When in 1609 Galileo first saw the moons of Jupiter, he must have been spellbound. I was certainly so enrapt when I saw Europa and her three companions strung like a line of jewels. Galileo may have appreciated the irony that my guide was a Jesuit priest, and the somewhat antiquated telescope we used was but a few yards from the Papal summer residence in Castel Gandolfo. Galileo prized open the door and before long, scientific imagination was fired by the prospect of innumerable inhabited worlds. As the centuries progressed, imagination raced ahead of facts, with the Moon optimistically colonized by Selenites, and Mars transformed by immense canals to supply the parched regions of a planet plunging into desertification. From this dying planet H.G. Wells propelled his aliens to terrorize southern England with immense tripods housing sinister octopoids.
Now we might be closer to knowing if Wells was in any sense on the right track. The spectacular success in detecting extrasolar planets has produced a roster in excess of 450, and this technology potentially allows us to detect Earth‐like planets. Even if many of the known planets are too large to be habitable and lie, for the most part, beyond the inferred ‘habitable zones’, before long we will get some clues as to how densely our galaxy is inhabited. The consensus points in two directions. First, life is a universal. Second, our biosphere will be of almost no use when it comes to comparisons. Let me draw your attention to a remarkably unappreciated fact: if you want to understand aliens, stay at home.
Am I serious? After all it is already clear that extrasolar planetary systems are vastly different to our Solar System. Immense planets orbit their suns every few days, their surfaces far more torrid than that of Venus. Other planets most likely possess giant oceans, hundreds of kilometres deep. The diversity of moons and planets in our Solar System is a reminder of what may await us light years from Earth. Even among our neighbours, a case can be made for possible life in the clouds of Venus and Jupiter, the oceans of Europa and hydrocarbon lakes of Titan, and—with perennial optimism—in the permafrost of Mars. We might assume, therefore, that the range of environments available to life, its ‘habitation box’, is gigantic, and that Earth's biosphere just nestles in one tiny corner. Oddly enough the evidence is exactly the opposite. Life on Earth has reached the limits of what is possible—anywhere.
Temperature? The current limit on Earth is 122 °C. Plunging in the opposite direction the evidence is just as remarkable. At temperatures well below freezing, life carries on cheerfully. Even far beyond the eutectic, in which free water cannot form, organisms remain in a state of suspended animation with rates of damage and repair almost precisely matched. What of extreme desiccation? Evidently life has reached the limits of water activity. Entertainingly some of the hardiest forms are fungi that inhabit the weird alien world of Blue Stilton cheese. So, too, the bright colouration of salt pans is a familiar sight, and these osmotic extremes not only host rich microbial faunas but life that can flourish in the most bitter of brines. What of the extremes of pH—bleach versus battery acid? Once again, alkaliphiles and acidophiles disport themselves in ponds and streams that would have the Health and Safety officers in a state of panic. Pressure, either crushingly high or extremely attenuated? Life, of course, exists in the deepest oceanic trenches, but how much deeper might be viable? The weakest link seems to be the pressure sensitivity of the phospholipid membranes, suggesting that even on planets with titanic oceans life won't survive much deeper than in the Mariana Trench. The same argument applies to the deep crust: at about 5 km the crushingly high pressures also coincide with the thermal limits imposed by the geothermal gradient. Shall we look to the skies? Clouds carry bacteria, but even at quite modest heights it seems to be accidental freight rather than a nebulous ecosystem.
Terrestrial life has conquered nearly all of the ‘habitation box’ and its evolution begs so many questions. Are some forms, such as the hyperthermophiles, survivors from the Earth's apocalyptic beginnings? Maybe, but most have clearly been reinvented several times. Getting to the limits of life isn't that difficult, but how do extremophiles not only survive but flourish in these environments? Often the adaptations seem minor, which merely means they are more subtle than we might realize. What of the future? So far as the Earth is concerned it must cope with ever increasing solar luminosity: the last men will long predecease the last microbe. Possibly long before, we will engage in the first great galactic diaspora; but wherever our biologists journey they may find that life ‘out there’ got no further than the blue jewel that is Earth.
- Copyright © 2010 European Molecular Biology Organization
Simon Conway Morris is an evolutionary palaeobiologist at the University of Cambridge, England. E‐mail: