Monday, August 04, 2008

Bacteria directing the course of life?

"Gut bugs may have guided the evolution of life" screams the headline in the New Scientist.

The article reports on a study by Jeff Gordon and Ruth Ley of Washington University in St Louis and published in the journal Science. The microbiologists analysed bacteria from the digestive systems of numerous mammals, then compared the samples with DNA proximity of the animals.

Surprise, surprise, they found that the closer the mammals were genetically, the closer the correlation between populations of gut bacteria.

The scientists speculate that the partnerships between bacteria and mammals could help explain the success in spread and diversity of mammals [over the 65 million years since the KT event gave us breathing space over the dinosaurs]. They say that any adjustments in diet (eg carnivorous to herbivorous to grass-based) would need to be accompanied by a change in internal bacterial populations.

I'm not convinced that this is saying much that is new and significant. That team was caught struggling with the question of causality - did a change in diet to the herbivorous necessitate a change in digestive bacteria, or did a change in internal bacterial populations enable a change in diet?

In fact, evolutionary reliance on useful bacteria is not an unusual phenomenon - quite the reverse. From insects to mammals to legumes, plants and animals have relied on bacteria to aid in essential living processes. For example, Richard Dawkins (Ancestor's Tale) recounts how some (but not all) termites do not - cannot - digest wood fibre (cellulose) unaided, and use internal bacteria to break it down into useful chemicals - the bacteria's waste products. There are several unusual aspects of that bacteria, Mixotricha Paradoxa, but suffice it to say it is found nowhere but in the gut of Darwin's Termite.

This suggests coevolution of bacteria and "higher" organisms can happen in strong partnership, with each essential to the other. In fact, it is simpler and more meaningful to conceptualise bacteria - and the toolkits they bring with them, including the capacity to evolve comparatively rapidly - as an inherent part of the environmental niche in which organisms evolve. Thus our evolutionary environment is both external and internal to us.

Again, there is nothing new in this. But in a built world in which humans usually respond to bacteria as inimical to life, it is another obligation of deanthropocentrism to reorient our thinking to regard bacterial life on the whole as fully essential to where we came from and to our continued existence.

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