Gaia: 'one tough bitch' of a theory

If anyone ever thought that science was dull, just get into the literature on Gaia and evolution. It rings with all the sound and fury of a battlefield. This is science played for high stakes, with radically new ideas challenging the established order - not only in science itself, but also in the political arena of environmental action. At stake is the question of how we are to live sustainably with the other occupants of the Earth.

In The Symbiotic Planet , Lynn Margulis takes some heavy swipes at what she sees as the dangerously mistaken assumption that humans act as thecaretakers of our beautiful blue marble in space: "Despite or perhaps because of Darwin, as a culture we still don't really understand the science of evolution."

The book presents a very different take on evolution and our position in the terrestrial scheme of things. In place of Darwin's focus on competition, there is a fundamental emphasis on the role of cooperation or symbiosis as the foundation of all significant evolutionary novelty.

The basic scientific ideas of the book will be familiar to most

biologists from Margulis's now-accepted theory of symbiogenesis as the key step in the evolution of life from bacteria to the stunning complexity of living forms that have evolved.

This describes how the ceaseless interactions of the simplest cellular creatures resulted in their cooperative union within a single more complex structure with a nucleus, the eukaryotic cell. The door was then open to the evolution of the multi-cellular life forms with which we are familiar - algae, fungi, plants and animals.

What is added in this volume is intriguing autobiographical material about the author's tortuous path to this discovery - the making of a rebel with scientific clout. What also emerges is a new taxonomy: tracing the evolutionary steps that have left their footprints in the organisation of different types of organism makes sense of these organisms' present properties in terms of their histories. Plants and animals turn out to be more similar to each other than to the rest of life.

Unfortunately there is some careless organisation of the text in telling this story, so it does not emerge as clearly as it should. But then rebels are rarely well organised.

The symbiotic theme is pursued throughout the book, including accounts of Margulis's own cooperative interactions with colleagues. We are given insights into transient evolutionary experiments, such as the extraordinary Ediacaran life forms of 600 million years ago. These are interpreted as a distinct lineage, neither plant nor animal, that emerged from early cooperative experiments and then disappeared entirely.

There is also the momentous occupation of land by unions of algae and fungi to form lichen-like partnerships that eventually evolved into plants. This union is still evident in the intimate relationship between plants and fungi, whereby the latter make minerals and basic salts available to plants while plants reciprocate with the gift of organic building blocks of life for the fungi. These are then distributed indiscriminately to all plants so that, for instance, a sunlit birch tree in a forest that is actively carrying out photosynthesis sends its sugars via the micorrhyzal fungal network to a spruce tree in the shade. No competitive struggle for life here.

The final chapter of the book deals with the history of the Gaia concept and Margulis's interpretation of this still-controversial hypothesis. She tells the story in her own terms, which are illuminating and clarifying. In her view, Gaia, the interacting planetary system of life and geophysical processes, is not an organism. No organism feeds on its own waste, for example, whereas Gaia is the genius of recycling: the waste of some organisms, such as the toxin oxygen, is used by others as an essential element for energy generation. A host of other gases are produced by millions of wood-devouring termites and released into the atmosphere to be used by other organisms as primary materials for life.

But Gaia is alive, if by this we mean a reproducing system capable of evolution by natural selection. Margulis describes a thought experiment to make the point: send a selection of microbes, fungi, animals and plants to Mars, which they might colonise after a period of time for adaptation to Martian conditions. If successful, this interacting system would have learned to use the minerals and gases on Mars to produce wastes that are recycled within the system. The only input would be sunlight. Gaia would have reproduced and evolved.

Margulis's punchy conclusion is a wake-up call to all of us to rethink our relationship to the planetary household: "Gaia, a tough bitch, is not at all threatened by humans," she says. "Our tenacious illusion of special dispensation belies our true status as upright mammalian weeds." Like it or hate it, this is relevant science.

Brian Goodwin teaches at Schumacher College, Dartington.