In an impressive evolutionary brainstorm, nature has come up with many solutions to the problem of terrestrial locomotion.
Snakes slither. Leeches ripple their bodies along the ground in an almost comically inefficient process. The South American pebble toad rolls away from predators. Then there’s the most familiar strategy: walking.
Even in that category, though, the evolutionary variety is impressive. There are two-, four-, six-, eight- and 10-legged creatures cavorting around the Earth. Millipede species have a few hundred legs — not quite as many as their hyperbolic name suggests, but still impressive. Some biologists refer to kangaroos as “functional monopods,” because of their hopping gait.
All this got me wondering why there is such variety in the number of legs animals possess, so I asked Boston University biological anthropologist Matt Cartmill. Turns out that modern legs evolved in two separate evolutionary events.
“In the Cambrian, creatures called lobopods used soft legs to move along the sea floor,” he said, referring to a geological period around 500 million years ago. “Lobopods likely gave rise to modern insects and arachnids. Mammals and other vertebrates trace their legs to different ancestors — marine vertebrates, which adapted their two pairs of fins to become legs when they dragged themselves onto land.”
This bit of evolutionary history goes a long way toward explaining the distribution of multi-pedality in the modern world. The body of a lobopod had a repeating series of mostly identical segments, each with its own pair of legs. Animals with this body arrangement are quite versatile in evolutionary terms.
Advertisement
As generations pass, they can add or remove segments — and, along with them, legs — in response to evolutionary pressures. As a result, the lobopods and their progeny gave rise to creepy-crawly creatures with a fascinating array of leg counts.
Six-legged insects are derived from lobopods, as are arachnids, even though their segmented bodies aren’t quite as apparent anymore. (If you’re skeptical, Norm Platnick, the curator of the new “Spiders Alive!” exhibit at the American Museum of Natural History in New York, points out that “Liphistiidae, a primitive family of spiders living in Southeast Asia, still have traces of segmentation on the outside of their bodies.”)
Despite this variation, bipedality and quadrupedality don’t work well for walkers of small stature. To help you understand why, Cartmill suggests that you attempt to balance a yardstick on its end in your palm. That’s pretty easy. Now try doing the same with a pencil. It’s much more difficult, because once a small object begins to teeter, it’s difficult to arrest its fall.
Advertisement
“Six-legged creatures walk with three feet on the ground at all times, keeping a stable triangle beneath them,” Cartmill explains.
There’s also something to be said for the eight-legged arachnid setup.
“In one word: versatility,” says Carlo Biancardi, a physiologist who studies octopedal locomotion at the University of Milan. “Eight-legged animals can move forward, backward and sideways with little loss in efficiency. They can negotiate a wide assortment of terrains, and they can turn in place better than other creatures. In addition, spiders can lose a pair of legs and easily survive.”
Dancing roaches
You’re probably wishing right now that you had a lot more legs. Unfortunately, mammals never had much chance of developing more than four legs, because our marine vertebrate ancestors — the ones who dragged themselves out of the sea and adapted their fins to become legs — had long since lost the repeating-segment body structure.
Advertisement
“Animals eventually found that they could derive real benefits from specializing their segments,” says Cartmill. “In fact, specialization is one of the great themes of evolution.”
Once our bodies developed heads and torsos, there was no opportunity to expand our leg count. After all, from where would your new legs spring? Your head? Your stomach? That’s an evolutionary bridge too far.
Since it’s much easier to lose legs than to gain them, we were stuck with a maximum of four legs, the number of fins that our marine ancestors brought from the sea.
That still leaves a big question: Why do we walk on two legs instead of four? It’s a question many scientists are still trying to answer.
“Many other creatures that run on two legs do so for speed,” says Cartmill. “It seems to be easier to coordinate two legs during a run than four.” Think of the basilisk, also known as the Jesus Christ lizard for its ability to run across water on its hind legs. Cockroaches also go bipedal when the lights suddenly turn on.
Advertisement
Unfortunately, that can’t explain human bipedalism. The human ancestors who learned to walk upright had short, stubby legs, and their musculature didn’t lend itself to speed. Early man was slow, no matter how many legs he used.
Evolutionary biologists have offered a wide variety of explanations for why we gave up on quadrupedalism, but none of them have been widely accepted.
Some believe that walking upright freed up the hands of humans to grab low-hanging fruit. Others believe that sexual monogamy led to bipedality. Under that theory, when our ancestors stopped battling constantly for females and settled down with that special proto-lady, they needed free hands to carry home food for the family — and maybe even flowers or little presents if proto-lady was miffed.
Palmer, a freelance writer based in New York, is a regular contributor to Slate.com’s Explainer column.
ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZLuiwMiopZqkX52yorjToWSsm5mau6Sxjp6tqKSlqbawusCrsGagmajBsL7YZp%2BepKCoeqbEz6WYoqZdrLW6eceupJqmo2K1osLEZquwp12hsqi%2FjmtnampfZYRwfpJonoOJcYSOhr%2BTkJasrJ%2Bnxm%2B006aj