More Biological Guessing: Walking on Four Legs

My last two posts were about “Discovering Ardi,” the oldest human fossil ever yet found.  And she walked on two legs!
    We know about evolution; the fossil and DNA evidence in support of Darwin is overwhelming.  But it’s hard to know the details.  So we make conjectures, based on sparse fossil evidence and present-day DNA.  Recently Svante Paabo has added much to what we know of Neanderthal humans by figuring out ways to construct a Neanderthal genome from ancient DNA.  The Neanderthals lived from a little more than 100,000 years ago to about 50,000 years ago.  That’s way long enough for DNA in Neanderthal fossils to deteriorate or become contaminated by the DNA of other creatures, including scientists who handle the fossils when they find them and put them on display.
    So when we contemplate millions of years of human evolution, we have lots of interesting skeletons to outline the big steps, but not enough to fill in all the tantalizing gaps.  This is one of the reasons why “Discovering Ardi” was so important and satisfying.  As I reported, Ardi presented some astonishing details that upset long held ideas about where we hominids first evolved and what the selection pressures were that produced our two-legged way of walking.
    Another such story has to do with the evolution, much longer ago, of four-legged walking.  Four-legged, land-going creatures evolved from lobe-finned fish: A friend of mine used to have a saltwater aquarium in his dining room, and in it he kept a very tiny lobe-finned fish.  It was amazing to watch that fish move around on the floor of the aquarium.  Its four fins were thick, blunt, and muscular, and it used them to “walk” just as dogs and cats use their legs.  The fish reminded me of a fat dachshund with no ears.
    As with hominids and their two-legged walking (see my posts for October 12 and 14), until recently we had to guess how four-legged walking evolved from four-lobe-finned navigation.  The guesses were excellent, convincing, and, as it turns out, wrong.  I’ll tell the story in my next post.