The animals of our planet walk on two legs, or four legs, or six, or even 750 for that matter. So why don’t any animals have three?

While truly three-legged animals don’t exist in nature, various creatures rest in a “tripod stance”, placing their weight on two legs plus a tail or beak. Meerkats perching upright to assess their surroundings rest on their hind legs and tail, and kangaroos prop themselves up with their muscular tails as they feed. Meanwhile, woodpeckers brace themselves with tail feathers when perched precariously on a tree trunk, and parrots climb using their two feet and beak as a trio of grippers. Even fish rest on three appendages; the aptly named tripod fish rests on the ocean floor using three spines – two protruding from its fins and one from its tail.

We know that legs can come in sets of threes, as insects have a total of six. As they walk, three legs are kept on the ground (two on one side and one on the other side) while the remaining three step forward. This mode of walking is known as the alternative tripod method.

Being positioned in a tripod is a fairly effective way of balancing and doesn’t cost the animal any extra energy, so why did three feet never evolve? In a piece published in the journal BioEssays, Tracy Thomson, a graduate student at the University of California, Davis, discusses the conundrum, noting that the explanation is likely rooted deep in our evolution.   

"If we're trying to understand evolution as a process we need to understand what it can and can't do," Thomson said in a statement. "Almost all animals are bilateral."

This means that the vast majority of species seem to have two symmetrical sides; we have an arm and leg on each side, fish have symmetrical fins on both sides of their bodies, and spiders have four legs on either side, for example. Thomson argues that being bilateral, when it comes to limbs at least, became rooted in our DNA very early on in the tree of life, potentially even before limbs and flippers evolved. After all, three-legged fossils are non-existent in the fossil record.

“The strongest constraint on the evolution of a triped is phylogenetic: namely, the early genetic adoption of a bilaterally symmetrical body plan occurring before the advent of limbs,” Thomson writes. “Presumably, this would greatly constrain any three?legged animal from ever-evolving.”

The occasional genetic mutation has led to the development of three legs in a few individuals. For example, there were reports of a three-legged duckling being born in China in 2008. A third leg has even been known to arise in humans, for example in Italian-American sideshow performer Frank Lentini who lived between 1889 and 1966. However, rather than a genetic mutation, Lentini’s extra limb was the result of a parasitic twin.

It seems evolution made way for a variety of leg numbers, but thanks to our symmetrical bodies, three just didn’t make the cut.