Signatures of History


There is a common misconception that the natural world is perfect — or at least that the creatures that occupy it are perfectly adapted to their life styles. This is not, in fact, true; organisms are rarely perfectly adapted to their ecological niches. The millions of imperfections of nature provide a huge body of evidence in support of evolution. This category of evidence could easily have been included above with Darwin's evidence, since he realized how competently nature demonstrated that all organisms are descended from ancestors which were adapted to a different life than their modern descendants.

For examples, we need go no further than our own skeleton. Our skeleton is obviously modified from one that was better fitted to quadrupedal locomotion than to our bipedal style. The modifications did not produce an ideal framework for locomotion on two feet instead of four, and we are intimately (and often painfully) acquainted with the result — the price we pay for bipedalism includes chronic lower back, knee, hip, ankle and foot problems. These are ailments from which our nearest cousin species, the chimpanzee, is largely free. His skeleton is still pretty much shaped in the older, quadrupedal style.

Chimp skeleton

So what are these structural anomalies that give us so much trouble? Let's use the knee as an example. If we use the chimp as a model for the older, ancestral skeletal style, we can see why he doesn't have knee problems. The knee is a joint between the upper and lower leg. The upper leg is formed around one large bone; the lower around two smaller bones.

The kneecap (patella) rests over the meeting point of these bones, held in place by small ridges of bone on the bottom of the femur (the thighbone) and by ligaments. The areas where bones would rub against each other are padded and lubricated by cartilage. In the chimp the angle of alignment between the upper and lower parts of the leg is straight, providing a sound design in which stress is pretty much equal in both directions.

Human skeleton

But in order for humans to walk upright, the feet had to move to a position underneath the center of balance. Otherwise humans would walk the way a chimp does when it waddles on two feet. This change in position of the feet was accomplished by changing the angle at which the femur hangs from the hip socket. In chimps, the femur hangs straight down; in humans the femur angles inward. The lower leg then hangs straight down from the knee.

Human knee

Thus the upper and lower parts of the human leg meet at an angle; the knee joint is crooked, and the stresses on the joint and on the patella are uneven

This makes dislocation much more common, and places a lot of stress on the cartilage cushioning, often resulting in abrasions and tears. This creates many of the problems we have with our knees. Not exactly a design to write home about.

Nature is riddled with millions of these legacies of the past. For example, baleen whales eat by filtering tiny life forms from the water. They don't have teeth. Instead, they have baleen, which is a filtering device. But baleen whale embryos develop a full set of teeth, which they then re-absorb. The embryo has no need for teeth; they are just a genetic remnant of an ancestor who did need teeth. Also, though no whale has functional back legs, many have the bones of the pelvis and even of the upper leg present, embedded in the muscle of their bodies. They serve no purpose — they are just evolutionary leftovers.

giraffe

Then there's the giraffe, with his long, long neck. Like all vertebrates, the giraffe has a nerve that connects his voice box to a particular nearby place on his spinal cord. And like the rest of us vertebrates, this nerve travels from the voice box, under the clavicle, and then to its connection point to the spinal cord.

The only problem is that, unlike the rest of us more ordinarily shaped vertebrates, the giraffe's clavicle is way, way down in its shoulder, while the voice box and the connection point are way, way up at the top of the neck.

So that nerve goes from the voice box, all the way down the neck, under the clavicle, and all the way back up the neck to its plug-in site.

Some signatures of history are behavioral. A lot of bird behavior is pretty hard wired, particularly some of the styles of mating display. The Blue Footed Booby is one of the Galapagos Island species. [In England, boobies and tits are birds ;^)] As is true for many kinds of birds, the males have to court females if they want to mate. When a male Blue Footed Booby wants to impress a female, he does a pretty little dance, all the while waving around a bit of grass or other nest building material to demonstrate what a good provider he is. When the female finally agrees to the match and they mate, they throw away the nest building material — and she lays her eggs on bare rock. Blue Footed Boobies don't make nests. Other species of boobies do, so the obvious conclusion we draw is that the Blue Footed Booby is descended from an ancestor which did make nests, and the mating display has persisted, even though the modern species has no need for nesting material.

The examples of this phenomenon would make a list hundreds of pages long. These odd and non-adaptive features constitute one of the most impressive bodies of evidence in favor of evolution.


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Updated 25 September 2004