Although earlier naturalists had realized that living things must have changed during the history of life on Earth, it was Jean Baptiste de Lamarck who first developed an evolutionary philosophy based on the key idea of adaptation. His theory of the "inheritance of acquired characteristics" remained popular in his home nation for most of the 19th century and influenced the speculations of Flammarion and Rosny concerning extraterrestrial life. Elsewhere, however, Lamarckism was called into question, undermined by the experiments of Weismann in Germany, and superseded by Darwin's theory of evolution by natural selection. According to Darwinism, the traits favored by natural selection vary in different environments and cause the genetic change in populations of organisms over generations. This view had an especially powerful effect on scientific thinking in Victorian Britain, where it was championed by Thomas Huxley and the sociologist Herbert Spencer, and used by H. G. Wells as the basis for his essays and stories about possible alien life-forms. In time, Darwinian ideas would transform and unify biology, helping it to become a modern, autonomous science, but they could not in themselves make biology into a universal science akin to Newtonian physics. Whereas the movements of the Moon and planets could be readily observed and therefore used as the basis for a generalization of the theory of gravity beyond the Earth, no data on extraterrestrial organisms were available from which the biologist could establish some cosmic principles of life. Consequently, there was no way of predicting if, on other Earthlike worlds, life would appear as a matter of course and, if it did, how far its development would progress and in what directions.

 

For the most part, biologists in the first half of the tentieth century steered clear of questions about extraterrestrial life, having problems enough in trying to understand how life had started here (see life, origin). The tendency among those biologists, such as W. D. Matthew, who did venture an opinion, was to argue that since life appeared to have originated on Earth only once or at most a few times over the course of many millions of years, and then as the result of an incredibly unlikely chain of chemical reactions, the chance of it happening elsewhere was very low and perhaps even zero. Astronomers, on the other hand, except during the 1920s and 1930s when the catastrophic hypothesis was resurgent, tended to assume that the vast number of stars and presumed accompanying planets virtually guaranteed that life was abundant, whatever the details of its origin. Greater interest and urgency was added to this debate in the 1950s by the results of laboratory work in prebiotic synthesis, including the Miller-Urey Experiment, and the imminent start of planetary exploration and proposals to search for life on Mars and other neighboring worlds. The laboratory creation of the basic chemical building blocks of life and also of structures that resembled cell membranes encouraged researchers involved in origin of life studies and in the nascent field of astrobiology to express optimism that the emergence and evolution of life is essentially deterministic – an inevitable consequence given certain starting conditions. This optimism was dented somewhat by the failure of the Viking spacecraft to find incontrovertible evidence of life on Mars and, more seriously, by the failure to synthesize in the lab more complex prebiotic chemicals, including proteins and nucleic acids. Jacques Monod, Francis Crick, Fred Hoyle, Norman Horowitz, and Ernst Mayr were among those who spoke out against the consensus view of astrobiologists. But more recently, the pendulum has swung again in favor of the idea that a "life principle" amy operate in the universe tending to produce increasingly complex organic systems from simple beginnings wherever conditions remotely allow it. This is one of the more spectacular suggestions arising from complexity theory.

 

There was also, and continues to be, disagreement over how different extraterrestrial life is likely to be from the varieties found on Earth (see extraterrestrial life, variety). Again, the dispute centers on the relative importance attributed to factors of chance and "contingency" on the one hand, and necessity and determinism on the other. How far up the ladder of life do significant differences begin to appear between the life-forms that have evolved on one world and those that have evolved elsewhere? The commonality in space of basic organic chemicals such as amino acids suggests that they and the larger units into which they polymerize, including proteins, may be general ingredients of life throughout the Universe. The phenomenon of convergence also argues in favor of cosmic similarities between life-forms. Set against this, however, is the vast morphological diversity of organisms on Earth and the major, unpredictable effects that random cosmic events such as asteroid impacts can have on the course of evolution (see cosmic collisions, biological effects). These factors suggest that, over all of space and time, life must be almost unimaginably diverse.