Over centuries, comparative anatomists marveled at the complexity of form and function in animals. For example, nineteenth century anatomist Charles Bell believed that the “designed perfection of the human hand could only have a divine origin.” His contemporary, Richard Owens, proposed that the remarkably similar patterns in the hands and feet of animals as diverse as whales, dinosaurs, bats and birds fit “the plan of the Creator.” Charles Darwin suggested that a common ancestor generated this diversity of the descendants that shared variations on the original limb pattern.
The comparative anatomy of the limbs takes us only so far. What if animals lack “limbs?” Can they still be related to us? Anatomist Neil Shubin turned to paleontology in search of the common ancestor between our limbed cousins and the fish. He discovered a fossil flat-headed fish he called Tiktaalik, with fins bearing a primitive version of the bone pattern found in modern vertebrate limbs. This “found” link offers one of many associations between the limbed and the limbless, yet there are deeper connections than naked eyes can see.
Shubin pursues the origins of anatomical commonality beyond paleontology into the realm of the genetic controls that guide the development of fish fins and human hands. We take for granted that each hand comes with one thumb, and that the resting thumb points toward the body. Strangely, that doesn’t happen in some human limbs nor in the equivalent structures in some fish. The answer lies in the genes that establish the number and position of fingers and fin rays—genes subject to the same disruptive forces. Not only do humans and fish share profound genetic similarities in the formation of limbs, but so does the housefly when it comes to the mechanisms controlling wing development. In other words, we share these same genetic controls as “bugs,” suggesting a common ancestor.
Humans and other animals develop from segmented embryos. Our segments morph into vertebrae and their associated skin, nerves and muscles. Segments “know” their relative position in the embryo as they transform into the appropriate adult structures. The question arises, “How did our segments ‘know’ what they should become and how did typically human segments derive from the segments of our non-human, non-vertebrate ancestors?” It’s clear that vertebrates share the ancestral control mechanisms of invertebrates, because molecular geneticists swapped genes between the embryos of remote descendants of the ancient common ancestors and found that donor control mechanisms can cause the host organism to respond by producing the organ proper to the host species, not the donor species.
Although we and our embryonic segments look nothing like those of insects or jelly fish, we and those animals share the same genetic control mechanisms that position body parts. Shubin explains that it’s “like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down, and modified for eons.” It seems to the reader that Owen’s plan of the Creator remains intact after all these millennia, and is itself a sign of the God’s genius.
Speaking of recipes, Shubin provides the directions for extracting DNA from peas, chicken liver or other menu items, using ingredients, containers and machines found in most kitchens. Sounds like a wonderful home-school science project.
An excellent teacher conveys complex information in a form that is understandable, accurate and complete. Neil Shubin qualifies as a master among teachers as he integrates the wonders of geology, paleontology, zoology, anatomy, embryology, genetics, molecular biology and evolution in order to reveal your inner fish. You may view many of Dr. Shubin’s “Your Inner Fish” lectures on YouTube and PBS on-line.