Is life too complex to have evolved without guidance or design? Proponents of Intelligent Design (ID) like Michael Behe and Kirk Cameron think so. They claim that even the simplest bacterium has so many interdependent parts -- like the gears and springs of a watch -- that it could not have sprung into being without the direct input of an intelligent creator.
At first glance, they seem to have a point. Deleting even a single gene in a developing fly embryo can be lethal to the fly. Changing just one or two letters out of 3 billion in the human genome can give rise to a variety of deadly diseases. If living things are so intricately balanced, how could they have evolved gradually, as evolutionary biologists claim? Wouldn't all of the parts have to spring into existence at the same time?
To answer this, consider the following analogy. Suppose a hunter-gatherer from the deep Amazon encounters an iPad. Opening its case to peer at the microelectronics inside, she is puzzled: who could design and build such intricate parts? Even when she finds out what each piece of silicon and metal is for, she is baffled. Nobody has small enough hands, or a large enough intellect, to build a microchip from scratch. They must have had the help of a computer -- but then, who built that one? She might conclude that the first computer must have been a gift from an ultra-intelligent, and perhaps divine, being.
Of course, our friend is overlooking history. As she investigates further, she learns that the predecessors of computers did not have microchips, but ran on manpower or steam. Engineers later capitalized on inventions from electronics and the materials industry, as well as centuries of advances in physics and mathematics. As computers became more complex, they helped people to design smaller, faster, and more complex computers, as well as the sophisticated microfabrication systems needed to build them. The first computers helped to improve the hardware, which in turn helped to build faster computers, and so on.
It's an imperfect analogy, of course. Unlike biology, even primitive computer technology is the product of intelligent designers driven by a set of goals. But that's giving modern computer engineers too much credit. The rise of the information age depended on the work of thousands of innovators spread over many centuries. Their ranks include mathematicians, physicists, and chemists who had no idea that their work would give rise to microprocessors, Google, or Twitter. In most cases, their goals had nothing to do with building a powerful computer, but were adapted for this purpose later.
Similarly, Biology has a goal: survival. To get an edge on their competitors, living things capitalize on whatever past innovations they carry in their genes. They re-purpose molecular bilge pumps for use as tiny propellers. They transform their ancestors' solar power plants into cameras. They even copy and build upon entire developmental blueprints from other living things, which is why it's so hard to tell the difference between the early embryos of humans and crocodiles.
Since evolution is blind, it can't predict whether an innovation will be successful. And it's usually not. Recall the supposed quote from Thomas Edison, "I have not failed; I've just found 10,000 ways that won't work." As long as a creature can sacrifice a little time and energy on mistakes -- and every genetic disorder could qualify as a mistake -- it will occasionally hit upon a great idea. Then comes the payoff.
In terms of our understanding of biology, we are like our fictitious hunter-gatherer: bright and curious, but almost completely ignorant of the past. To worsen matters, no one was around to document evolution during 99.99% of its history. If we want to understand how life arose and evolved, we have to take every little miraculous piece of biological machinery, and examine it. What are its genetics? What evolutionary path could it have taken? Are there any paths we can rule out? It takes a long time.
Understanding the natural world is tedious. But it's infinitely more satisfying than exclaiming "Impossible!" and walking away.