Sunday, March 16, 2014

How quickly did life arise on earth?

One of the great mysteries of life on earth is how life was able to appear so quickly on a newly formed planet. Earth was recently confirmed to be at least 4.375 billion year old. And yet there is evidence of life existing on Earth 3.8 billion years ago. Which means early earth had a chance to cool, and then form life, in a period of "only" 575 million years or so. No one knows how long the cool-down period took, but biogenesis may well have had considerably less than 575 million years in which to take place.
Francis Crick

Sir Francis Crick (one of the discoverers of the structure of DNA) found it hard to believe DNA-based life could get started in the time available. He famously quipped: “You would be more likely to assemble a fully functioning and flying jumbo jet by passing a hurricane through a junk yard than you would be to assemble the DNA molecule by chance in any kind of primeval sea of soup in 500 to 600 million years. It is just not possible.”

There are two aspects to rapid biogensis that bear emphasis. First, it's possible early earth was bombarded with comets, meteors, and/or asteroids containing significant quantities of ice and organic molecules. Meteors similar to the Murcheson meteorite (which is older than Earth) may have brought amino acids and complex organic compounds to the earth fully formed.

As recently as a billion years ago, there were no life forms bigger than a jellyfish on earth, and no vascular plants.

Also important is the fact that chemical reactions increase (logarithmically) in speed with increases in temperature (a relationship made quantitative by Arrhenius in the late 1800s). It is possible that the early chemistry leading to the precursors of life took place under high-temperature conditions, perhaps in deep ocean waters, near thermal vents, where (due to the great ambient pressure) water boils at much higher temperature than at the surface. (Just ten meters deep, water boils at 246°F or 120°C.) Chemistry at these high temperatures and pressures would have been very rapid.

It's interesting to note that by some estimates, all of the water in all the oceans on Earth can cycle through all the hydrothermal vents in the sea in only 10 million years. That's a relatively short cycle time compared to the 500+ million year time frame in which life appeared.

If monomeric molecules formed quickly under high-temperature/high-pressure conditions, they would possibly also have degraded quickly. Some researchers say that life may have had only a short window in which to appear before monomers broke down again into simpler solutes. In other words, high-temperature conditions favored a situation of rapid appearance of life, or no appearance.

Getting from simple monomers to stable macromolecules almost certainly required lower temperatures, since high temperatures disrupt the hydrogen bonds on which macromolecular 3D conformations depend. Nevertheless, the boiling point of water at sea level needn't be considered an impediment to the creation of life. We know that some types of bacteria on earth can survive (and even thrive) at temperatures of 122°C. So it's theoretically possible that early life on Earth could have emerged even in boiling seas.

A harder problem (for astrobiologists) is imaging how life could form in ultra-cold conditions, as in the methane oceans on Titan. Under super-cold conditions, hydrogen bonds might be too sticky to allow conventional life to appear; instead, low-temperature bio-molecules might interact based solely on van der Waals forces. Exactly what form such molecules would take is anyone's guess.

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