When I talk to Creationists, I generally run into a lot of ignorance about how we “prove” evolution to be true. I think a lot of them really do believe that we invented the whole thing because we found some rocks that look like fish. Young Earth Creationists are a particularly funny bunch because they get so hung up on things like carbon dating. By raising the objections they do, they prove their own ignorance of not only evolution, but the scientific method itself. Today, I want to share with you what I think is one of the neatest mechanisms in embryonic development, and show you how it points squarely to shared evolution between humans and other animals.
In the 1950s and 60s, biologists John Saunders and Edgar Zwilling discovered something extraordinary. In species as diverse as whales, birds, and humans, developing embryos contained two tiny patches of tissue that seem to control the development of the appendages, whether they were flippers or wings or hands. Remove the tissue and development stops. Take tissue from one side of a growing wing and put it on the other and you get a “double wing” that mirrored itself on each side.
This “zone of polarizing activity” — or ZPA — remained a mystery until the 1990s, when three separate laboratories worked collaboratively to discover a gene in flies that made one end of the body develop differently from the other. They named this gene “hedgehog.” Before long, a version of the same gene was found in chickens. The scientists named this gene “Sonic hedgehog,” after the video game character.
Once the chicken gene had been found, it became a rather easy matter to look for it in other creatures. And guess what… All animals with limbs have it. In the developing embryo of every limbed creature on earth, the Sonic hedgehog gene literally controls what goes where and how it goes there. The ZPA is actually a patch of tissue in which this particular gene is active, regardless of what kind of animal embryo it is.
We have since discovered that Sonic is one of several dozen genes that work in conjunction to make limbs, and in every creature that has limbs, the DNA recipe is virtually identical. This points squarely to a common ancestor for all limbed creatures, but even more than that, it points to a common ancestor even farther back linking limbed animals to flies. This genetic mechanism is truly ancient.
But it goes even farther. As we all know, Evolutionary theory states that life began in the water and moved onto land. And as it turns out, experiments with sharks and skates have backed up that claim. Obviously, fins are very dissimilar to limbs. Other than the fact that they protrude from the torso, there really isn’t much we can find to suggest that limbs and fins have anything at all in common.
Shark fins have tiny cartilage skeletal rods, and all of them look essentially alike. Just from looking at them, it’s very hard to think of any reason we should expect them to form the same way as bony human hands. But sometimes nature surprises us. When scientists placed a tiny bead of mouse Sonic hedgehog in between several of the embryonic skeletal rods, guess what happened.
Yep. Shark fingers.
What a crazy thing, don’t you think? A warm-blooded, four legged live-birthing mammal with fingers and toes, and a shark with not a single bone in its body (remember, sharks have cartilaginous skeletons!), and yet, the genetic recipe for embryonic development is so similar that you can literally plug one into the other and it will still work!
Flies, chickens, humans, and sharks. It would be hard to find four more dissimilar animals, and yet, all of them use the same biological mechanism to become what they are. Once evolution discovered that it could build appendages, it didn’t need to invent them again. Instead, descent with modification worked on the core process for billions of years, creating staggering diversity, from insects to birds to mammals. But it never needed to change the formula that worked. Sonic hedgehog is the proof.
SOURCE: Your Inner Fish, by Neil Shubin, paleontologist and professor of anatomy.