Wednesday, October 23, 2013

The Cambrian Explosion: Could Such Rapid Evolution Happen Naturally?

The Cambrian Explosion refers to a period around 540 to 520 million years ago during which arose an abrupt increase in the number and types of animal groups as evidenced by the fossil record. This sudden increase in the rate of evolution has caused some to argue that it could not have happened naturally. But there has never been an accurate measurement of the actual rate of evolution during this period, until now.

A new study in Current Biology (1,2) has found the rates of “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.” The study focused on arthropods, the most diverse animal group in the Cambrian period and today, but the results are considered to be generally applicable to the other animal groups. 

Theories of how the Cambrian Explosion happened generally fall into three main categories: genetic, ecological and environmental/geochemical. In the latest edition of Science (3,4) Professor Paul Smith of Oxford and Professor David Harper of Durham University propose that no one theory explains it all, but that a ‘cascade of events’ led to the sudden explosion in the number and diversity of species. It is becoming clear, however, that the oxygenation of the earth’s atmosphere played a major role. Large organisms cannot exist without a minimum amount of available oxygen (5). Large amount of evidence has shown that two geological periods of oxygenation occurred, one in the Precambrian period and another two billion years earlier (6,7). During the Neoproterozoic era that preceded the Cambrian Explosion, the geological conditions were chaotic, with the breakup of the supercontinent Rodinia and a period of prolonged global cold during which oceans were covered with ice from pole to pole and were low in oxygen content. During this time oxygenation occurred non-linearly and organisms evolved that were able to survive both aerobically and anaerobically, perhaps leading to even greater diversity.