Evolution in the Mirror
by Myra Saturen,
What do we have in common with fish, frogs and Tyrannosaurus Rex?
To celebrate International Darwin Day, Charles E. Rinehimer, NCC professor of biology, answered this question with a presentation, "Evolution in the Mirror: How Our Bodies Reflect the Past," at Northampton Community College (NCC) on February 11.
Today there are several ways we can find ourselves on the evolutionary tree of life: through fossils, genetics and comparing our bodies with those of other vertebrates. Rinehimer chose the third way to give vivid illustrations of how much we share with cats, flounder and fruit flies.
"All vertebrates have the same toolkit, the same master building genes," Rinehimer explained. These genes directed body parts to appear in an embryo in a fixed order, in precise places. All other vertebrates have the same genes and structural plan, but some genes are programmed to "turn off" in some beings. Tails, for example, appear on many animals. In humans, all that is left of a tail is a vestigial bone, the coccyx-with the exception of 150,000 infants a year born with a remnant of this relic.
Rinehimer showed how evolution can be traced through examining embryos. Female fish lay eggs in water, to be fertilized later by males. A membrane protects the embryo as it grows. Likewise, amphibian females shed eggs in water that is in easy access to males. The female gestates on land and water but must return to the water to give birth. Reptiles, on the other hand, do not have to venture into water; they reproduce internally and have a chorion, a fluid-filled sac that eliminates the need for a shell. "We can thank reptiles for sex," Rinehimer said. The fetuses float in a sac of embryonic liquid that prevents it from drying out. "Snakes, birds, crocodiles, turtles, and mammals use this protective shell, which evolved 375 million years ago," he said.
All vertebrate history recapitulates, visibly, over and over, in each animal's embryos.
Yolks, for instance, exist in all vertebrates. In fish, the yolks nourish the embryo. As animals have evolved, however, yolks have diminished in size and function. Human embryos have a yolk, but they are non-functional, and our genes do not express them. They remain an artifact from our ancestral past.
Vertebrate genes, while the same, express themselves in varied ways. In fish, genes direct the growth of pharyngeal slits into gills. In reptiles, these slits morph into the lower jaw. In birds, the slits become the larynx, the voice box. And in humans, they develop into the larynx and the bones of the inner ear. Among the world's people, about .15% bear a reminder of the pharyngeal slit: a tiny indentation near their ears.
Through an interactive "quiz," Rinehimer challenged students to name limb bones from a dog, an elephant, a mouse, and a Tyrannosaurus Rex. Each animal had identical bones, in identical order in the limb. Even a bird's wings correspond to a human's arms. A "sonic hedgehog gene" signals cells to form and place digits into a pattern. The same gene that causes paws to form on mice instructs fingers to form on our hands. Genes are "turned on and off," in different beings. In a few families, babies are born with six digits, the inhibiting gene having remained active. Rinehimer used this phenomenon to show how evolution works through natural and sexual selection. If six-fingered humans derive functional advantages from an extra digit, or other humans find this feature attractive in a prospective mate, the characteristic will slowly spread throughout humanity.
Scientists have transplanted sonic hedgehog genes from one part of an embryo to another, resulting in a mirror image of wing pairs on one insect.
Sometimes, Rinehimer showed, the same bone in the same order may be located in a slightly different place. The calcaneus, the human heel bone- a hock in a horse or dog-is positioned lower in humans, to enable them to walk on two legs.
Walking on two legs, however, has some disadvantages, Rinehimer pointed out. Eighty percent of Americans suffer back pain as a result of human spines having curves. "You can blame evolution," he said. "Human evolution had to use the tools in the toolkit." Femoral neck (usually referred to as hip bone) fractures, occur frequently because of human anatomy. "Evolution does not result in perfection because of its limited toolbox."
Do you ever get goose bumps while watching a scary movie? We have this mechanism in common with other vertebrates. Cats' and dogs' fur, for example, stands on end when the animals are frightened. When this happens, a gene is causing the skin to pull back, making the creature look larger and meaner. Humans have the same muscle. But our goose bumps will not scare away a lion. The muscle exists in humans, but no longer serves a purpose.
Human skin color is also a road map of our evolution, Rinehimer explained. Our absorption of vitamin D produced by ultraviolet light is a fragile balancing act. Too much, and skin can become cancerous; too little, and the body will lack enough calcium. Originally, humans, who evolved in Africa near the equator, developed melanin-rich skin as protection from the sun's strong rays. Over tens of thousands of years, emigrants to higher latitudes developed lighter skin, which allows more calcium absorption.
"We all come from the same background," he said. "We are all related to other humans and to other animals through evolution."
Rinehimer played video clips from Your Inner Fish, a PBS program by evolutionary biologist Neil Shubin.
So, what do we have in common with fish, frogs and Tyrannosaurus Rex? Everything! "The past is who we are," Rinehimer said.
The talk was sponsored by the Secular Student Alliance and the Science Club.