Astrobiology, cynics say, is the only science with nothing to study. We know of no extraterrestrial life, so how do its followers spend their time? In fact, astrobiology involves not only searching for life elsewhere, but also the quest for an understanding of how life began on Earth, and its subsequent development. Unless and until we comprehend how replicating organisms originated on our own planet, we have no firm basis for estimating the abundance of aliens, if they exist at all.
The evolutionary track culminating in our own emergence has involved numerous discrete steps, occasional mass extinctions (of dinosaurs, trilobites and other less famous beasts) leaving niches free for new animals to burst forth. These extinctions have involved asteroid impacts, huge volcanic eruptions, variations in atmospheric composition, and other influences. We should expect the factors at work elsewhere -- on arid Mars, or in the ocean thought to exist beneath the icy crust of Jupiter's moon Europa -- to be different, but the question may be moot. If there is no life there at all, what difference do planetary cataclysms make?
Searches for extraterrestrial life therefore involve attempts to detect the very simplest forms (although the camera on board the British-led Beagle 2 probe would also spot any little green Martians that may come gambolling over the horizon). For clues about to what to look for, we sensibly consider the history of life on Earth.
Our planet formed about 4.5 billion years ago, its pelted surface took the next half-billion to solidify, and the oldest signs of microbial life date from around 3.8 billion aeons. There is then a huge interval during which slime ruled. That is, nothing existed except for single-celled organisms, laying down fossil mats (called stromatolites) that continue to be produced today.
Admittedly this slime lived in some extravagant places, including deep underground (if not, asteroid impacts could have sterilized the Earth, putting an end to our evolution before it started), but it's difficult to get too excited about life forms on to which we commonly pour disinfectant. The real action did not start until 600 million years ago, when the first multicellular life appeared in the oceans: things looking like modern-day jellyfish, for example, and soft corals.
Being soft-bodied, the fossils these left are rare, and difficult to spot. They were first recognized in the Flinders Ranges of South Australia by a geologist named Reg Sprigg, in 1946. (Personal disclosure: I was so amazed by the significance of these fossils that a few years back I arranged for an asteroid to be named Sprigg.)
This story is far from complete. A new era will dawn next year, when the International Geological Congress is expected to approve the definition of the first new palaeontological period in over a century. Sprigg's fossils are called the Ediacara fauna, for the low, sunbaked hills where he found them. The interval from 600 million down to 543 million years ago is to be termed henceforth the Ediacaran period.
This idea is in itself contentious, because all subsequent geological periods (the Jurassic, Cretaceous, Carboniferous and so on) are defined in terms of the types of fossils that appear in their strata. There are precious few fossils in the Ediacaran, making a definition of its beginning problematic, although the end is easy: starting 543 million years back, life suddenly proliferated, in what is called the Cambrian Explosion. Within five to 10 million years -- a blink of an eye, palaeontologically -- a vast panoply of life forms arose, many of them hard-bodied, so there are plentiful fossils available in the rocks.
What does this mean for extraterrestrial life searches? It implies that even if life did get started on some other planet, either within our solar system or else orbiting some distant star, we should anticipate it to be monocellular slime, rather than polycellular animals -- let alone sentient beings. Pick a random epoch in Earth's history, and slime is the most likely thing to be found.
On the other hand, once conditions were right, complex life did bloom here -- and it seemed to thrive on calamities (we would not be here if the dinosaurs were not gone). Where might we expect to find little green men, then? I'd say on a naturally violent planet, circuiting some distant star.
Duncan Steel is reader in space technology at the University of Salford, UK.
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