Introduction

Evolution refers to the heritable changes that accumulate in one or more characteristics of a population of a species from one generation to the next. The present state of humankind on Earth is the outcome of three intertwined kinds of evolution — chemical, organic, and social or cultural.

Radiometric dating of meteorites puts the age of the solar system, and of Earth itself, at roughly 4.5–4.6 billion years. The newborn planet stayed inhospitable for several hundred million years: it was intensely hot at first, because the collisions of the planetesimals that came together to form it released enough heat to melt the entire planet. Only once the outer surface cooled and solidified into a crust, and water vapour released from the interior condensed to form the oceans, did conditions allow life to begin. Because no direct record of that moment survives, the origin of life has to be pieced together from indirect evidence, and several competing theories have been proposed to explain it — a few of which this chapter takes up.