Exuberant bacteria

13th December 1996 at 00:00
Michael Rowan-Robinson is inspired by an account of the complexity and variety of the evolutionary process. LIFE'S GRANDEUR By Stephen Jay Gould Cape Pounds 16.99

The astronomers with whom I work tend to believe that intelligent life is common in the universe, that it it is likely to evolve wherever there are stars, like our sun, with planets around them - and even that it is worth spending time and money trying to listen for their signals. Some of them believe that the universe has such a beautiful underlying structure that a creator's hand must be at work, and that the main purpose of the universe must have been for intelligent life to arise.

Biologists paint a very different picture. They see the evolution of intelligent life on earth as an extremely improbable chance event. Homo sapiens is not the end-point of evolution, but a twig on one of several vigorous evolutionary bushes, most of which are concerned with the rich variety of single-celled organisms on earth. The meaning of the universe cannot therefore have much to do with our existence: it seems more reasonable to face the fact that the universe does not have a meaning. We are here by chance and had better make the best of it.

In his preface, Stephen Jay Gould describes Life's Grandeur as "a companion volume of sorts to my earlier book Wonderful Life". Together they present an integrated and unconventional view of life's history and meaning - one that forces us to reconceptualise our notion of human status in this history. Wonderful Life asserted the unpredictability and contingency of any particular event in evolution - and emphasised that the origin of Homo sapiens must be viewed as an unrepeatable particular, not as an expected consequence. Life's Grandeur presents the general argument for denying that progress defines the history of life or even exists as a trend at all. In such a view of life-as-a-whole, humans cannot have the status of a culmination. Life has been dominated by its bacterial mode.

Wonderful Life used the amazing fossils of the Burgess Shale in Canada to demonstrate the many directions that evolution might have taken but did not. Life's Grandeur continues the theme of the non-inevitability of our existence by demolishing the concept that evolution results in a progression to more complex species.

The conventional view is that evolution embodies a fundamental trend towards increasing size, anatomical complexity and neurological elaboration, a trend that places Homo sapiens at the top of the evolutionary heap. Gould sets out to demolish this myth with subtle statistical arguments and witty analogies. He emphasises that in studying the evolution of some characteristic of a population over time, great care has to be taken in how the change is measured.

As an example of how misleading the median (or middle) value of some characteristic of a population can be, he cites a personal example, the life expectancy quoted for a rare form of cancer, which he was diagnosed as having in 1982. The quoted median mortality is only eight months, but he discovered that the distribution of mortality is skewed towards longer values. This was important for him personally in facing the disease, but it also gave him insight into the need to look at whole distributions, not just mean or median values.

The mean value for a population can appear to increase when all that is really happening is that the relevant characteristic is bounded on one side and the dispersion is increasing. Gould applies this to the alleged trend for the size of species to increase with time, illustrating it by analysis of the size of a particular type of plankton that is ubiquitous in the fossil record.

The central point of Life's Grandeur is important. The dominant organisms on earth remain today, as from the beginning, bacteria. The sequence of candidates for most complex organism over time does not represent a tree, a ladder, or even a connected line: it goes something like: bacterium, eukaryotic cell, marine alga, jellyfish, trilobite, nautiloid, placoderm fish, dinosaur, sabre-toothed cat and Homo sapiens. For sheer numbers, variety of species and range of environments they have colonised, nothing competes with bacteria.

Especially interesting are the bacteria found deep in the oceans at thermal vents, which do not rely on energy from sunlight. Some thrive at temperatures of more than 500 degrees Fahrenheit.

Recently, bacteria have been found from oil drillings deep in the earth's mantle, inhabiting fluid-filled pores, cracks, and interstices of rocks while living off the Earth's interior heat and chemicals. Gould gives quite a bit of space to speculative estimates that these mantle bacteria constitute the dominant fraction of the earth's biomass. I guess the naive enthusiasms of astronomers penetrate everywhere.

Michael Rowan-Robinson is professor of astro-physics at Imperial College, London.

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