Charles Darwin
The biggest name in biology, period.
Charles Darwin was an English naturalist who first put forward the theory of evolution due to natural selection, stating that the spectrum of differences seen in life on Earth were due to a slow accumulation of change over many generations, selected via environmental pressures. This theory, though it has been significantly fine-tuned since Darwin's day, is the cornerstone of modern biology.
Darwin was not the most interesting of children. By all accounts he was quite lazy, more interested with foxhunting than anything else. He flunked out of medical school because he couldn't stand the sight of blood. He eventually settled down to study theology for becoming a country parson, a job that would have given him the easy living that he wanted.
By 1831, Darwin was all set for a life of peace and obscurity when he received an invitation to join a two-year around the world voyage on the HMS Beagle as the captain's companion (not like that, or that; at the time, being captain was a very lonely job as they could not fraternize with the crew, and a companion was usually hired to provide a source of conversation). Captain FitzRoy was particularly keen to have a companion, since the previous master of the Beagle had committed suicide. During the trip, he sent back massive amounts of fossils and specimens, and filled journal after journal with the observations (especially those on the Galapagos Islands) of common traits that would eventually lead to his initial theories. Despite all this, the trip was not that pleasant for Darwin, as he spent most of his time on the water violently seasick, fell out with FitzRoy on several occasions (despite the arguments, they remained friends for years after the voyage) and contracted a debilitating disease. The disease in question was possibly Chagas disease, since Darwin studied the feeding habits of bloodsucking Asassin Bugs by letting them bite him. Whatever the illness was, it plagued him for the rest of him life.
It took Darwin about 20 years of work before he would publish his theory of evolution. He had completed several drafts of On the Origin of Species, and made arrangements for it to be published after his death, but was spurred to action when another scientist, Alfred Russel Wallace, having arrived at a very similar idea, wrote to him to ask his advice on it. Darwin forwarded Wallace's paper to Charles Lyell with a letter remarking of the similarities, "he could not have made a better short abstract! Even his terms now stand as heads of my chapters ... he does not say he wishes me to publish [Wallace's paper], but I shall, of course, at once write and offer to send to any journal." Distraught and distracted over the illness of his baby son, Darwin put the problem of assigning credit into the hands of his friends Charles Lyell and Joseph Hooker, who proposed that Wallace's essay be put forward in a joint publication with unpublished work by Darwin, highlighting the fact that Darwin had got there first, and presenting the article to the Linnean Society in May of 1859. When the reaction to the presentation and publication was muted, sparking no great controversy, Darwin proceeded with publication, and before 1859 was out, the first edition of On the Origin of Species was released.
The book was an instant bestseller, and debates over God, creation, science, ethics, the place of man, the meaning in life and other such philosophical concepts began almost immediately, continuing to this day. Darwin never actively joined in with the debate, leaving the fighting to his more pugnacious friends; T.H. Huxley was dubbed "Darwin's Bulldog" for his staunch defence of the theory, leading eventually to Richard Dawkins being dubbed "Darwin's Rottweiler" by some. Interestingly, the initial controversy over the theory had little to do with religion directly, and focused more on the revelation by Darwin that the green and pleasant scenes so familiar to the English country gentleman of the time were, in fact, vast battlefields where species and individuals were locked into an unending cycle of conflict; if this seems surprising to you, consider that the issue of the evolution of mankind as a species was barely touched on in Origin, that subject being tackled instead by a later volume, The Descent of Man (Not to be confused with The Ascent of Man, which evolved its title from that work). His name has also been associated with Social Darwinism, which is the application of his ideas to nations and the human race, notably advocated by his cousin, Sir Francis Galton, as well as Herbert Spencer.
Darwin would be absolutely disgusted by these Social Darwinists misinterpreting his work for use in Realpolitik, and was disgusted and horrified by the slave trade, as had been his grandfather Erasmus Darwin and wife Emma's grandfather, Josiah Wedgewood, both prominent abolitionists. He considered this interpretation impractical, and disagreed with it on scientific grounds, as well as moral ones. Although he analyzed the supposed negative effect that the "weak" caused by "propagating their kind," he cautioned that to allow hard reason to override sympathy would have an even worse effect: "A deterioration in the noblest part of our nature." He and Emma were first cousins - Darwin felt guilty about this because he believed it might have led to their children being weak, one daughter dying tragically of illness while very young. He is usually seen in fiction as an old man with a Badass Beard, but he wasn't always like that. The picture at the top of the page shows he was a bit of Mr. Fanservice in his younger days (think Nerds Are Sexy).
Oh, and Darwin was also an expert in pigeon breeding, orchids, earthworms and (of all things) sea barnacles.
Oh, and his ideas did not inspire either the Soviet Union or the Nazis - the Nazis banned his work (On the Origin of Species was one of the first books to be burned by them), and the Soviet denounced it as "bourgeois science", instead promoting a neo-Lamarckian view of evolution, which hampered their biological research for decades, until they had to abandon it (they also attempted something similar with Albert Einstein's theories, but thanks in large part to Vladimir Fock, it didn't stick).
Oh, and he did not recant on his death bed - the woman calling herself "Lady Hope" who claimed he had done so was not there during his last illness or any of his illnesses, according to Darwin's daughter, and indeed it is doubted that Darwin ever even met "Lady Hope".
The film Creation is a dramatisation of part of his life.