There isn’t any! Evolution is not “goal oriented,” and that’s the point. There is no goal to the evolutionary process, no sense of direction, no preference for a particular path or result, no intention—other than to choose what works under current conditions.
If a random genetic mutation causes a protein change that works in the context of the animal’s or plant’s structure, lifestyle, or environment, then that mutation tends to stick around. If the change the mutation creates works better than the existing protein regime, then the creature tends to live better, surpass what had come before, and the mutation becomes a platform to build on. If the change works just as well as the existing regime, the mutation might hang around for a while and eventually point the protein in a new direction, or not. If the change works poorly, the mutation might stay for a few generations as a recessive form of the gene and then—when enough ill effects have collected in a single organism—kill it off and foreclose its future lineage. This process does not require any kind of protein-coding genius, anticipation of effects, or foreknowledge about outcomes to render this judgment. It only needs the iron hand of survival as a criterion for accepting or rejecting the change in the context of the particular organism in its current environment.1
We humans are not ideal beings, not the products of any genius’s design or expectations. We are evolved apes, brothers to chimpanzees and bonobos, and second cousins to all other mammals. Our intelligence is of a different magnitude, a different order of complexity, but not a different kind from that of a chimp or a dog or an elephant. Are we the highest order of being on Earth? Well, yes, for now, and until we meet savants who shine brighter and who have come from other worlds. But that is not because we were chosen for our intelligence or beauty. We just ended up on the top of a steep and ragged heap.2
Theorists of creationism and intelligent design say that the development of a complex feature like an eyeball, all at once, with a lens to focus light, a retina to capture the image, and a visual cortex in the brain to interpret it—that such a cluster of developments happening simultaneously by random chance is impossible. And of course they are impossible—but that is not what evolution proposes.
Nothing in evolution happens all at once. Instead, the process is an accretion of small changes at the protein level, baby steps in molecular biology, tweaks to what has gone before. You start with a cell in which the chance mutation of a protein has yielded a fleck of material that happens to respond chemically to light waves. The reaction this enables might help a microbe move toward or away from sunlight. Then you build on that minor and almost overlooked capability over generations, with a random improvement here and a random innovation there: one might link those proteins together into a structure; another might differentiate the proteins to detect light in various wavelengths and so detect different colors. If you happen to develop from a single-celled microbe to a multi-celled organism, you might encircle those light-sensitive parts with a ring structure that channels the light waves, then add a surface that focuses them. And the brain that grows up in that creature might benefit by extending neural axons to detect and analyze the reactions in those light-sensing cells. These changes may all be happening simultaneously, developing different structures in parallel, but they do not come about all at one time in a single organism or even a single species.
Working this way, from a fleck of protein, you could, eventually, after a few million years, get an eyeball … or that microbe with its light-sensitive patch might encounter a period of drought, perish when its pond or puddle dries out, and never set the animal kingdom on the road to hawks and eagles that can spot a field mouse from a distance of a thousand feet.3
Of course there are many intermediate steps which are not improvements, even many that are backward and lethal. But they don’t survive compared to the steps that offer improvement. The “selection” part of “natural selection” is the criterion of what works. Mutations that don’t add to the system—or that detract from it—might hang around for a while and perhaps compete as a recessive form of the gene. But sooner or later, the recessives are weeded out—again through natural selection and by disabling or killing off their owners. What’s left, over time, are the improvements.
Evolution is no respecter of individuals. On the way to the best possible wing or eyeball, evolution will make many backward steps.4 And unless they confer a benefit on the individual, new mutations will either join the legion of genetic recessives in the species gene pool or die out completely. The fact that evolution can be destructive as well as constructive is shown by individuals in the current human population who suffer from eye defects, inflamed appendices, bone and joint defects, auto-immune diseases, and all sorts of other problems in this supposedly idealized human body.
To look at one body part or system and wonder how that intricate structure could have come about without careful and intelligent planning is like going into the desert, noticing a balanced rock on a ridgeline, or a natural bridge with an incredibly thin upper span, and wondering how nature could have carved such a delicate structure. It must have been planned, because nothing random like erosion with drops of water and grains of sand could leave such a perfectly poised sculpture. But the observer must remember that all he or she is seeing is the surviving member of a long line of trials and failures. For every balanced rock you see, there are thousands of rocks from that same ridge that have fallen over, smashed, and worn away, thousands of bridges that have collapsed into the chasm. You are looking at a snapshot in time from a process that’s been going on for millions of years. You only see the occasions when the forces of erosion and gravity came together in the right way and for a short while—in geological terms, for no more than an instant.
