HENRY

One of the big debates in evolution has been about the diversification of life on earth and how rapidly this occured. Around 580 million years ago, organisms tended to be quite simple and single-celled. However, shortly afterwards, we start to see a rapid burst of diversification leading to precursors of all the complex organisms we know and love today (rapid, that is, geologically, so around 80 million years).

This “Cambrian explosion” has always been a huge area of debate. Did this explosion really occur, or is it an artifact of the fossil record and the difficulty of finding fossil single-celled organisms? If the explosion did occur, then we need to re-think the idea of gradualistic change which has been central to evolutionary theory since the modern synthesis in the 50s. More importantly what caused this explosion? increase in oxygen levels? development of bodyplan genes like Hox? the end of “Snowball Earth” caused by global warming? Increased ecological specialisation and the spread into niches by the organisms?

In short - we don’t have a clue. However, today in Science a paper provides strong evidence that something unusual did happen during the Cambrian. Mark Webster, in A Cambrian Peak in Morphological Variation Within Trilobite Species (doi:10.1126/science.1142964), compares differences in almost 1,000 species of trilobyte, and shows that prior to the explosion, the variation in trilobites was much greater than after the explosion.

Science Daily says:

(Webster) focused on actively evolving characteristics. The trilobite head alone, for example, displays many such characteristics. These include differences in ornamentation, number and placement of spines, and the shape of head segments. His findings: Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so.

Unfortunately, no explanations of what caused this, but give it time. This is fascinating stuff, I highly recommend keeping an eye on this work, and reading up about the debates. The wikipedia has a nice sampler and Stephen Jay Gould’s Wonderful Life is a must read.

Posted on July 27th, 2007 by Simon Greenhill