A hundred years ago this day in Sarajevo, disgruntled nationalist Gavrilo Princip fired a shot. An Archduke and his wife died, the world mourned and fulminated, and in a rash of misunderstanding and patriotic throes the nations of Europe went to war with each other, a war that in its calculated butchery exceeded all that came before it and changed the course of history. Even today the fields of Ypres and the current of the Marne call out to us and demand an explanation. How could a lowly nobody like Princip change everything?

When you read the story of the shots that led to World War 1, what strikes you is how staggering the gulf between cause and effect was, how little it takes for history to change, how utterly subject to accidental and unlikely events the fickle fortunes of men are. Reading the story of Princip and the Archduke, one sometimes gets the feeling of being no more than wood chips being cast adrift on the roaring river of history.

The dark comedy of the assassination of the Archduke and his wife is succinctly narrated in skeptic and writer Michael Shermer's highly readable book "The Believing Brain", and the story is as good an example of the roots of conspiracy theories as any other. It sheds light on human psychology and illuminates conspiracy theorizing in all scientific quarters, ranging from creationism to climate change denial.

Shermer recounts how, on that fateful day, six conspirators waited in the shadows to carry out their deed. When the Archduke's motorcade passed close by, the first two conspirators failed to take any shots because of the crowds and an inadequate line of sight. The next conspirator managed to throw a bomb at the Archduke's car but it simply bounced off and fell into the car behind. The two conspirators quietly disappeared while the third tried to commit suicide by ingesting cyanide but simply vomited and was captured by the police. Unlucky Princip and the other two insurgents gave up and sauntered away. Meanwhile the Archduke made it all the way to the city hall and gave a speech, expressing outrage to the mayor that he had just been subjected to an assassination attempt.

Since the Archduke had just expressed outrage at an attempted assassination, he should have known better than to drive back the same way he came. However it seems that only one of the generals in his entourage suggested taking an alternative route back. But in the heat of the moment, for some reason this timely advice was not communicated to the driver who decided to again drive back through the city center. While this was happening Princip had purportedly given up and was hanging around a bakery, maybe enjoying a pastry. However when he saw the car return on the same route the opportunity was too good to pass; more so since the transmission seemed to be jammed and the driver could not back up. The rest is very much history.

Even after Princip's arrest World War 1 was not foreordained. Nothing is. But as Barbara Tuchman recounts in her marvelous book "The Guns of August", an almost surreal comedy of errors and a mountain of human stupidity on the part of Europe's leaders and diplomats followed the Archduke's murder and led to the Great War. But part of Shermer's motive in recounting Princip's story is to illustrate the absurdity of most conspiracy theories. A lot of conspiracy theorists, including those who deny climate change or evolution, try to convince everyone of some grand machinations going on in the highest reaches of government/industry/secret syndicates that lead to reality being either hidden from the public or being shamelessly manipulated for nefarious ends. But Princip's story tells us how messy reality is; the assassination almost failed, and at every turn its success or failure depended on events that ultimately were a function as much of chance as anything else. Anyone who believes in well-oiled conspiracy theories flawlessly functioning in the dark has simply ignored the great role of historical contingency in the operation of human affairs and the natural world.

But the murder of the Archduke provides us with another valuable window into the fickle nature of history and the minds of conspiracy theorists. This window illuminates the fact that staggeringly important events can result from trivial causes. Even a relative nobody like Gavrilo Princip or Lee Harvey Oswald can change history because of the unpredictable effects of chance and circumstance. But the problem is that the human mind being what is, it looks for causal patterns that are as large as the effects they produce. We find it easy to accept the incalculably evil Nazis as the cause of World War 2 but find it hard to swallow the lowly Princip as the pivotal cause of World War 1. We find it even harder to accept the inconsequential Lee Harvey Oswald as the causal factor for the murder of the consequential John F Kennedy. In the face of disparate differences between cause and effect our mind resorts to what Shermer calls “patternicity” and “agenticity”. Since we believe that the agents responsible for historic effects should be as major as the events themselves, we start conjuring them up to soothe our psychology. So, since Oswald does not fit the right profile as an agent for JFK’s assassination we start invoking the CIA, the Cubans, the Mafia and LBJ as more plausible agents, even if the evidence implicating these entities is thinner than the other evidence. The pattern fits, but only in the comfortable confines of our minds.

It is this inability to grasp the disparities between cause and effect that leads to some of the most prominent conspiracy theories involving science, including climate change and evolution denial. For instance, consider some of the questions that both camps raise when confronted with the evidence: How can puny humans cause the global climate to change? How can “microevolution” be responsible for “macroevolution”? How can minor policies that we undertake today be useful for ameliorating the untoward influences of climate change tomorrow? Even when the mountain of evidence is monumental, conspiracy theorists will try to discredit the entire edifice based on tiny details. Transitional fossils? Too sparse to say anything about evolution. Melting of ice sheets? Too inconsequential to say anything about global climate change. Bacterial change (as demonstrated massively through the patient experiments of Richard Lenski)? Too minor to account for change in higher animals. Conspiracy theorists either cannot accept or actively deny the role of simple details in the larger picture, although the former trait is definitely widespread.

But the tiny details matter. Over the ten decades following that unfortunate day in June, it is science itself that has provided some of the answers to these conspiracy theorists. Not that the evidence will make most of them change their mind, but we have found for instance the sensitive dependence of natural phenomena on initial conditions, a finding which is at the heart of chaos theory; as demonstrated by the famous “butterfly effect”, even slight changes in initial conditions can lead to enormous changes in the outcome. There is an entire science of complex systems now devoted to such effects. Through physics we have also discovered the ultrasensitive dependence of the features of the known universe on the slightest differences in the values of the fundamental constants; change the strong force in nuclei by one percent and it may make the difference between a universe with or without life. As recounted in the recent book “The Butterfly Defect”, we have also realized the complex web of interdependencies between both natural and human events that globalization has stitched together; for instance a recent natural catastrophe in Hong Kong affected the shipping and distribution of a significant percentage of hard drives around the world because the major manufacturers of these drives happened to be located there. And none of these phenomena are really predictable; they are really the product of chance and contingency.

Science therefore has now provided at least some justification for what the human mind always suspected, but what some human minds refuse to believe; that small changes can lead to big changes, that these small changes can be random and unpredictable, that fickle accidents of history can affect both the human and the natural worlds. Today as we contemplate Princip’s actions and the rupturing of world affairs that followed, it is wise to also contemplate this web of interconnections and to use it as a bulwark against those who would deny its implications and instead try to foist their own deterministic prejudices upon its gossamer threads. As the old proverb goes, we now know and can even rationalize how for want of a horseshoe an entire kingdom can be lost. Whether we find this fact fascinating or heartbreaking, we need to accept it as a fact at the heart of reality itself.