Friday, June 26, 2015

Eight-Legged Envy

This morning, on my way to my car, I walked through a gargantuan spiderweb. A diligent arachnid had been hard at work, industriously spinning his beautiful web, only to have some bumbling human destroy it in one fell swoop.

Of course I had my hands full, since these incidents never occur when one is unencumbered. I tried swiping the web from my face, only to realize its owner had conveniently hopped aboard my person. I suddenly became aware of a chunky spider the size of a malt ball taking a leisurely stroll down my arm.

Despite the burden of my computer, a coffee mug, and my purse, I managed to flail my limbs with enough vigor to dislodge him. He gracefully sailed down his web, landing gently at my feet and then scampering off into the undergrowth. After giving myself a thorough rubdown to ensure I wasn’t toting a giant egg sack on my back, I gathered my belongings and went on my way.

I’m not particularly afraid of spiders. I hold them in the same regard I hold snakes: cautious respect and deep admiration for the way they ambulate. Image what humans would be like with eight legs instead of two? It would probably render automobiles obsolete.

My eight-legged encounter got me thinking about our own mode of locomotion. In the animal kingdom, walking on two legs (bipedalism) is pretty unique. Only two other bipeds readily come to mind – penguins and kangaroos, both of which have devised their own strategies for getting around. Penguins have sacrificed efficient walking for swimming, and kangaroos took to hopping, which sure beats walking across the Australian bush.

So why did humans evolve such an unusual gait? Perhaps we should first ask, "when?"

Ancient fossils are hard to come by. The older they are, the less chance they have of being preserved intact. But there are clues to bipedalism among the fragmented remains of our earliest ancestors, and some of the best evidence has nothing to do with legs.

The hole in the base of the skull where our spine enters is called the foramen magnum. And it’s the position of this hole that provides a clue to upright walking. When it’s oriented at the base of the skull, it shows that the creature stood upright. If the hole is located toward the back of the skull, it indicates a quadruped (think about your dog or cat).

And it turns out our bipedal gait evolved much earlier than once believed. It was once thought that walking on two legs evolved in concert with our large brains. But what we find in the fossil record is that bipedalism was in place millions of years before our big brains arrived on scene. Even the seven million-year-old Sahelanthropus tchadensis, unearthed by a group of French paleoanthropologists in 2001, is believed to have been bipedal, based on his foramen magnum. Although scientists are still quibbling…

But the bigger question is why? Why did humans switch from four to two legs?

Theories of bipedalism go all the way back to Darwin, who believed the freeing of our arms allowed us to concentrate on the production of tools and weapons. This makes sense until you take into account that stone tools don’t show up until many millions of years after we started scooting around on two legs.

Others believe climate change had a hand in it. Perhaps humans took to walking as their forests were reduced and food became harder to come by, prompting males and females to partner up for provisioning. Males could gather food (in their arms, of course), and provide for their female and offspring, which would cement their bond and benefit both parties.

Or perhaps the reduction in forests required our ancestors to traverse longer distances. Walking upright, or better yet, running, has been shown to be more energy efficient than the knuckle-walking of our primate cousins, and there’s a whole new line of research examining the role running may have played in the evolution of humans.

Whatever the reason, we humans wouldn’t be human without our unusual gait. Sure, spiders have it made, what with their eight legs and their ability to walk on water. But it’s hard to imagine how humans could have accomplished all we accomplished if we were still ambling about on all fours. Stone tools, pottery, weapons, and art would have been quite a challenge without free hands, as would carrying, whether it be food, firewood, or children.

So you can keep your eight legs, Mr. Spider, and I’ll stick with my two. Your arachnid abilities may grace you with unusual gifts, but it only takes one of my two feet to squash you like a pancake.

Post Script - I would never dream of stepping on a spider...

In case you didn't get enough.