By Chet
Raymo '58, '64Ph.D.
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.
(July 2003)
