Conditions on Early Earth and the Beginnings of Life

[#paragraph1]A little more than 3.8 billion years ago is a good estimate of when life began on Earth. How it began remains speculative. There is no standard theory; there is instead a confusion of conflicting theories that attack the problem from different angles. [#insert1]This is a change from 1953 when a classic experiment on the origin of life was published. Then, Stanley Miller and Harold Urey had just completed their famous laboratory simulation of the conditions of an early Earth at the University of Chicago. [#insert2]When Miller and Urey let electric sparks course like lightning through an “atmosphere” of methane, ammonia, and hydrogen, which circulated above an “ocean” of boiling water, they found that a reddish substance, rich in amino acids, accumulated in their glass apparatus. [#insert3]Amino acids, when strung together in long folded chains, form proteins, and proteins are the building blocks of the living cell. [#insert4]From the spontaneous synthesis of amino acids to the spontaneous origin of life on the primitive Earth did not seem such a long way to go.
 

[#paragraph2]That early optimism has proven profoundly mistaken, for at least two reasons. The first is simply that it is, in fact, a long way from amino acids to life. The hardest part about creating life is not making the amino acids that go into proteins; or the sugars, phosphates, and bases that go into DNA, which carries the cell’s genetic blueprint; or the lipids that form its protective membrane. The hardest part about creating life is not making the “bricks”: it is assembling them into a finished structure. That is what all the theories that have emerged since the Miller-Urey experiment are primarily about, and the conflict among them shows no signs of being [#highlight2]resolved[/highlight2] soon.
 

[#paragraph3]Furthermore, in recent years even the fundamental premise of that landmark experiment has been called into question. [#highlight3]Today most researchers who study early Earth do not believe that its atmosphere was primarily methane and ammonia, which would have been a strongly reducing atmosphere, where reducing means hydrogen-rich.[/highlight3] Methane and ammonia are both comparatively fragile molecules that might easily have been broken apart by the ultraviolet sunlight that bathed the young Earth, which had not yet evolved an ozone shield. More important, the idea that Earth was hot to begin with as a result of its violent birth, when large asteroids collided to form it, implies that its early atmosphere was rich in carbon dioxide rather than methane. That is the form in which carbon would be released by exploding asteroids.
 

[#paragraph4]The bottom line is that the early atmosphere is not likely to have been a giant Miller-Urey experiment; it would have been mostly nitrogen and carbon dioxide. In such an atmosphere it is indeed hard to make the molecular bricks of life, let alone a living organism. It is hard even to make the chemical compounds necessary for life. The most important compounds are formaldehyde and hydrogen cyanide, which, brought together in the presence of water, react to produce amino acids, from which the bricks are made. Formaldehyde and hydrogen cyanide, then, seem to be essential stages on the chemical road to life, and hydrogen cyanide especially cannot be made in great quantities in a carbon-dioxide atmosphere. Both compounds, however, are abundant in comets like Halley, Hyakutake, and Hale-Bopp. [#highlight7]Presumably[/highlight7] they are in other comets as well.
 

[#paragraph5]Here, then, is an elegant solution to the dilemma. The dilemma is that the old view of how life began conflicts with the new view of how Earth began and how it acquired an ocean. The solution, perhaps, is to deliver the organic precursors of life with the same vehicles that almost certainly helped create the ocean: icy comets. Researchers have calculated that over the course of Earth’s history, comets have delivered an amount of organic matter to the planet that is nearly a million times its present biomass—the total mass of all living things. Most of the organic matter would have arrived during the heavy bombardment that ended 3.8 billion years ago.