Famed neurologist Oliver Sacks is the author of many books, including The Man Who Mistook His Wife for a Hat and Uncle Tungsten. In this first of a two-part essay on astrobiology, Dr. Sacks muses on the possiblity of life on other worlds.

One of the first books I read as a boy was H. G. Wellss 1901 fable, The First Men in the Moon. The men land in an apparently barren and lifeless crater, just before the lunar dawn; then, as the sun rises, they realize there is an atmosphere; they spot small pools and eddies of water, and then little round objects scattered on the ground. One of these, as it is warmed by the sun, bursts and reveals a sliver of green. ("A seed, says Cavor . . . and then, very softly, Life!") They light a piece of paper and throw in onto the surface of the moonit glows and sends up a thread of smoke, indicating that the atmosphere, though thin, is rich in oxygen, and will support life as they know it.

This, then, was how Wells conceived the prerequisites of life: water, oxygen, and a source of energy (sunlight). "A Lunar Morning," the eighth chapter in his book, was my first introduction to astrobiology.

If Wells envisaged the beginning of life in The First Men on the Moon, he envisaged its ending in The War of the Worlds, where the Martians, confronting increasing desiccation and loss of atmosphere on their own planet, make a desperate bid to take over the Earth (only to perish from infection by terrestrial bacteria). Wells, who had trained as a biologist, was very aware of both the toughness and the vulnerability of life, and all the vicissitudes, which could befall it.

It was apparent, even in Wellss day, that most of the other planets in our solar system were not possible homes for life. But Mars was a solid planet of reasonable size, in stable orbit, at a reasonable distance from the sun, and with a temperature range, it was thought, which would allow liquid water to exist; so, it seemed, a fair bet for life.

But free oxygen gashow would this survive in a planets atmosphere without being mopped up by ferrous iron and other oxygen-hungry chemicals on the surface . . . unless it were pumped out in huge quantities, continually, enough to oxidize all the surface minerals, and then to keep the atmosphere charged? The obvious raw materials for oxygen production are water and carbon dioxide, but the photochemical decomposition of water (to yield oxygen) requires not only energy but metal-containing catalysts or enzymes, such as only occur in living matter. Thus the presence of free oxygen in a planets atmosphere would be an infallible marker of life.

It was the blue-green algae, the cyanobacteria, which over a vast period infused the earths atmosphere with oxygen, a process that took between a billion and two billion years. The fossil record shows that blue-green algae go back three and a half billion years, but, amazingly, some still thrive today, in odd corners of the world, forming strange, cushionlike colonies called stromatolites. It is an extraordinary experience to go to Shark Bay in Australia, where stromatolites flourish in the hypersaline waters, to watch them slowly bubbling oxygen, and to reflect that this, three billion years ago, was how the earth was transformed. (Oxygen, of course, is a mere by-product, a waste product, so far as the blue-green algae are concerned. The virtue of photosynthesis for them is that it enables them to use the suns energy to bond carbon and hydrogen and oxygen together to form complex moleculessugars, carbohydrateswhich can then be stored and tapped for their energy as needed.)

But astrobiologists should not see atmospheric oxygen as a necessity for life. Planets, after all, start without free oxygen, and may remain without it for all of their lives. But this does not negate the possibility of life. Anaerobic bacteria swarmed before oxygen was available, perfectly at home in the reducing atmosphere of the early Earth, converting nitrogen to ammonia, sulphur to hydrogen sulphide, carbon dioxide to formaldehyde, etc. (From formaldehyde and ammonia, they could make every organic compound they needed.) There may be planets in our solar system (and elsewhere) that lack an atmosphere of oxygen but are nonetheless teeming with anaerobes. And such anaerobes need not be on the surface of the planet; they could occur well below the surface, in boiling vents and sulphurous hot pots (as they still occur on the Earth today), to say nothing of subterranean oceans and lakes. (There is such a subsurface ocean on Europa, locked beneath a kilometres-thick shell of ice, and its exploration is one of the astrobiological priorities of this century. One would like to think of it teeming with great squids and whalesor the equivalent of these in an alien evolutionbut it would be exciting enough even it if just contained bacteria. Curiously, Wells, in The First Men on the Moon, imagines life originating in a central sea in the middle of the moon, and then spreading outwards to its inhospitable periphery.)

It is not clear whether life has to "advance," whether evolution has to take place, if there is a satisfactory status quobrachiopods, lampshells, for example, have remained virtually unchanged since they first appeared in the Cambrian. But there does seem to be a drive to gain ground, to become more widespread, more efficient, if this is possible. Thus the primitive anaerobes that represent the first signs of life we can find on the earth consisted of very small and simple cells, cytoplasm bounded by a cell wall, but without any internal structure at all. Such prokaryotes, as they are called, survive to the present day, along with the more complex organisms that arose from them.

(Primitive as they are, these prokaryotes are still highly sophisticated, with formidable genetic and metabolic machinery. They contain around 3000 proteins, and their DNA upwards of a million base pairs. It is certain that still more primitive life forms must have preceded themperhaps, as Freeman Dyson has suggested, organisms capable of metabolizing, growing, and dividing, but lacking any genetic mechanism for precise replication. But we have, as yet, no evidence concerning such precursors, nor of the abiotic chemical cycles that must have come still earlier, in the primordial sea.)

But by degreesthis happened with glacial slownessprokaryotes became more complex, acquired internal structure, nuclei, mitochondria, etc. (such nucleated cells are called eukaryotes) and acquired the capacity to utilize what was originally a noxious poison: oxygen. (Lynn Margulis, in the 1970s, championed the astounding suggestion that eukaryotes arose by incorporating other bacteria, which eventually became symbiotic-- functioning parts, organelles, of their hosts. This certainly seems to be true of mitochondria, etc., which are genetically different from the rest of the cell.)

These evolutionary changesfrom prokaryote to eukaryote, from anaerobic to aerobicoccupied the better part of two billion years. And there than had to pass another 1200 or 1300 million years before life rose above the microscopic, and the first "higher," multicellular organisms appeared.

(c) 2002 Oliver Sacks