Posted by Bruce on December 27, 2011
Posted in Uncategorized |
It’s been a good month of December for devotees of the Cambrian “explosion”. First, Doug Erwin and his colleagues published an excellent paper, bringing together the paleontological record, sophisticated molecular clock analyses, and the latest in Evo-Devo research (including analysis of both mRNA and miRNA in the developmental toolkit of metazoa) to create a fascinating (albeit somewhat speculative) overview of the current state of our understanding of evolution during the Ediacaran and early Cambrian. Then, as a Christmas treat, Science, in its December 23 issue, published this paper, by Therese Huldtgren and colleagues, in which they use advanced microscopy to reach the conclusion that some fossils from China, hitherto believed to be examples of early metazoan embryos, are in fact much more likely to be clusters of protist cells, thus moving them outside of the metazoan crown group.
N. J. Butterfield, in a Perspective piece accompanying the paper, summarizes the significance of the paper as follows:
Wherever the Doushantuo fossils eventually end up, it will clearly not be within “crown-group” Metazoa. Does this then mean there were no early Ediacaran animals? Not at all. No fossil assemblage, however well preserved, provides a full account of past diversity, particularly when the local conditions are so extraordinary as to fossilize nuclei and other intracellular constituents. The “exceptional” fossil record is, by any measure, woefully unrepresentative and incomplete (15). When it comes to assessing the first appearance of early, difficult-to-preserve animals, the most reliable signal will be found in the conspicuous coevolutionary features that they have induced in other organisms with higher preservation potential (16). Early Ediacaran microfossils—with or without included “embryos”—may no longer include animals, but their dramatic radiation clearly marks the arrival of this revolutionary new clade.
So how does this get translated into the popular media?
The first is from an online site devoted to state politics; the second from the Voice of America. I invite you to read one or both of these articles – they are actually not bad. However, both headlines no doubt suggest to some that the new findings in some way threaten the entire theory of evolution. Quite the contrary – what the authors did was to test a very specific hypothesis – that the Doushantuo fossils are the remains of early metazoans – by applying new methods, and in so doing appear to have disproven that specific hypothesis. This is in fact what happens in the normal course of science. Most experimental scientists (including me) have had similar experiences. However, we also recognize that to a great extent, scientific progress is dependent on such testing, and as it occurs, many hypotheses will be found wanting.
So again, it’s been a great month for the Cambrian. And no, the theory of evolution has not been upended.