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Flash in the Phan


The last 530 million years are a bit of a blip, really, say geologists

Geoscientist Online Monday 7 September 2009

The Earth has supported an active biosphere for at least the past 3500 million years, but the obvious fossil record is limited to just the last 530Ma. This Phanerozoic record documents a wealth of large scale (macro)evolutionary patterns, such as mass extinction and adaptive radiation, and sheds important light on the functioning and potential fate of the modern biosphere. Nick Butterfield (Cambridge University) told the British Science Association meeting in Guildford today: “Not only is the present the key to the past, but the past can be the key to the present. In this seemingly uniformitarian light, the macroecological and macroevolutionary "rules" of the Phanerozoic have commonly been extrapolated uncritically into the much deeper, pre-Cambrian record. “

Study of the pre-Cambrian fossil record over the past few decades has yielded a wealth of prokaryotic and eukaryotic fossils, but the emerging patterns differ fundamentally from those of the Phanerozoic. Prior to about 630Ma, not only were all organisms effectively microscopic, but diversity appears to have been fundamentally lower - and evolutionary turnover fundamentally slower - than at any time since. The principal picture is one of profound evolutionary stasis - and no measurable extinction - over hundreds of millions of years.

All this changed with the onset of the Ediacaran, which begins with the first measurable biotic “radiation” in the whole of the fossil record, followed closely by the appearance and relatively rapid turnover of Ediacaran macrofossils, the first sedimentary trace fossils and the first biomineralized macrofossils. The Ediacaran marks a fundamental shift towards macroevolutionary patterns typical of the Phanerozoic.

No shortage of hypotheses have been put forward to explain the shift of evolutionary dynamics at the beginning of the Ediacaran; but most of these (e.g., Snowball Earth, meteorite impact, global oxygen increase) fail to provide a proximal explanation for the observed phenomena, according to Butterfield. He believes instead that the key innovation was the evolution of Eumetazoa - diploblastic and triploblastic animals with a differentiated gut and nervous system - and their unique impact on ecology and evolution.

BSA “Unlike all other types of organisms, eumetazoans are capable of building multi-tiered trophic structures, and driving the morphology-based co-evolutionary arms races that give the Phanerozoic biosphere its peculiar character - not least large organismal size, complex behaviour, biomineralization, high diversity, high standing biomass, rapid evolutionary turnover, dynamic instability, mass extinction, biogeographic partitioning, and eukaryote-dominated primary productivity. As such, the history of life on Earth can be divided into two, more-or-less mutually exclusive phases, separated by the (newly ratified) Ediacaran Period (see fig). It was during this critical, 100 million-year transition that of animal-based ecosystems were developed, along with the decidedly non-uniformitarian rules of Phanerozoic macroecology and macroevolution” says Butterfield.

Punch line

“The punch line” says Butterfield, “ is that animals fundamentally changed the way that both evolution and ecology work; that the principle of uniformitarianism cannot be applied to a world without animals; that the biosphere of the past 500 million years is a bizarre situation quite unlike what the planet has usually hosted; and that the pronounced geochemical perturbations associated with the Cambrian explosion are almost certainly the consequence of animal evolution rather than its cause."

This also means that if we ever encounter an alien biosphere, the chances are that it will not resemble ours but the one that prevailed before the great “explosion” Most biospheres in the universe are likely to have the character of Earth's pre-animal biosphere” says Butterfield.  The highest organism on such a planet is more likely to stick to your boot than take you to its leader.

Butterfield says the next challenge for him will be to “get geochemists to appreciate the powerful role that animals play in driving biogeochemical cycles, including both the carbon cycle and the history of atmospheric oxygen. They tend to hold that evolution is driven more or less exclusively by geochemistry!”