Cambrian smackdown

There are few things more satisfying than having one of your long-held views vindicated. Today's WUN seminar, actually broadcast out of Penn State, gave me one of those moments.

The Cambrian explosion is the name given to the geologically "sudden" appearance of most major animal groups about 530 million years ago. As fossil strata go, it represents a strikingly singular event, when life on Earth appeared to go from nothing to being prodigiously well-stocked with a diverse plethora of forms. Sort-of like the "big bang" of biology. However, I've never bought it.

It probably stems in part from a deep-seated unease with a seemingly ex nihilo creation event. But I was also largely won over by molecular clock evidence years ago, and have been perennially sceptical of over-confident palaeontological assertions about the lack of Precambrian fossils. This in spite of being trained up as a "whole organism" biologist with only a scant grasp of things molecular.

Anyway, today's seminar was delivered by the evolutionary biologist Blair Hedges. And he played something of a blinder with his seminar. He started off in an ever-so-slightly dull fashion with evolutionary trees, and how they can be redrawn to show how the separation of branches has occurred through time. So far, so pedestrian. He then drew in some work he's done with nobody's favourite group of organisms: the eubacteria. Through some clever analysis of all sorts of factors (including GC-content), he showed how the evolution of bacteria splits modern groups into terrabacteria, who display phenotypic features that favour life on land, and hydrobacteria, who display traits more suited to aquatic existence. He also showed when the split between these groups occurred: around 3 or so billion years ago. This was news to me, but it was the tip of the iceberg in his talk.

This appearance of a specifically land-loving clade of bacteria way back in deep time points to the correspondingly early colonisation of land. Traditionally, this has been viewed as a relatively recent phenomenon, but the ancient origin of bacteria adapted to life in the dry paints the history of the Earth very differently. Expanding into the eukaryotes, Hedges showed similarly ancient diversification in plants, animals and fungi, and drew out links between evolutionary milestones and changes in life's relationship with the Earth. Specifically changes in the atmospheric concentrations of carbon dioxide and oxygen, and the succession of "Snowball Earth" events in the Neoproterozoic.

From here Hedges skipped onto specifically animal evolution, and again showed ancient splits between the major lineages; splits that have been suggested before, but which now have much greater weight as the quantity and diversity of molecular evidence relentlessly grows. He also made an observation which was new to me, but is pretty damning of fossil evidence: most modern fauna, especially small, soft-bodied animals, have a shockingly poor fossil record. Groups which must date back to at least the Cambrian only crop up in very recent strata, and some don't show up at all. So the "cast-iron" evidence of missing fossils starts looking extremely moth-eaten to say the least. Hedges posits that the Cambrian explosion, far from being a revolution in biology, actually represents a more modest change in animal diversity driven by larger size (in turn driven by atmospheric oxygen) and the appearance of preservation-conducive "hard parts".

Another interesting piece of analysis that Hedges brought to the table was the evolution of complexity. Using his revised phylogenetic and temporal trees, Hedges and his colleagues have estimated the complexity of lifeforms over the history of the Earth based on the total number of cell types. So the appearance of particular groups at specific times adds to the complexity of life on Earth. Viewed this way, the Cambrian explosion is a slight rise in a much longer trend of increasing biological complexity. Hedges used this to make the much wider claim that our apparently "Rare Earth" may not be so rare after all. Something to raise a smile (and hopes) in astrobiology circles.

Obviously, such a sweeping assessment of the evidence isn't without its critics, and I'm probably blinded by my own prejudices in my reaction to it. But it just felt good to have such long-held hunches "confirmed", slide by Powerpoint slide.

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In passing, along with several colleagues, Hedges is responsible for a remarkable project called www.timetree.org. This brings together papers on phylogeny to both enable easy searches, and the construction of novel meta-phylogenies (trees of trees) based on the data collated to date. This includes a fun application which allows you to estimate the date of the last common ancestor between arbitrary organisms. For instance, for cats and humans this was about 98 million years ago. For humans and my favourite invertebrate this is a massive 981 million years ago.