Author and Title
Shubin, Neil. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body
Sir Richard Owen
Sir Richard Owen noticed similarities between the limb bones of various species, before Darwin. The general pattern was one bone in the upper arm, two bones in the lower arm, several small bones in the wrist, then the bones of each of the fingers.
The Fins of Fish
The fish fin does not follow this pattern. Instead of one bone attaching to the shoulder, it has four or more parallel bones attaching to the shoulder. The fin of the lungfish also has several bones, but arranged serially, going away from the shoulder, rather than in parallel. Only one of these bones attaches to the shoulder. The fin of the fossil fish Eusthenopteron has one bone attaching at the shoulder, followed by two bones further out, followed by several more bones even further out.
The Wrist and Hand
Shubin, with his student Ted Daeschler and mentor Farish A. Jenkins, Jr., discovered the first fish with a wrist, the Tiktaalik, in the Canadian Arctic. Next in the course of evolution, after the Tiktaalik, came the amphibian Acanthostega fossil. It has one bone at the shoulder, followed by two bones after the elbow, followed by several digits in parallel.
Genetics of Asymmetry
The author also talks about the genetics of limb development in the embryo. The concentration gradient of the Sonic hedgehog (sic) protein influences the zone of polarizing activity (ZPA). The ZPA is what causes the hand to be asymmetrical, with the pinky and one side and the thumb at the other. The gene for Sonic hedgehog was discovered in 1980 by German scientist Christiane Nüsslein-Volhard and American scientist Eric Wieschaus. The relationship between the Sonic hedgehog protein and the ZPA was discovered by Robert D. Riddle, Cliff Tabin and colleagues at Harvard in the early 1990s.
The fact that invertebrate eyes, such as those of insects, and vertebrate eyes are so different has always posed a problem for theorists of evolution. Invertebrate eyes increase surface area by having many folds in their light-gathering tissue, while vertebrate eyes have bristle-like projections. Detlev Arendt discovered a marine annelid worm called the polychaete that has two kinds of eyes, an invertebrate eye and a vertebrate proto-eye. It has also been discovered that the gene, Pax 6, that controls the differentiation of tissue into eyes is very similar in invertebrates and vertebrates.
Fish have no middle-ear bones. Reptiles and amphibians have one middle-ear bone. Mammals have three ear bones (and a pinna). All of the middle-ear bones evolved from the curved gill arch bones of fish. The stapes (stirrup) evolved from the hyomandibula of the reptile, which connects the jaw to the skull. The hymandibula, in turn, evolved from the second gill arch bone of fish. The malleus (hammer) and incus (anvil) evolved from bones in the back of the reptilian jaw, which in turn evolved from the first gill arch bone.
Three percent of the mammalian genome is devoted to genes that code for receptor proteins that bind odor molecules. In primates with color vision, many of these genes have become non-functional. We have traded smell for vision.
The Hard Parts
For their hard parts, mollusk and crustacean invertebrates, use chitin and calcium carbonate. In vertebrates, hydroxyapatite is the mineral that gives bones and tooth enamel their hardness. Hydroxyapatite first appeared in teeth, not bones, which were then made of cartilage. The upper and lower teeth of mammals fit well together (occlusion), but this is not true of reptiles.