Extraterrestrial Civilizations - Isaac Asimov [84]
So it went. Over two dozen different kinds of molecules have now been detected in interstellar space. The exact mechanism by which these atom combinations are formed is not as yet clear, but they are there.
And even in outer space, the direction of formation would seem to be in the direction of life.* In fact, both in meteorites and in interstellar clouds it is interesting that the carbon chains are forming and that there is no sign of complex molecules that do not involve carbon. This is evidence in favor of our assumption that life (as we know it) always involves carbon compounds.
All of this evidence—in the laboratory, in meteorites, in interstellar clouds—makes it look as though the Haldane-Oparin suggestions are correct. Life did start spontaneously on the primordial Earth, and all indications would seem to be that it must have started readily, that the reactions in that direction were inevitable.
It follows that life would therefore start, sooner or later, on any habitable planet.
WHEN LIFE STARTED
But how much sooner, or later, is “sooner or later”? When did life start on the Earth?
Our knowledge of ancient life forms upon the Earth comes almost entirely from our study of fossils—remnants of shells, bones, teeth, wood, scales, even fecal matter—that have withstood at least some of the ravages of time and have done so sufficiently to tell us something about the structure, appearance, even behavior of the organisms of which they were once part.
Fossils can be dated in various ways, and the oldest ones that we can deal with easily are from the Cambrian period (so called because the rocks from that period were first studied in Wales, which in Roman times was called Cambria).
The oldest Cambrian fossils are 600 million years old, and it was tempting to assume that that was when life on Earth began, more or less. However, since we know Earth is 4,600,000,000 years old, that would mean it lay for 4 billion years without life. Why so long? And if lifelessness continued for that long, why did life suddenly appear? Why is Earth not still lifeless?
Then, too, at the time the fossil record begins in the Cambrian, life is already plentiful, complex, and varied. To be sure, all the life of which we have a record from that period is marine; there is no freshwater life or land life. Then, too, it is all invertebrate. The earliest chordates (the group to which we belong) did not appear for another 100 million years.
Nevertheless, what does exist seems quite advanced. Thousands of species of trilobites are found in the Cambrian period; these are complex arthropods very much like the horseshoe crabs of today. It is impossible to suppose that they sprang out of nothing and split up into many species. Before the Cambrian time, there must stretch long ages of simpler life. In that case, why is there no record of it?
The most likely answer is that the simpler life was not particularly prone to fossilization. It lacked the kind of parts—shells, bones—that survive easily. And yet despite that, traces of earlier life have been found.
The American botanist Elso Sterreberg Barghoorn (1915–), who in the 1960s was working with very ancient rocks, came across faint traces of carbon that, as he could demonstrate, were the remains of microscopic life.
The dim evidence of such microscopic life has now been traced back as far as 3,200,000,000 years, and it probably extended back a few hundred million years before that.
We might conclude, then, that recognizable life forms existed by the time the Earth was one billion years old.
This sounds reasonable intuitively. We can well imagine that during the first half-billion years of Earth’s history the planet may have been in a pretty unsettled state. The crust