According to modern science’s version of Genesis -- less colorful than the biblical story, but no less wonderful -- Earth was born, together with the Sun and the other planets, in a whirlwind of gas and dust, some 4.5 billion years ago, a little more than nine billion years after the Big Bang. A half-billion years later, our planet had recovered sufficiently from the pangs of its violent birth to become physically capable of harboring life. After less than another half-billion years, it did indeed harbor life, in particular an entity, called the last universal common ancestor (LUCA), that gave rise through evolution to all known living creatures, including microbes of various kinds, plants, fungi, animals, and humans.
Primitive organisms arose from nonliving matter in what were probably hot, sulfurous, metal-laden, volcanic waters. This unsavory brew was likely “spiced” with abundant small organic molecules such as amino acids, sugars, nitrogenous bases, and other typical components of biological constituents. One of the most astonishing discoveries of the last decades, revealed by exploration of space, nearby celestial objects, and especially meteorites that fell to Earth, is that many of the chemical building blocks of life form spontaneously throughout the universe. Organic chemistry, so named because it was believed to be a prerogative of living organisms, has turned out to be the most widespread and banal chemistry: the chemistry of carbon.
How this “cosmic chemistry” gave rise to the first living cells is not known in detail, but the process may be summed up in two words. The first is chemistry , the essence of life. Living beings continually manufacture their own constituents from small inorganic and organic building blocks, with the help of catalysts called enzymes and of energy derived from sunlight, mineral sources, or foodstuffs made by other organisms. Something similar happened in the origin of life, but along pathways, by the action of catalysts, and with sources of energy that remain to be identified.
Enormous research efforts have already been devoted to this problem. Much has been learned, but no solution is in sight. All that can be said is that the processes involved must, being chemical, have been highly deterministic and reproducible, that is , bound to occur under prevailing conditions. If chemistry admitted even a small element of chance, there could be no chemical laboratories, no chemical factories.
The second key word is replicability , the ability of certain information-bearing molecules to induce the making of (complementary) copies of themselves by the machineries responsible for the synthesis of their kind. This function, fulfilled mostly by DNA today, was probably first carried out by RNA, a close relative of DNA.
In the beginning, replication concerned only RNA molecules. Soon, RNA molecules became involved in the synthesis of proteins according to RNA-supplied blueprints, so that replication extended to proteins, by way of RNA (eventually DNA). In turn, replication came to affect, by way of proteins, increasingly complex objects, up to cells and multi-cellular organisms.
Replication allowed the endless reproduction of the same entities, generation after generation, which is the basis of genetic continuity. Furthermore, because of the inevitable failures in the fidelity of the process, replication necessarily led also to variation (in replicable form), hence to competition among variant lineages for available resources. The necessary outcome, as first divined by Charles Darwin, was the selection of those lineages most apt to survive and , especially , produce progeny under existing conditions. This process became added to chemistry as soon as replicability appeared, operating first on molecules and, subsequently, on increasingly complex assemblages, until the present day.
With replication, chance made its appearance, by way of the variations, or mutations, that were offered to the screening action of natural selection. According to all we know, these variations are strictly accidental, totally devoid of any intentionality or foresight – hence the widespread notion that the history of life was ruled by contingency.
But this view ignores the possibility that the array of choices offered by chance to natural selection may be sufficiently extensive to allow an optimal or near-optimal solution to emerge, in which case the process is actually close to obligatory and reproducible under the prevailing conditions.
Indeed, there are strong reasons to believe that optimizing selection may have occurred in the origin and evolution of life more often than is generally assumed. This implies that life, to the extent that it is the product of deterministic chemistry and of optimizing selection, is likely to arise, in a form similar to life as we know it, wherever conditions mimic those that surrounded its birth on Earth, thus justifying today’s interest in extraterrestrial life.
But this optimizing selection during evolution is nothing like proof of intelligent design. Irrespective of the arguments put forward in support of ID, which have been abundantly refuted, let it simply be stated that a theory based on an a priori declaration that things are not naturally explainable is not a scientific theory. By definition, the science is based on the idea that the object of study is naturally explainable. Why look for an explanation otherwise? What is truly wonderful is how much of nature, including the fundamental features of life, has already proven to be explainable.