Astrobiologists aim to work out how life fits into the wider universe, uniting biology with the planetary and space sciences to draw a unified picture of the cosmos and our place in it. This ambition is, of course, far older than the discipline itself.
The atomist philosopher Anaxagoras (c. 510–c. 428 BC) was perhaps the first to detect a kinship between the sun and other stars, proposing that they were fiery stones and thus objects of the same fundamental kind. He was also among many thinkers of his time to conjecture that the moon reflected rather than emitted light, that it was rocky and mountainous like the Earth, and that it might therefore be inhabited. Many atomists spoke of an infinite “plurality of worlds” each with its own inhabitants, but this usually meant a multitude (even an infinity) of distinct cosmoses — each a concentric world-system with its own central “Earth” (perhaps inhabited), and each with its own stars fixed in a great, all-encompassing sphere. Various Greek philosophers recognised the rotation of the Earth, and a few argued that it orbited the Sun, but Plato and Aristotle had little time for these far-fetched ideas, which clashed with their metaphysical views.
Several thinkers in the Islamic Golden Age recognised the rotation of the Earth, and the Persian polymath Fakhr Al-Din Al-Razi (1150–1210), who was probably familiar with Greek atomist texts, argued strongly for the plurality of worlds. However, the early Christian Church adopted Aristotle’s geocentric system together with the Ptolemaic “epicycles” needed to explain retrograde planetary motions, and these ideas were not robustly challenged in Europe until the fifteenth century when Scholastic thinkers began to diverge radically from Aristotle. Thus, Nicholas of Cusa (1401–1464), a German cardinal and theologian, claimed that the universe was infinite and centreless, and that all celestial bodies — moon, sun, planets and stars — were inhabited. Copernicus’ De Revolutionibus (1543), placed Earth among the planets as fellow travellers around the sun and so implicitly raised the question whether other planets might also be Earth-like. Drawing on Copernicus along with various other scientific, mystical, and occult sources, the Neapolitan philosopher Giordano Bruno (1548–1600) argued that stars are other suns, some like ours orbited by “aqueous or crystalline” planets. Both stars and planets, he thought, were inhabited. Kepler, who placed Copernicanism on a firmer mathematical, theoretical, and empirical foundation, agreed with Bruno about the ubiquity of intelligent life on celestial bodies and discussed at length the probable inhabitants of the moon.
Despite early attempts by the religious authorities to suppress Copernicanism and its implications, speculation about extraterrestrial life flourished after the sensational discoveries of Galileo and gathered pace during the Enlightenment of the 17th and 18th centuries. Descartes and Newton realised that each star was the gravitational centre of its own, discrete planetary system. Books such as Bernard de Fontenelle’s Entretiens sur la pluralité des mondes (1686) and Huygens’ Cosmotheoros (1695) argued that the inhabitants of other planets probably resembled life on Earth. Improved telescopes made it possible to map the near side of the moon, to resolve surface features on Mars, and even to catalogue nebulae (the latter were eventually identified as the birthplaces of new worlds by none other than Immanuel Kant, a firm believer in extraterrestrial life). Other revolutions took place in biology; Aristotle’s views on the spontaneous generation of life from inanimate matter were cast into doubt, microorganisms were discovered, and sweeping evolutionary theories were proposed to explain the development of the cosmos and its inhabitants through time.
Many of the key advances underpinning a modern scientific approach to astrobiology were made in the nineteenth century. The discovery of natural selection, elaborated in Darwin’s 1859 On the Origin of Species, showed that life was moulded by its environment, and added to a growing weight of geological evidence that Earth was at least a hundred million years old. Pasteur elegantly refuted Aristotelian spontaneous generation (also in 1859). Proteins and nucleic acids were discovered, and the discipline of biochemistry was inaugurated. Maxwell’s equations, atomic theory, and the periodic table were formulated. The development of spectroscopic methods in astronomy confirmed the ancient suspicion that the sun is a star (and revealed it to contain helium), and it finally became clear that the moon lacked an atmosphere and was probably uninhabited. In many ways the universe became less mysterious, although in others mysteries multiplied and deepened. The source of stellar heat and light was unknown; nor was it clear how stars are distributed through space. The fossil record seemed riven by inexplicable gaps; complex animals simply appeared at the base of the Cambrian with no evolutionary precursors. Genetic heredity could not be explained chemically. There seemed no direct way to test the widely held assumption that intelligent life had evolved on Mars (although there were plenty of sceptics, among them Alfred Russell Wallace). Powered human flight — let alone space flight — seemed a long way off.
Progress in the twentieth century solved these problems and thereby brought the fundamental questions of astrobiology firmly within the scope of rigorous scientific investigation. How and when did life originate? How many worlds are habitable, and how many of those are inhabited? Could the seeds of life spread between worlds? Can life on other planets be detected from Earth? In the late twentieth century, spurred on by Russian and American advances in space exploration, the new discipline of “astrobiology” coalesced around these questions. In striving to answer them, we are — like all scientists — carrying on a project begun in antiquity, whose greatest achievements may still lie ahead of us.