New Delhi: The answer to why there aren’t any animals with three legs may be at the core of animal evolution, a new essay argues. Tracy Thomson, graduate student in the earth and planetary sciences department at the University of California, Davis, has been pondering the non-existence of tripeds. Thomson got the idea after taking a graduate class on evolution with paleontologist Geerat Vermeij, who challenged the students to come up with a “forbidden phenotype:” an animal or plant that does not and cannot exist.
Thomson points out that there are lots of animals that use a tripod stance to rest. Meerkats in an upright stance rest on their tail and rear feet; woodpeckers use tail feathers to brace themselves against a tree-trunk. A tripod stance does not require any energy to be stable, Thomson notes. Unlike, for example, standing upright on two feet, which does require some muscle work as well as relatively large feet.
Three-limbed movement is less common. Insects, which have six legs, have a mode of movement where their legs move in sets of three: two legs on one side and one on the opposite side are on the ground, with the opposite legs moving, at any time. This is called the “alternating tripod” gait.
“Almost all animals are bilateral,” he says. The code for having two sides to everything seems to have got embedded in our DNA very early in the evolution of life—perhaps before appendages like legs, fins, or flippers even evolved. Once that trait for bilateral symmetry was baked in, it was hard to change.