All afternoon I have been watching a pair of hummingbirds play about our porch. They live somewhere nearby, though I haven’t found their nest. They are attracted to our hummingbird feeder, which we keep full of sugar water.
What perfect little machines they are! No other bird can perform their tricks of flight—flying backward, hovering in place. Zip. Zip. From perch to perch in a blur of iridescence.
If you want a symbol of freedom, the hummingbird is it. Exuberant. Unpredictable. A streak of pure fun. It is the speed, of course, that gives the impression of perfect spontaneity. The bird can perform a dozen intricate maneuvers more quickly than I can turn my head.
Is the hummingbird’s apparent freedom illusory, a biochemically determined response to stimuli from the environment? Or is the hummingbird’s flight what it seems to be, willful and unpredictable? If I can answer that question, I will be learning as much about myself as about the hummingbird.
So I watch. And I consider what I know of biochemistry.
The hummingbird is awash in signals from its environment— visual, olfactory, auditory and tactile cues that it processes and responds to with lightning speed.
How does it do it? Proteins, mostly.
Every cell of the hummingbird’s body is a buzzing conversation of proteins, each protein a chain of hundreds of amino acids folded into a complex shape like a piece of a three-dimensional jigsaw puzzle. The shapes are as various as the words of a human vocabulary.
An odor molecule from a blossom, for example, binds to a protein receptor on a cell membrane of the hummingbird’s olfactory organ — like a jigsaw-puzzle piece with its neighbor. This causes the receptor molecule to change that part of its shape that extends inside the cell. Another protein now binds with the new configuration of the receptor and changes its own shape. And so on, in a sequence of shape-shifting and binding—called a signal-transduction cascade — until the hummingbird’s brain “experiences” the odor.
Now appropriate signals must be sent from the brain to the body —ion flows established along neural axons, synapses activated. Wing muscles must respond to direct the hummingbird to the source of nourishment. Tens of thousands of proteins in a myriad of cells talk to each other, each protein genetically prefigured by the hummingbird’s DNA to carry on its conversation in a particular part of the body. All of this happens continuously and so quickly that to my eye the bird’s movements are a blur.
Molecular biologists have pretty much solved the problem of how genes make proteins. They are now embarked upon the far more challenging task of deciphering the language of proteins — compiling the dictionary of shapes and the grammar of shape-shifting that lets the hummingbird respond to signals from its environment. There is much left to learn. But this much is clear: There is no ghost in the machine, no hummingbird pilot making moment by moment decisions out of the whiffy stuff of spirit. Every detail of the hummingbird’s apparently willful flight is biochemistry.
How much of this applies to my own actions? All of it.
Between myself and the hummingbird there is a difference of complexity, but not of kind; this is the firm conclusion of contemporary biology, and certainly the most important scientific discovery of the 20th century. The book of human freedom may be The Complete Works of Shakespeare compared to the hummingbird’s Harry Potter, but the chemical vocabulary and grammar are the same.
Of course, complexity is not without consequence. The human brain contains about 100 billion neurons, and each neuron has approximately 1,000 synaptic contacts with other neurons, a web of interconnectivity exceeding that of any other creature. If we humans have assumed the role of lords of nature, it is because of the unparalleled tangle of neurons that sits atop our spines.
The brain of a simple organism like a worm is hardwired; that is, the neural connections are genetically determined and much the same from worm to worm. It is difficult to speak of a worm self. By contrast, the brains of more complex animals are partly hardwired by genes and partly wire themselves. As a human brain develops, cells move to locations that are only loosely specified by genes. The migration of any particular cell is dependent upon the cells it moves past and by local hormones those cells secrete, which in turn depend upon an individual’s past experience. Every human brain is continuously engaged in the construction of a self.
Most of this chemical commerce takes place unconsciously, but our conscious brains are alert to some mental states. As biologist Ursula Goodenough writes: “We are spectators to our own awareness.” It is difficult to say to what extent we share this characteristic with other species. Self-awareness appears to have originated with the great apes. Certainly, it has its most spectacular development in Homo sapiens.
But to reiterate: Between the worm, the hummingbird and the human there is a biochemical continuum, no difference that is not a consequence of complexity.
What does this mean about human freedom? If we are self-programming biochemical machines in interaction with our environments, in what sense can we be said to be free? What happens to “free will”?
Trying to escape the bugaboo of determinism, some commentators — such as the mathematical physicist Roger Penrose — have looked for the source of human freedom in quantum indeterminacy, the intrinsic stochastic skitter of subatomic particles. But there is no evidence that quantum randomness plays any role in biochemical reactions. Which is just as well, since few of us want to think our much-vaunted freedom is merely quantum noise.
A more satisfying place to look for free will is in what is sometimes called chaos theory. In sufficiently complex systems with many feedback loops—the global economy, the weather, the human nervous system—small perturbations can lead to unpredictable large-scale consequences, though every part of the system is individually deterministic. This has sometimes been called—somewhat facetiously —the butterfly effect: A butterfly flaps its wings in China and triggers a cascade of events that results in a snowstorm in Chicago. Chaos theory has taught us that determinism does not imply predictability.
An example: Photons of light and odor molecules from a piece of candy stimulate neurons in my optical and olfactory organs. Signal-transduction cascades inform my brain. Mmm, candy! Do I pick it up? Do I put it in my mouth? My action depends not only upon the external stimuli and my genetically inborn taste for sweets but also upon prior experiences and anticipations of future consequences as recorded in the soft-wired sections of my brain. I pick up the candy or I do not, depending upon a hugely complex—and to an outside observer unpredictable—conversation of molecules. This is not what traditional philosophers meant by free will, but is indistinguishable from what traditional philosophers meant by free will, i.e., the power to make free choices unconstrained by external agencies. If it walks like a duck and quacks like a duck, it’s a duck. When all is said and done, free will is a social construct, not a scientific hypothesis. Humans long ago discovered that living peaceably in groups required a notion of individual responsibility. Responsibility implies freedom. In contemporary society, it is the judicial system that ultimately decides to what extent our actions are “free.” The defense “my genes made me do it” probably won’t help in a court of law, but a claim of mental impairment might. Both assertions of diminished responsibility are at root biochemical. Society negotiates responsibility. Science delves.
I watch the hummingbirds at the feeder. Their hearts beat 10 times faster than a human’s. They have the highest metabolic rate of any animal, a dozen times higher than a pigeon, a hundred times higher than an elephant. Hummingbirds live at the edge of what is biologically possible, and it’s that, the fierce intenseness of their aliveness, that makes them appear so exuberantly free.
But there are no metaphysical pilots in these little flying machines. The machines are the pilots. You give me carbon, oxygen, hydrogen, nitrogen and a few billion years of evolution, and I’ll give you a bird that burns like a luminous flame. The miracle of the hummingbird’s freedom was built into the universe from the first moment of creation.
Chet Raymo is a science columnist for The Boston Globe. His most recent book is The Path: A One-Mile Walk Through the Universe.