A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why has no animal species ever evolved wheels?

May 15, 1998

Dear Cecil:

I've just begun reading your latest book, The Straight Dope Tells All. On page 2, while ruminating on the subject of "questions … that give you pause," you write, "The other day someone writes in and says, 'If making a robot limb is so hard but other types of machines are easy, how come no animal species has ever evolved wheels?' Had to think about that for a while." You then go on to answer a completely unrelated question. Could I trouble you to answer the question you were asked?

Cecil replies:

At first I didn't answer this question because I thought it was too, you know, out there. Then I thought, 25 years I've been doing this, and I'm worrying about being out there now?

For starters let's concede the original writer's premise. Robot limbs that faithfully duplicate animal motion are a design nightmare. Coordinating multiple limbs, maintaining balance — if you think that's easy, try it after you've had a few brewskis. Remember the walking war machines in the Star Wars movies? One reason they were so striking is that they had no real-world counterpart. (That is, we don't have real walking machines. I know some of the Star Wars machines were based on real animals.) We use wheels for our vehicles because rolling is so simple. Why doesn't nature?

You might say: Because it's impossible. How would such a wheel evolve? Many intermediate steps would be required, but until the proto-wheel became functional (semi-techie talk coming up here), it would be useless baggage offering no selective advantage.

Just one problem. Some critters already have evolved wheels, sort of. Take the mother-of-pearl moth, Pleurotya ruralis. While in the larval stage, this bug is generally content to amble along in the we'll-get-there-when-we-get-there manner of all caterpillars. However, when sufficiently startled, P. ruralis hoists itself into a wheel shape and rolls out of harm's way — up to five full revolutions at 40 times its normal walking speed. (OK, so I previously denied there were hoop snakes. Who said anything about hoop caterpillars?)

You're not impressed. "Armadillos, tumbleweeds, freaking rocks roll," you say. "What I want to see is a creature with a wheel and axle."

Coming right up. The bacterium Escherichia coli, among others, moves by spinning whiplike filaments called flagella like tiny propellers. The typical flagellum is rotated up to several hundred times per second by what is basically an organic electric motor. We know it spins (rather than, say, twisting back and forth like a washing machine agitator) because researchers glued down an E. coli flagellum and the critter's body spun around like an eggbeater. If this thing isn't a wheel, it's pretty darn close. For an illustration, see www.arn.org/docs/mm/flag_labels.jpg. (This phenomenon is often used to make an argument about intelligent design to which Cecil doesn't subscribe, but our interest here is in the illustration, not the argument.)

You object: "Who cares about a germ? A more complex creature couldn't evolve the wheel. Every time the thing turned, the nerves and blood vessels serving it would get hopelessly twisted." Evolutionary biologist Stephen Jay Gould makes essentially this argument in his book Hen's Teeth and Horse's Toes.

But this may not be an insurmountable obstacle. A flesh-and-blood wheel might use the umbilical hookup found on some merry-go-rounds. Tape one end of a piece of ribbon to a tabletop and the other to the bottom of a compact disc. Turn the CD over so that the ribbon drapes over the side. Now move the CD so that it "orbits" the ribbon clockwise, at the same time rotating the disc clockwise, two rotations per orbit. (Not the easiest thing to explain without diagrams, but think of it as an IQ test.) The wheel turns, but the ribbon doesn't twist. Would it be easy for a living wheel to evolve something along these lines? Maybe not, but who's to say it's impossible?

"I give up," you say. "Why didn't animals evolve wheels?" Best guess: no interstates. Wheels are fine if you've got roads but next to useless on rough terrain. For quick starts, stops, turns, climbing, etc., legs are hard to beat. (For more, see McGeer, "Principles of Walking and Running," in Advances in Comparative and Environmental Physiology, volume 11, 1992.) We've got plenty of roads now, though, and natural selection presumably continues apace, for us as well as our forest friends. Aeons hence, who knows? There may be a whole new meaning for the expression, "Hey, nice wheels."

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