杏吧原创

鈥淢OST writers of biology texts imply that an organism exists apart from its environment, and that the environment is mostly a static, unliving backdrop.鈥� That, at least, is the contention of Lynn Margulis and Dorion Sagan. You may not agree but few will dispute the contrast between the pedestrian prose of most textbooks and this visionary portrait of the diversity, dimensions and interdependence of terrestrial life.

Margulis and Sagan explore the essence of life with tremendous brio, borrowing from the Gaia hypothesis 鈥� James Lovelock鈥檚 analysis of the Earth as a self-regulating organism and from Russian scientist Vladimir Vernadsky鈥檚 portrayal of living matter as a geological force. But they are also tough-minded advocates of science. Whether contemplating bowerbirds or honeybees, they show a true sense of wonder, without lapsing into the quasi- mysticism of a Rupert Sheldrake or a Lyall Watson.

Metaphors and astutely selected facts stir our imagination and intellect. Every five days, the authors point out, we have a new stomach lining, every two months a fresh liver. Meanwhile, each breath connects us to the rest of the biosphere. It too 鈥渂reathes鈥�, though at a slower rate, with increasing carbon dioxide concentrations on the dark side of the globe each day and decreasing concentrations on the lighted side. Life, as 鈥渇lying phosphorus-rich gulls, racing schools of mackerel, and sediment-churning polychaete worms, moves and chemically transforms the planet鈥檚 surface鈥�.

In What is Life? (consciously named to capture the spirit of the book of the same name written half a century ago by Austrian physicist Erwin Schr枚dinger), Margulis and Sagan reject the view that chance mutations, blind and undirected, generate all evolutionary novelty. 鈥淕reat gaps in evolution have been leaped by symbiotic incorporation of previously refined components 鈥� that have been honed in separate lineages. Pre-existing modules (which turn out to be primarily bacteria), already generated by mutation and retained by natural selection, come together and interface.鈥� In contrast to other sections of the book, which do not always explain how we know that life evolved in particular ways, there is an outstanding chapter demonstrating that, for example, the ancestors of our mitochondria were oxygen-respiring purple bacteria.

One of the most telling themes is the unity of vital processes over both space and time. Although we normally burn glucose aerobically, using sugars from the air, strenuous activity compels our muscles to ferment glucose in the anaerobic way invented by early bacteria. 鈥淥ur bodies thus 鈥榬emember鈥� the times before the atmosphere became suffused with oxygen. Such physiological flashbacks represent past environmental conditions and the bodies that evolved to live in them 鈥� All beings today retain traces of Earth鈥檚 earliest biosphere.鈥�

Likewise the discovery some years ago of haemoglobin in legume roots prompted the suggestion that these plants had somehow appropriated the oxygen-carrying pigment from animals feeding on them. But, as the authors point out, haemoglobin has now been found in the filamentous, sulphur- oxidising bacterium Vitreoscilla, indicating that it evolved in the bacterial ancestors of both humans and plants. 鈥淗aemoglobin is chemical evidence of a 鈥榖lood tie鈥� to early life 鈥� a blood tie that evolved long before blood.鈥�

This colourful and colourfully written book will not please everyone. Some will demur at the alleged emergence of 鈥渟uperhumanity鈥�, ingesting not only food but also coal, oil, iron and silicon. A few will be irritated by the misspelling of Alexander Fleming鈥檚 name and his identification as an Englishman, or be dismayed by the omission of any reference to N. W. Pirie鈥檚 highly apposite 1937 essay on 鈥淭he Meaninglessness of the Terms Life and Living鈥�. Others may have preferred more about the social processes of science.

All, however, will be won over by the persuasiveness of the book鈥檚 central theme and stylish presentation. Try this, as a final taster: 鈥淲e share more than 98 per cent of our genes with chimpanzees, sweat fluids reminiscent of seawater, and crave sugar that provided our ancestors with energy 3000 million years before the first space station had evolved. We carry the past with us.鈥