If I were permitted only one question about science, with a guaranteed correct answer, I would ask: How did life begin on Earth?

I wrote a column on this subject once before. My focus then was on how two completely different processes, building an organism and reproducing an organism (roughly speaking, the business of protein production and of DNA development) could possibly have developed at exactly the same time. I suggested, as many others have done, that RNA, which plays a role in both building proteins and in reproduction, might have been the agent, but I was not, and am not, at all satisfied with that answer.

Today I want to concentrate on a different element of the same mystery, and one that at first sight may seem much simpler: Where did life begin on Earth?

In the 1960s, scientists believed that they knew the answer. The story went as follows:

Ten billion years ago, neither this planet nor our Sun existed. Something like five billion years ago, they and the other planets were created from a great cloud of dust and gas known as the Primitive Solar Nebula. At that time there was no chance that life could have survived on the Earth. The surface was molten, torn by massive volcanic activity, and battered from outside by the impact of massive meteorites.

However, by maybe four billion years ago the planet had settled down into a form that we would recognize. The surface had cooled and oceans had formed beneath an atmosphere that was largely hydrogen. The first life developed in the warm bath of that primordial ocean, about three billion years ago.

There seemed to be experimental evidence supporting this scenario. In 1953, Stanley Miller had performed experiments in which he passed an electric spark through a mixture of hydrogen, ammonia, methane, and water vapor. The effect of this simulated lightning on the simulated early atmosphere was surprising and persuasive. More complex "prebiotic" molecules, including several of the amino acids needed to build up proteins, were formed. No one knew exactly when and how life had begun, but at least the circumstances to permit it seemed there.

In the 1970s and 1980s, that clear picture began to blur. The meteorite bombardment had gone on longer than expected, until at least 3.8 billion years ago. How can we know this, since the surface of the Earth from that era is long gone? We know because rocks brought back from the Moon show direct evidence of such battering, and the Earth and Moon are so close together in the solar system that while the Moon was being hit this planet could not have been spared.

Second, the date of first life was steadily pushed back. Geological evidence now suggests that primitive life forms already were present on Earth 3.8 billion years ago - while the heavy meteorite bombardment from space still was going on.

Third, certain sedimentary rocks and iron oxides can be reliably dated to the same distant era. Such rocks could not have formed with an atmosphere that was largely hydrogen. Also, today's atmosphere, with its substantial fraction of oxygen, could only form after life was established on Earth and photosynthesis had begun. The evidence now suggests that the atmosphere 3.8 billion years ago was a chemically neutral one, without much hydrogen or much oxygen. Unfortunately, repeating Miller's experiments in such a neutral atmosphere produces negligible amounts of amino acids. The idea of prebiotic molecules, formed by lightning and drifting down to combine into life itself in the placid primordial ocean, is no longer tenable.

What can replace the earlier picture? Life formed somehow on a turbulent planet, still shaken by impacts from space. And it formed astonishingly quickly. How?

Two theories are currently in vogue. The first is an old idea, proposed by Svante Arrhenius in 1907: Life did not originate on Earth; it was carried to Earth from elsewhere, perhaps formed on another planet, perhaps beginning in space itself. Arrhenius's idea seems less improbable now than it would have thirty years ago, because astronomers are constantly discovering more complex molecules in the great gas clouds between the stars. The notion that life was carried here does nothing to solve the problem of how life first began; it does, however, explain how we might find evidence of life at almost the first moment that the surface of Earth could sustain it.

The other theory - or rather theories, since there exist several versions - argue that life began not so much on Earth as in Earth. It is supported by several lines of evidence. First, we find plenty of life around the hot undersea vents known as "gray smokers" and "black smokers." This life, miles below the surface, does not rely on sunlight to provide energy through photosynthesis. It employs chemosynthesis, using the chemical energy in the sulphides emitted from the vents.

Second, deep drilling has discovered plenty of bacterial life in rock strata many miles down, far below the level where there can ever be easy contact with the surface. These bacteria, like the most ancient types of bacteria near the surface, are thermophilic, meaning that they prefer and thrive in hot surroundings.

The idea of a deep, hot biosphere may seem alien and difficult to accept, but it has one great advantage: the environment far underground might have been peaceful and stable, even while meteorites still ravaged the surface. In fact, even today the deep underground remains a possible locale for widespread life. Thomas Gold argues that not only did life on Earth originate there, but as much life is to be found at great depths as on or near the surface. His ideas are supported by the fact that deep drilling continues to find bacteria belonging to no known species.

The main conclusion that you could draw from all this is that when it comes to the question of where life began, our current state is one of profound ignorance. I can't disagree, but I want to point out one other implication of both the life-from-space and the hot-deep-life theories.

When astronomers examine the other planets of the solar system, they often conclude that those worlds are not likely to support life. So long as we require that life resemble what is on the surface of the Earth, that conclusion remains true. But if we are willing to entertain the idea that life can exist in open space, or far below a planetary surface, the picture changes.

Habitats for living things could be commonplace throughout the solar system. But those life forms may not much resemble us. It is we - multicellular objects, oxygen-tolerant and depending on sunlight for energy - who provide not the rule but the exception.

So if you feel that you are something special and out-of-the-ordinary, hang onto that thought. Maybe you are.

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