After the Earth had formed and its surface had cooled enough for liquid water to accumulate, it remained a very hostile environment from our perspective. Temperatures near the surface must have been around the boiling point of water, volcanoes and earthquakes were ubiquitous and almost everyday occurrences, the atmosphere consisted largely of nitrogen, carbon dioxide and water vapour (there was no oxygen), massive thunderstorms were almost continuous, drastic fluctuations of atmospheric pressure were frequent, and the weak young sun poured ultraviolet irradiation through any breaks in the storm clouds. Worst of all, comets, cometary debris and meteorites battered the young planet continually. On the face of it, conditions seemed appropriate for mass extinction rather than the beginning of life.
Where in this Dante-esque world could the first organisms have acquired a foothold? The "warm little pond" of Darwin's speculation did not exist. The boiling oceans, popularly considered the cradle of life, are implausible candidates: organic molecules in hot dilute solution are far more likely to break down than to assemble into more complex structures, and the oceans were unlikely to favour the coalescence of anything resembling cells. The arid and unstable land surface was riven by volcanoes and comet collisions. Therefore, it seems implausible that life began anywhere on the Earth's surface. This leaves us with candidate locations above or below the surface: (1) in deep ocean hydrothermal vents, (2) in subterranean rocks or (3) in the atmosphere.
The first candidate - hydrothermal vents - has been the most popular since "dark smoker" ecosystems were discovered. Brands and his colleagues, Russell and Hall and other authors maintain that the earliest organisms resembled archaea rather than bacteria; archaea contain very slowly evolving genes, use energy-producing chemistry consistent with the hydrothermal vent environment and are intolerant of oxygen. These authors point out that hydrothermal vent archaea are chemoautotrophs, taking energy and material sources from their immediate environment, requiring neither sunlight nor atmosphere. In the hydrothermal vent environment, the formation of organic compounds is thermodynamically favoured. The small channels have a high total surface area suitable for catalysis. However, there are difficulties with this view. Modern deep-ocean archaea may have an indirect requirement for photosynthetic products falling from the ocean surface; if so, they do not wholly resemble the earliest organisms. Also, it is hard to see how cellular structure could form from molecular components in such a turbulent environment as a hydrothermal vent. And it is by no means certain that deep ocean vents on the primitive Earth were immune to sterilisation by cometary impacts. None of these objections is fatal to the hypothesis, but they would all have to be answered convincingly before the idea could attain consensus.
The second candidate - subterranean rocks - has so far gained only minority support. Thomas Gold believes that the newly-formed planet contained hydrocarbons, the components of natural oil and gas. According to Gold, the world's oil and gas reserves are made of just this material. Received wisdom tells us that oil and gas are the remains of once-living organisms. Gold says the opposite: the first organisms were made from the hydrocarbons in oil and gas deposits. There is some evidence for this. In Australia, oil has been discovered that is 3,000 million years old; it could hardly have formed from fossil organisms. In Sweden, oil has been found under nearly seven kilometres of rock, below what is usually considered the biosphere. Helium, a widespread product of radioactive decomposition in rocks, is always found with oil and gas deposits, never on its own. The deep subterranean environment was well protected from comets, meteorites, ultraviolet irradiation, atmospheric changes and other threats. The raw materials of life were available - hydrogen, iron, manganese, sulphur and other elements, in addition to the organic compounds.
Was life forged deep underground from the Earth's primitive organic constituents, only reaching the surface later? Not many people agree with Gold, but so far as the formation of biological molecules is concerned, his argument is hard to fault. What is less clear is whether cells could have formed in subterranean rocks. However, substantial and thriving populations of archaea have been discovered in rocks several kilometres underground, and they might live independently of the products of photosynthesis. Are these subterranean organisms the direct progeny of the earliest life on Earth? The possibility is exciting because it implies that life originated in an essentially solid medium (rock) rather than a liquid one (sea water), as has usually been supposed. But current opinion is sceptical.
What of the third candidate - that life began in the atmosphere? A few years ago, Tuck and Murphy made the striking observation that the stratosphere holds droplets of ocean water containing up to 50% organic matter. Some of these droplets are remarkably stable; their stability depends on their size. Big ones soon fall, very small ones fuse together, but droplets a micrometre or two in diameter remain suspended for many days. Their organic contents are concentrated by evaporation. These include greasy molecules that cover the droplet surface. If such a droplet falls through a similar greasy surface layer when it re-enters the ocean, the layers will fuse to produce something very like a cell membrane.
Granted that greasy molecules were present in the turbulent oceans of the primitive Earth, this droplet process would certainly have been commonplace. Could the molecules necessary for forming the first organisms have been trapped in high-altitude droplets? Exposure to intense ultraviolet radiation in the upper atmosphere would promote some chemical reactions, though nucleic acids would probably have been damaged. Coincidentally, the most stable suspended atmospheric droplets are almost exactly the size of prokaryotes32, and with a greasy membrane around them they bear a closer structural resemblance to cells than anything else so far conjectured about the origin of life. This possibility has not been widely considered, but it has no less intrinsic merit than the other alternatives.
32 The stable size depends on gravity and atmospheric pressure. If, say, the same process had taken place on the newly-formed Mars, the stable droplet size would have been considerably smaller. It would have conformed to the size of the deposits found a year or two ago on a certain Mars-derived meteorite recovered from Antarctica, amid much public excitement. Coincidences do happen.
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