Origin of Life

Every organism alive today, and every organism that has ever lived, is connected through an unbroken chain of ancestry stretching back roughly 3.5 to 4 billion years, to a time when Earth held no life at all. Understanding evolution — the process by which populations of organisms change over successive generations, and by which new species arise from earlier ones — must therefore begin with the deeper question of where the very first life came from in the first place: the origin of life itself.

When Earth first formed, roughly 4.6 billion years ago, its surface was an inhospitable mixture of molten rock, volcanic activity and an atmosphere with essentially nothing in common with the air we breathe today. There was no free oxygen gas at all; instead, the earliest atmosphere is thought to have consisted mainly of simple gases such as methane, ammonia, hydrogen and water vapour, released from volcanic activity and the gradual cooling of the planet's crust. As the planet slowly cooled over millions of years, water vapour condensed into rain that, over an immense span of time, filled the low-lying basins of the crust to form the first oceans. It was somewhere within this early, chemically very different world — in shallow tidal pools, around volcanic vents, or dissolved within the primitive ocean itself — that the very first steps toward life are believed to have taken place: not the sudden appearance of a fully formed living cell, but a slow, gradual, step-by-step process of chemical change, often called chemical evolution, in which simple inorganic and organic molecules reacted with one another, driven by energy from lightning, ultraviolet sunlight (unfiltered by the ozone layer that did not yet exist) and volcanic heat, to build up progressively larger and more complex organic molecules.

Two of the most important scientific ideas that first gave this general notion a testable, scientific footing were the Oparin-Haldane hypothesis and its subsequent experimental support from the Miller-Urey experiment (both discussed in the next section, §6.2), which together proposed a plausible chemical pathway by which the simple molecules of the young Earth's reducing atmosphere could have combined to form amino acids and other organic building blocks — the essential raw materials from which the first simple, self-replicating living systems eventually arose. This chapter takes that single chemical starting point and follows the whole subsequent arc of evolutionary biology: the several independent lines of evidence — palaeontological, anatomical, embryological and molecular — that together convince biologists evolution really has occurred; Charles Darwin's own theory of evolution by natural selection and its later synthesis with the science of genetics; the specific population-level mechanisms, from mutation and recombination to gene flow, genetic drift and the mathematics of the Hardy-Weinberg principle, that drive and let biologists measure evolutionary change; the phenomenon of adaptive radiation, in which a single ancestral species diversifies rapidly to fill many ecological niches; and finally, the specific evolutionary history of our own species, Homo sapiens.