One of the first books I read as a boy was H. G. Wells's 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 it onto the surface of the moon it 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 Wells's 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 gas - how would this survive in a planet's 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 planet's atmosphere would be an infallible marker of life.
The early Earth had very little free oxygen until cyanobacteria (shown above) and other photosynthetic life forms began producing it about 2 billion years ago. Image Credit: Y. Tsukii, Oct., 2000
It was the blue-green algae, the cyanobacteria, which over a vast period infused the earth's 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 sun's energy to bond carbon and hydrogen and oxygen together to form complex molecules - sugars, carbohydrates - which 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 whales - or the equivalent of these in an alien evolution - but 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 quo - brachiopods, 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 them perhaps, 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 degrees this happened with glacial slowness - prokaryotes 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 changes from prokaryote to eukaryote, from anaerobic to aerobic occupied 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.
So if the Earth's history is anything to go by, we should not even expect any higher life in a planet which is still young it may take, even if life has appeared, and all goes well, many billion years for evolutionary processes to get that far. Moreover, all these "steps" - the evolution of aerobes from anaerobes; of eukaryotes from prokaryotes; of multicellular organisms from unicellular ones; and of complex beings with intelligence and consciousness from the first multicellular forms - may have occurred against daunting orders of probability, as Stephen Jay Gould and Richard Dawkins, in their different ways, have brought out. Gould speaks here of life as "a glorious accident," and Dawkins of evolution as "climbing Mount Improbable." And life, once started, is subject to vicissitudes of all kinds, from meteors and volcanic eruptions to global overheating and cooling; from dead ends in evolution to mysterious mass extinctions; and finally (if things get that far) from the fateful proclivities of a species like ourselves.
Yet since there are microfossils almost four billion years old, life must have appeared within one or two hundred million years of the earth's cooling off sufficiently to have liquid water. This makes one think that life may develop readily, perhaps inevitably, given the right physical and chemical conditions on a planet.
Image of the Viking Lander. Credit: NSSDC Photo Gallery
Such conditions could easily have occurred on Mars (before it dried and froze) or Venus (before it boiled). For Mars was once wet and warm, with seas and hydrothermal vents, and perhaps deposits of clay and iron ore, and it is especially in such places that we might hope to find evidence of past life. NASA has examined samples on Mars with its robot Explorers, but it has not been possible yet to make on-the-spot searches for microfossils, only to test for biogenic oxidation or metabolic products. No Martian sample has been returned to Earth. (There was, of course, great excitement a few years ago when it seemed that ALH 84001, the Martian meteorite, might contain minute fossilized structures.)
There must be thousands of Martian meteorites on Earth, and the notion of "seed-bearing meteoritic stones" was raised by Lord Kelvin in 1871, and the notion of free spores drifting through space and seeding life on other planets ("panspermia") was postulated by the Swedish chemist Arrhenius a few years later (an idea revived in the twentieth century by Francis Crick and Fred Hoyle). The idea was considered implausible for more than a century, but is once again a hot subject for discussion. For now, it is evident that the insides of sizeable meteors do not get heated to sterilizing temperatures, and that bacterial spores, or other resistant forms, could, in principle, survive within them, protected by the body of the meteor not only from heat but from radiations deadly to life. Meteors were being flung in all directions during the period of Heavy Bombardment four million years ago. Chunks of the Earth must have been ejected into space then, as well as chunks of Mars and Venus a Mars and Venus which might, at the time, have been more hospitable to life than Earth itself.
But we may not need to look too far afield for such meteors. We already know, from the samples returned by the Apollo missions, that there are early Earth and Martian meteors on the moon in considerable quantities. Now, perhaps, the time has come to plan a new mission to the moon, to allow mining and soil-concentration experiments that utilize technology unimaginable in the 1970s (such as portable polymerase machines to search the soil for ancient DNA). Here, perhaps, more easily than anywhere else, we may hope to find traces of the earliest life forms from Earth, or Venus, or Mars, and determine how life first started in our solar system.
And yet a romantic part of us cries out for more, for evidence of higher life, of human-order beings who can communicate their existence to us directly. So we need to keep SETI, our electromagnetic ears, open for more distant signs of life, as well as sampling our neighbors in this solar system. Who is to say what the next few years or decades will uncover?
For myself, since I cannot wait, I turn to science fiction on occasion; and, not least, back to my favorite Wells. Though it was written a hundred years ago, "A Lunar Morning" has the freshness of a new dawn, and it remains for me, as when I first read it, the most poetic evocation of how it may be when, finally, we encounter alien life.
