Answer: A lot. Most specifically, contingency (chance), and the implications of contingency upon meaning.
Prior to Charles Darwin there were a lot of 19th century people who believed that evolution must have occurred in some form, but they thought about evolution in theistic, deistic, or Platonic terms. In other words, evolution had occurred, but it was guided by telos (some sort of purpose).
But here’s the kicker.
Friedrich Nietzsche saw that Charles Darwin had landed upon a mechanism, natural selection, that accounted for evolution absent purpose. And Nietzsche saw that the metaphysical consequences of this discovery could not be more profound. Darwin, by putting an evolutionary mechanism in place of telos had irretrievably set human beings into the realm of the contingent. Darwin had divorced life—and by extension, humankind—from metaphysics and wedded it to history. Thus human morality was no more grounded in some transcendent purpose than any other natural phenomenon. Instead of tracing morality to heaven, Nietzsche traced it to earth—to the historically contingent (hence Nietzsche’s On the Geneology of Morals ). Here’s how one of Nietzsche’s seminal biographers, R.J. Hollingdale, put it in his now classic, and still much read and praised biography, Nietzsche: The Man and His Philosophy (Cambridge, 1965, rev. 1999, 72-73):
Darwin had shown that the higher animals and man could have evolved in just the way they did entirely by fortuitous variations in individuals. Natural selection was for Nietzsche essentially evolution freed from every metaphysical implication: before Darwin’s simple but fundamental discovery it had been difficult to deny that the world seemed to be following some course laid down by a directing agency; after it, the necessity for such a directing agency disappeared, and what seemed to be order could be explained as random change. ‘The total nature of the world,’ Nietzsche wrote in Die frohliche Wissenschaft, ‘is . . . to all eternity chaos’ (FW 109), and this thought, basic to his philosophy, arose directly from his interpretation of Darwin.
The implications of this were enormous. Hollingdale again (73):
Nothing that existed in the phenomenal world could have come from ‘outside': if the universe were intelligible, it must be intelligible from within. Part of that intelligibility had been expounded by Darwin: the ‘divine’ attributes of man had in reality descended to him from the animals. Man was in touch with no ‘beyond’, and was no different from any other creature. But, as God had been the meaning of the universe, so man had been the meaning of the earth. Now God and man, as hitherto understood, no longer existed. The universe and the earth were without meaning. The sense that meaning had evaporated was what seemed to escape those who welcomed Darwin as a benefactor of mankind. Nietzsche considered that evolution presented a correct picture of the world, but that it was a disastrous picture. His philosophy was an attempt to produce a new world-picture which took Darwinism into account but was not nullified by it.
And where did Nietzsche, in the light of Darwin, go to produce his “new world-picture”? Answer: pre-Socratic Greece. Nietzsche saw in pre-Socratic Greece a period in history comparable to the post-Darwinian late 19th century. Pre-Socratic Greece was a time prior to the Platonic idea of telos (just as the late 19th century was a time after the collapse of Platonic telos ). As Hollingdale puts it (74):
Philosophy from Plato onwards had been founded upon suppositions that were no longer valid, and to find philosophers who had faced their problems without presuppositions one had to go back to Heraclitus, Pythagoras, and Socrates.
And what did Nietzsche find there? Survival of the fittest and cruelty (74-75):
Not only did Nietzsche discover the nature of his dilemma among the Greeks, he also discovered the key to its solution. Long before he had formulated the theory of the will to power he had discovered that the driving force behind the culture of Hellas had been contest, agon, the striving to surpass. . . . The Greeks were cruel, savage, and predatory; yet they had become the most humane people of antiquity, the inventors of philosophy, science and tragedy, the first and finest European nation.
In short, concludes Hollingdale, it was Darwin, and his leading of Nietzsche to antique Greece, that led to mature Nietzschean philosophy (78):
Darwin and the Greeks, then, and not Wagner and Schopenhauer, were the starting-point of Nietzsche’s philosophy; and the reason his early works represent a false start is that he sought to interpret the Darwinian problem and the evidence of ancient Greece in the light of Wagnerian aesthetics and Schopenhaueran metaphysics. Not until he had put both behind him did he enter into his own proper field.
And what Nietzsche also put behind him, in his attempt to work out the consequences of Darwin’s discovery, was “slave morality” (Christianity) and democracy. As Thomas Nagel, in his essay on Nietzsche in his recent excellent collection of essays, puts it (37-38):
The conflict of perspectives and competing wills that is the true reality is obscured and flattened out by the social imposition of a common standpoint, in language, thought, morality, and politics, which presents itself as simply or cosmically true by concealing its true sources. The inquiry into the geneology of these ruling ideas is therefore a vital part of their unmasking. The proposed geneology of Christian morality, as the expression not of universal love but of the slave revolt of the base against the noble, motivated by fear, hatred, and envy, is Nietzsche’s most famous thesis, expounded in Beyond Good and Evil and On the Geneology of Morals. . . . He regarded modern morality, which speaks with the voice of the community or even of humanity as a whole, as particularly dangerous, because it requires suppression of the cruelty and recklessness that distinguishes the strong individual. The height of self-realization cannot be reached by someone who is too concerned with the reactions of others, or his effects on them. There is a fundamental conflict between the pursuit of individual creativity and perfection and the claims of the general welfare.
For this reason, Nietzsche was not a democrat. Already at the time of writing The Birth of Tragedy, he defended slavery as a condition of the possibility of great cultural achievement by the few, as in ancient Athens. And he defended its modern counterpart, the economic oppression of the masses, for the same reason. He opposed shortening the workday from twelve hours to eleven when it was proposed in Basel, he approved of child labor, and he opposed the educational groups for workers. When in 1871 he heard the false rumor that the Paris communards had pillaged the Louvre, he called it ‘the worst day of my life.’ Equality meant nothing to him; he believed it would inevitably push everything down to the lowest common denominator, that of the ‘democratic herd animal.’ Life, he insisted, is tragic; it is necessary to choose between justice and aesthetic perfection. And in his latest writings he expressed fantasies of annihilation, with ‘degenerates’ being got rid of to make room for the highest type of man.
So Charles Darwin’s survival of the fittest brought Nietzsche to antique Greece (prior to democracy), and thence to his notion of the lone and cruel Ubermensch, the “highest type of man”, the creative supremacist triumphant over the degenerate masses. Though Nietzsche might—and I emphasize might here—have regarded the Nazis as (in Nagel’s phrasing) “an extreme manifestation of the herd instinct” (39), it’s hardly surprising that the aesthetic loving and scientistic Nazis found in him—and in Charles Darwin—key pieces to the puzzle of their intellectual justification for being.