Imagine that an astronaut returns to Earth after a trip to deep space, and reports that he has discovered a planet whose inhabitants are so intelligent that — a long time ago — they succeeded in designing a very primitive car which carried its own completely automated automobile factory around in its trunk.
Every once in a while, the trunk opens and out pops a new little automobile, which slowly grows into a full-sized car, complete with its own fully automated auto factory in the trunk. The cars on this planet continue to reproduce, and make nearly perfect copies of themselves, generation after generation. The inhabitants don’t have to do anything but watch.
How many generations could this continue without any help from the intelligent inhabitants? Surely without mechanics to fix the problems that arise, errors would accumulate and put an end to this process after a few generations, you say. The duplication errors were actually beneficial in the long run, he replies.
Errors occasionally result in tiny improvements, and the slightly improved cars are more likely to survive and reproduce. So in the eons since the first primitive car was designed, the car fleet on this planet has actually made great advances, gradually evolving internal combustion engines to replace the early steam engines, and automatic transmissions to replace manual ones, and hydraulic brake systems to replace mechanical ones. Now they are even self-driving.
You say, I can’t imagine human technology ever advancing to the point that we could produce cars with fully automated auto factories inside, able to construct new cars — with fully automated auto factories inside them. And able to preserve this complex machinery generation after generation, resisting the natural tendency that plagues our simple cars to, in the absence of constant maintenance, degrade over time. The people on that planet must be much smarter than we are.
Well, the astronaut says, the first very primitive, small, self-reproducing car actually formed by pure chance, perhaps when an asteroid hit the planet a long time ago. There aren’t really any intelligent inhabitants on this planet. I just made up that part of the story because I was afraid no one would believe how the first car really formed. But once that first self-reproducing primitive car appeared, the evolution of much more advanced models was inevitable and easy to explain. And the appearance of that first primitive car was really a relatively small step in the whole process.
Perhaps your response to this story is: Some of the major changes required for primitive automobiles to evolve into modern self-driving cars are irreducibly complex — there is no way to gradually transition from steam engines to internal combustion engines, for example, without the development of new, but not yet useful, features.
If so, you have certainly spotted one major problem with the astronaut’s story. But there is a much simpler and more fundamental problem. If you can’t spot it, here are two videos that might help: