natural selection. The study results were published in the journal Current Biology.

According to University of Adelaide’s Mike Lee, for a long time, this simultaneous burst of life, with little to no precursors, appeared at odds the concept of gradual evolution through natural selection.  However, the study findings reveal that this pattern could have been produced through moderately accelerated evolution, sustained over a few tens of millions of years. A five-fold increase in evolutionary rates would have compressed almost 100 million years’ worth of change into a relatively brief 20 million years.

Considered to be the most diverse and successful animals of the Cambrian explosion, arthropods, which include such organisms as insects, spiders, and crustaceans, currently account for over 80 percent of all animal species. Thus, to understand major patterns in animal evolution, the researchers focused their efforts on arthropods as the key study group.

Making use of the fossil record and molecular dating methods, the research team quantified the anatomical and genetic differences between living arthropods in order to infer past evolutionary rates. The analysis revealed that changes during the Cambrian explosion occurred four to five times faster than present-day rates. During the same period, genetic evolution and anatomical evolution increased by approximately the same amount.

These faster evolutionary rates can likely be explained by innovations such as predation, vision, and active swimming, which opened up a new range of possibilities for arthropods. The same kind of evolutionary jump can also be seen when new environments are colonized, for instance when birds or mammals end up on islands. Thus, when a lineage acquires a novel adaptation, a subsequent burst of evolution often takes place to fill up the newly opened range of environments and niches.