Monday, January 14, 2008

Evolution: Wonderful Life 2: the pattern of early diversification

Recapping on the Cambrian explosion: this was pretty much the beginning of modern multicellular animal life about 530 million years ago. In The Lying Stones Of Marrakesh, Stephen Jay Gould characterised this as constituting the greatest of all mysteries: "the causes of both the anatomical explosion itself and the 'turning off'of evolutionary fecundity for generating new phyla thereafter."

This has troubled biologists since before Darwin, and is less likely to be due to a simple gap in the exposed fossil record than to other phenomena, such as an extinction event covering the preceding Ediacaran fauna (sessile - non-mobile sponge-like marine animals). That's just a guess, but it parallels what happened 65 million years ago (the KT event, the end of the dinosaurs), when a meteor caused mass extinctions and specifically allowed mammals to proliferate.

The Cambrian Explosion saw the emergence of most major animal body types (phyla), bar the relatively insignificant marine bryzoa.

I'll restate a key passage of Gould's book Wonderful Life:

"The major argument of this book holds that contingency is immesurably advanced by the primary insight won from the Burgess Shale - that current patterns were not slowly evolved by continuous proliferation and advance, but set by a pronounced decimation (after a rapid initial diversification of anatomical designs), probably accomplished with a strong, perhaps controlling, component of lottery." (p301)

(Here, decimation doesn't refer to the literal reduction by one-tenth, but more the common understanding of much more dramatic paring to a relatively small fraction.)

If you can picture this as a spindle diagram, as Gould suggests, the direction of time would be bottom to top, and the spindle's width at various levels would represent diversity. And the appearance of animal body types would be like a christmas tree - the bottom would depict a small diversity of type, followed by a rapid (relatively instantaneous) expansion, followed by a gradual reduction. This is probably contrary to popular understandings of evolution of animal variety, as being either like an inverted cone - a constant increase in variety - or perhaps like a diamond, where a constant increase is followed by a constant decrease.

example of a spindle diagram

Gould depicted the Cambrian phenomenon as one of sudden explosion of so-called disparity - that is, sudden appearance of a variety of fundamental animal body types, or phyla.

This is where it gets interesting: he notes that in recent studies, he "concluded that the pattern of maximal early breadth is a characteristic of lineages at several scales and times, not only of major groups at the Cambrian explosion". In effect, the bottom-heavy christmas tree shape happens often. He "surveyed the entire history of marine invertebrate life - 708 spindle diagrams at the level of genera within families". With only one exception, he found lineages that arose early in the history of multicellular life - Cambrian or Ordovician periods - had spindles with 'centre of gravity' less than 0.5 - ie more like a christmas tree than a diamond.

This is certainly food for thought. Why such a regular pattern?

The KT experience does suggest that mass extinction permits remaining life to proliferate into the ecological vacuums, but there is no immediate suggestion that this happened in the Cambrian explosion. Yet the disappearance of the Ediacarans shortly beforehand (in geological terms) must have left a gap.

In The Lying Stones Of Marrakesh, Gould details evidence of phosphorised embryos in precambrian rock [the phosphorisation process can preserve soft tissue that would otherwise decay, but it works only on very small entities], with some suggestion that modern animal phyla had emerged before the Ediacarans disappeared. Then there is the SSF (small shelly fauna) of early Cambrian times to account for.

The story continues, and there are obviously more discoveries to come, more theorising. The mystery is too big to remain unaccounted for too much longer.

Gould, SJ (1989): Wonderful Life. Penguin, London
Gould, SJ (2002): The Lying Stones Of Marrakesh. Vintage, London.

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