In the same way, when you look at a complex mechanism like the human eye or a bird’s wing, you’re only looking at the successful examples which have survived the myriad cullings of evolution and which proved beneficial to the organism. You don’t see the incidents of retinal disease, cornea deformation, and other defects that are constantly plaguing this perfectly complex system. And you don’t see the millennial creep of developments from the light-sensitive spot in a single-celled microbe, to the hex-combed eye structures of an insect or arachnid’s eye, to the familiar but less discriminating eyes and brains of fish, frogs, and reptiles, parallel to the differently evolved but similar eye structures of mollusks like octopi and squids, until you reach the really wondrous and effective eyes of latecomers like the dinosaurs, birds, and mammals.
A species or a family that evolved, say, on a planet with cloud cover so thick that light from its sun never reached the surface would find some other means of “seeing” and navigating through its world. It might develop cells with resonant crystals to detect radio waves, along with the structures to focus and enhance those signals. It might develop systems to detect heat in the infrared wave lengths, or high-energy waves in the x-ray and gamma ray frequencies. The rule governing such development would not be what was most elegant, or complex, or intelligent to provide for sight in these creatures, but what worked in the context of their environment. And if a family or a phylum of radio-seeing creatures had already evolved, it would be difficult for a new species of x-ray-seeing creatures to develop and compete with them unless the x-ray adaptation offered a distinct advantage.
No organic system or body part is perfect. None fits some pre-conceived, Platonic ideal that makes perfect sense. The sense we bring to the argument—that we are designed to fit this wondrous world—is a circular argument after the fact. We evolved to live on this world and to perceive it as wonderful. We will all understand this much better when we begin to voyage out into the universe, and not just to airless places like the Moon that we know are not for us. We’ll understand evolution better when we land on a near-perfect planet, with fresh air containing the right mix of nitrogen, oxygen, and trace gases, sunlight of the right color and intensity, gravity almost the same as Earth’s, something underfoot that looks like grass—and water with a consistent pH of 3 that burns the tongue right out of your mouth. So near and yet so far.5
Evolution is the most delicate process in the history of human science and the imagination. I find it endlessly fascinating and satisfying. Those who doubt its ability to create great complexity generally question the randomness of the changes on which evolution depends. Or they seek a straight-line development from one individual to the next over an unimaginably short time. Instead, they should focus on the criterion of selection: what works, survives; what hurts, dies. It’s as simple and as beautifully precise as that. And it’s the only way to create a creature that finds its current surrounding both pleasing and beautiful, sufficient and sustaining, wonderful and benevolent.
2. Some would question whether we humans are really smarter than dolphins, whales, or elephants—or do we just have physical improvements like thumbs and vocal cords that let us work and communicate better? And some would say we lose out to butterflies and birds on the scale of beauty—although our eyes and senses have evolved to detect and admire their kind of beauty in the first place and for a reason.
3. Why didn’t plants evolve eyes that see by sunlight, too? Probably because they didn’t need them. Many plants are sensitive to light, and will turn their stems and leaves to follow the sun—it’s called phototropism. But you don’t need an eyeball that produces a coherent image to do this. Evolution is remarkably efficient and doesn’t put eyeballs on elm trees, legs and arms on corn stalks that make their living by absorbing sunlight from a single patch of ground, or hair on the body parts of dolphins and whales that don’t need it to stay warm and protected.
4. The problem of deleterious and damaging effects in a single system is compounded by the fact that every organism is a complex of competing systems. All are developing at once and at different rates: nervous system, sensory systems, digestive system, skeletal and musculature systems. The tragedy comes when an organism with a very promising mutation in its eyeball is taken out by a lethal mutation in its digestive system or other body part. That’s why evolution is no respecter of individuals, and also why so many systems have been developed and improved many times over the course of life on Earth—such as the wings of pterodactyls, birds, and bats. Same function, same structural adaptations, but completely different family lineages.
5. We humans couldn’t have lived on the early Earth, either: wrong atmosphere without the foundation of the green plants to provide oxygen, wrong soil chemistry without an evolution of microbes to provide nourishment, wrong temperature without the stabilizing effects of greenhouse gases. We like the world into which we evolved, but we would hate the one in which our kind of life started.