Free as a bird

Originally published 17 April 2005

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 back­wards, 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. Shapes 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 shapeshift­ing 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.

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.

Between the hum­ming­bird and myself there is a dif­fer­ence of com­plex­i­ty, but not of kind. If humans are the lords of ter­res­tri­al cre­ation, it is because of the huge tan­gle of nerves that sits atop our spines.

So what does this mean about human free­dom? If we are bio­chem­i­cal machines in inter­ac­tion with our envi­ron­ments, in what sense can we be said to be free? What hap­pens to “free will”?

Per­haps the most 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.

Of course, this is not what philoso­phers tra­di­tion­al­ly meant by free will, but it is indis­tin­guish­able from what philoso­phers tra­di­tion­al­ly meant by free will. If it walks like a duck, and quacks like a duck, it’s a duck.

I watch the hum­ming­birds at the feed­er. Their hearts beat ten 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 hum­ming­bird’s free­dom was built into the uni­verse from the first moment of creation.