Edward Norton Lorenz died on April 16, 2008. Lorenz has been called "The father of Chaos Theory," and it was he who delivered the paper, "Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas" at a AAAS Conference thereby creating the conditions for the phrase "The Butterfly Effect." It helped that a graphing of the "Lorenz Attractor" looked sufficiently like a butterfly.
I myself used Lorenz's butterfly image when describing a storm in SunSmoke, not realizing that I was just ahead of an avalanche of such usages. It wasn't a cliché when I used it in 1983, honest.
It's also been noted that Ray Bradbury used a crushed butterfly to set off all the change-the-past stuff in "The Sound of Thunder." From a scientific point of view, Bradbury was far too conservative. He had meddling in the Jurassic merely change human history; it could have erased human history entirely.
People have as much trouble with chaos theory as they do with quantum mechanics and parallel worlds. There is a tendency to underestimate the effects, to bring them down to human scale, for example. But the point is that small changes in initial conditions can have, under certain circumstance, large changes in outcome. It's also important to understand that this isn't always the case. Not all systems are chaotic.
Suppose you have a very round ball bearing and a very smooth surface. Drop the bearing straight down onto the surface and you can be pretty sure that once it stops bouncing, it is going to be very close to where it first hit the surface. If there is a depression in the surface, you can be even more certain. The bearing is going to wind up at the bottom of the depression.
Now put another ball bearing down below the first, and drop the one onto the top of the other, as best you can. Where will it wind up?
You can be pretty sure that you aren't going to get two ball bearings stacked onto each other. Past that, well, it's anybody's guess, and guess is the operative word. Conservation of momentum says that the two bearings will ultimately be on opposite sides of your starting point, but they could be very far apart if there isn't much friction in the system. The smallest offset between the centers of the two ball bearings get multiplied very quickly by the bouncing.
Multiply this situation by a few dozen orders of magnitude and you have atoms colliding in a liquid or gas. Look at the system in fine enough detail and you can see "Brownian movement," the effect of bunches of atoms randomly hitting one or the other side of something preferentially for brief periods of time.
In truly chaotic systems, like those showing fluid turbulence, the small effects can magnify as time progresses, and produce major, macroscale phenomena. It's not just the butterfly wing that can set off the tornado, Brownian movement can also. So can a single quantum fluctuation, the radioactive decay of a single atom, the ionization shower from a single cosmic ray, the heating of a single molecule by a single solar photon.
Or maybe not. Sometimes things do cancel out, perhaps. We don't have access to all those alternate quantum universes, so we don't know how many there are, nor do we know how different they would have to be to no longer be here. Identity is a slippery thing, after all.
But weather is chaotic, so all possible weather events probably happen in the Great Beyond. Read any history and count the number of times when weather played a big role in the life of a nation, a people, or just individuals. Crops fail, and famine is a chaotic event.
War is chaotic, of course. Every soldier is a fatalist, knowing that the difference between life and death is often a matter of seconds, or inches, or a single random impulse. Plagues are chaotic, with disease vectors jumping around (literally sometimes) like fleas.
That's three of the Four Horsemen. The fourth one is Death, and he looks like Chaos to me.
But Life is also chaotic, even at the beginning. It's sometimes said that the fastest sperm gets to fertilize the egg, but in fact, it takes a mass of sperm, containing enzymes that break down what is called the "zona pellucida" to allow a single sperm to get through. So it's more like "We're taking the 3,887,996 caller."
Every conception is a random throw of the dice. Every birth is a door from chaos into chaos. Every individual creates a myriad universes, just by existing.
Is that enough? I mean, what more do you want?