Duke researcher credits nature with inventing the wheel

What's seen as the greatest invention in human history may not have originated in our brains at all.

Mother Nature, not humans, created the wheel and stuck it in all sorts of places: in insects, in birds, and even in our own bodies.

That's the claim made by Adrian Bejan, professor of mechanical engineering at Duke University, in an article published last week in the American Journal of Physics.

"Beliefs we take for granted are shattered," he said. "The idea that nature did not invent the wheel happens to be wrong."

The study is another step in Bejan's long quest to find simple, unifying physical explanations to account for the complexities of biology.

In 1996, Bejan invented the idea of constructal law, which spells out, according to a single principle, what it means to survive.

The idea is this: For any system, animate or inanimate, to persist in time, it must evolve to move with greater ease.

Nature's wheel fits nicely into this theory. According to Bejan, all forms of animal locomotion - running, swimming and flying - can be predicted and unified by the underlying wheel.

"This is a physics story of what it means to be the fittest, to survive in nature," he said. "I do not have to know Darwinian order to predict these biological systems. ... Biology is a subchapter of science. Laws of physics are for everything."

Round and round we go

Every animal can be seen as a rolling body, said Bejan - as both a set of wheels and a vehicle to carry the weight of the animal.

The optimal balance of efforts is achieved by a type of motion called "falling forward" - otherwise known as rolling.

"The body falls forward, lifts itself up and falls again," Bejan said. "All motion is the cyclical movement of leaning forward and catching oneself."

For example, when people walk, they use energy to lift their bodies partially off the ground. As they take a step forward, their body weight "falls" forward with the leg. The reason people don't crash to the ground, Bejan said, is that the second leg swings around from behind, essentially catching the fall.

Each leg is analogous to the spoke of a wheel, said Bejan. Viewing legs in this way can explain why animals have them to begin with.

"The origin of skeleton comes from something extremely basic - something that people do not recognize as the wheel is, in fact, the best wheel."

Evolution of revolutions

The evolution of nature's wheel could have been predicted by viewing the changes made to the human-made wheel over time, Bejan said.

The first artificial wheels, often attributed to the Sumerians in about 3500 B.C., were solid disks. By around 2000 B.C., Egyptian chariots boasted the first spoked wheels.

Spokes were beneficial because they allowed stresses applied to the wheel to be distributed uniformly. The same advantages were won when animals moved to land, developing spokes (legs) of their own.

But not all numbers of legs are created equal.

The best wheel is the one with the fewest spokes, Bejan explained, as it can carry the same load using a lighter design.

And just as human-made wheels improved to contain fewer spokes, so too, over millions of years, did animals evolve with fewer legs.

A set of two legs is like a single wheel, and each wheel touches the ground at one contact point. As a result, a two-legged animal like a human is anchored to the ground at only one contact point, compared with, for instance, a 100-legged centipede's 50.

With fewer contact points tethering it down, a creature has the freedom to grow up. And nature favors height because it allows speedier travel.

"Taller runners run faster," said Bejan. "Winning swimmers tend to be taller because longer bodies in the water make taller waves.

"It was evolution toward the obvious."

Nature's best

The wheel-based story of animal movement is compelling for another reason: It can explain the mystery of our alleged imperfections.

It's a mystery of size, one that has puzzled biologists in many circles. In theory, said Bejan, bigger is better.

"The heart would have been better if it were larger," he said. "Blood vessels would be better if larger as well, so as to be less constricted."

But looking at each organ by itself is not the way to go about it, he said. Nature's goal is to create the best possible whole - not the best possible individual parts.

Reaching the optimum involves a tug-of-war between nature's competing interests. On one hand, an animal wants larger organs to be more efficient at doing their jobs. On the flip side, an animal wants to move more easily - meaning it wants its entire load lighter.

Nature solves this battle diplomatically. Animal sizes are not imperfections, but rather carefully constructed compromises.

"Biologists claim nature makes mistakes. When you examine organs in isolation, you might scratch your head," said Bejan. "But all of us are suitcases of the right size, not too large or too small."

The idea spreads

For almost 15 years, Bejan has been telling the tale of animal life from an unconventional perspective. Now, he is no longer alone.

The idea that life is about flow has spread to scientists around the globe. And just like the wheel he's described, it's gaining momentum.

Antonio Heitor Reis, an engineer at the University of Évora in Portugal, has adopted Bejan's theories of flow into his own research.

"We can see these laws everywhere in nature. I have applied [them] to predict changes in the human brain and in river basins," he said.

For others, the beauty of Bejan's work lies in its simplicity.

"For the first time ever, a theory unifying design in nature and design in engineering is proposed," said Sylvie Lorente, an engineer at the University of Toulouse in France.

"And it is so simple that one asks: Why didn't I think of it before?"