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 9 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 a half-billion years later, 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 and 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: chemistry and replicability.
Chemistry is the essence of life. Living beings continually manufacture their own constituents from small inorganic and organic building blocks. Catalysts called enzymes and energy derived from sunlight, mineral sources or foodstuffs made by other organisms assist in the process.
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. This means they are bound to occur under prevailing conditions. If chemistry admitted even a small element of chance, there could be no chemical laboratories and 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, 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 and eventually DNA. In turn, replication came to affect, by way of proteins, making increasingly complex objects that extended 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 -- and then to competition among the variant lineages for available resources.