Roughly 550 million years ago, life on earth consisted of bacteria and single celled organisms that plied the primordial ocean ooze. About 520 million years ago, primitive representatives of all but one existing animal phylum (and some which don’t exist anymore) skittered, crawled or swam on a very different Earth. The diversity of species was suddenly equivalent to the diversity of today. Speedy evolution such as this has never been repeated since.
Scientists don’t know for sure how or why the success and diversification of animal life happened so quickly – over about 40 million years, a geological and biological blink of an eye. Darwin himself reportedly wondered if this rapid diversification, referred to as the Cambrian explosion, represented a flaw in his theory of natural selection. Could natural selection alone account for such an explosion of life?
“The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life,” stated Michael Lee, Associate Professor of the University of Adelaide’s School of Earth and Environmental in South Australia. Yet the questions remain. What happened? Why during the Cambrian? And how did it happen so fast? One thing scientists do know, however, is how fast it happened.
In a paper published recently in the online journal Current Biology, Dr. Lee, Dr. Greg Edgecombe of the Natural History Museum, and their associates apply a stopwatch to the Cambrian explosion.
Their study analyzes the genetic and anatomical differences among modern arthropods – a phylum which includes crustaceans, insects, and other creepy crawlies – and determines how long it took those differences to accumulate. This allowed them to establish a timeline for evolution, which can be applied to the ancient arthropod ancestors from the Cambrian.
Though fossil evidence is scarce, scientists know that arthropods were as numerous during the Cambrian explosion as they are today. Fossils of trilobites are particularly prolific. Using their model, Lee and Edgecombe have been able to clock the rapid evolution of arthropods during the explosion and lay Darwin’s concerns to rest.
A centipede compared next to its ancient forerunner the trilobite, a fossil arthropod that lived 515 million years ago
“In this study” said Lee, “we’ve estimated that rates of both morphological and genetic evolution during the Cambrian explosion were five times faster than today – quite rapid, but perfectly consistent with Darwin’s theory of evolution.”
Theories still abound about the Cambrian explosion, but an understanding of how rapid the rate really was will go a long way in helping scientists to answer the remaining questions in the future.
Source: Lee, M. S., Soubrier, J., & Edgecombe, G. D. (2013). Rates of phenotypic and genomic evolution during the Cambrian explosion. Current Biology.
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