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CAVEMAN LOGIC

Prometheus Books

Chapter One

THE CRADLE OF LIFE

Human beings evolved in a world unlike the one we inhabit today. Our sense of history, as it is taught in high school and portrayed in films, doesn't begin to do justice to what our species has been through.

Going back two hundred years for an entertaining "historical drama" leaves us squarely in modern times, evolutionarily speaking. Other than different hair and clothing styles, that brief journey hasn't begun to move us out of the modern world. Even if we go back to the time of Jesus or, further yet, to Greek civilization, we're still in modern times. Gone are the trains, planes, automobiles, and iPods, but if we could magically visit these times and places, we'd still see recognizable humans acting in recognizable ways. That is why stories set in so-called ancient times still resonate with us today. Their settings are very recent.

Human beings may be a very new species in terms of life on Earth, but we still go back a long time. The hominids that would eventually become humans split away from a common ancestor, leaving the apes behind about 6 million years ago. Those early proto-humans went through a hell of a lot so you could download music and buyairline tickets online. The selection pressure on them to survive and reproduce was tremendous, and living conditions were harsh beyond our imaginations. The physical appearance of these early hominids was also different from what we take for granted today. Physical changes were evolving and accumulating slowly in both the minds and the bodies of our early ancestors.

Natural selection was in no hurry. On one hand, it is a ruthless efficiency expert, heartlessly excluding those features that do not maximize reproductive success. On the other hand, it can only select among existing alternatives. That slows things down. Natural selection can't cause giraffes to grow long necks just because the food supply happens to be located above their heads. But if a relatively long-necked individual does show up, he or she is more likely to experience reproductive success. Because that longer neck is coded in the individual's genetic material, and because that individual may be more successful than its short-necked competitors, long-necked giraffes may gradually become more numerous in the population.

Plainly, evolution through natural selection only works on traits that have a genetic basis. Heredity is the cornerstone of Darwinian theory. Without it, the effects of natural selection would be confined to a single generation. Every generation would be starting from scratch. If some trait (e.g., a snazzy new hairstyle) were acquired during an individual's lifetime, then natural selection could do nothing with it other than conferring some transient rewards (perhaps a few more sexual partners). Here on Earth, almost every important behavioral and physical feature of plant and animal life is transmitted genetically and is directly vulnerable to selection pressure. Thus, traits that lead to greater reproductive success eventually spread and often become standard equipment for a species.

Traits that meet environmental challenges and enhance reproductive success are known as adaptations. When the person carrying the genes for these traits reproduces, there is a chance the traits will be carried into the next generation. The final results also depend on the genome of the sexual partner. For all the pleasure it produces, sexual reproduction also carries some disadvantages. The most obvious is that each partner contributes only half of the new genome.

It makes sense that individuals do not mate indiscriminately. No one, whether a Nobel laureate or a tree frog, wants a substandard partner. All forms of animal life have evolved methods for "screening" potential mates in order to select those with the best genomes. Such screening need not be conscious; indeed, it rarely is, even among our own species. Human mating rituals, whether practiced at high school dances or in so-called primitive societies in the Amazon basin, are only one example of this. A zoologist could dazzle us with stories of how other animal species choose or attract partners. One of my favorites involves the gladiator frog (Hyla rosenbergi). As in most species, the female is very particular when it comes to choosing the father of her offspring. Since "toughness" is a desirable trait in this pugnacious species, the female practices a very simple screening technique. She approaches her potential mate and literally does her best to knock him off his feet. If she succeeds, that's one Mr. Froggie that won't get lucky with her.

Natural selection is not really a very contentious process despite the uninformed political debate that swirls around it. At its most basic level, natural selection simply means that heritable differences within a species lead to different levels of reproductive success. The most successful adaptations tend to spread in the population. That hardly seems like a difficult or dangerous idea. Dog breeders routinely take advantage of it, substituting their own preferences for the traits that nature might select.

According to most surveys, the majority of Americans do not "believe in" Darwinian evolution. This seems an unfathomable state of affairs in the twenty-first century. Obviously, someone has to understand a viewpoint before deciding whether to accept or reject it. I've talked to enough high school students and teachers to conclude that Darwinian evolution is neither well taught nor well understood. It is entirely possible that the view of Darwin many Americans reject would also be rejected by most scientists. It is simply wrong. The misunderstanding is so pervasive. Most people can't even tell you the name of Darwin's famous book. Ask someone and, if they know the book at all, they are likely to report Origin of the Species rather than On the Origin of Species. It is a subtle difference, but quite telling. Even Spencer Tracy, appearing as Clarence Darrow in the award-winning 1960 film Inherit the Wind, got it wrong.

At a time when debates over the teaching of evolution are turning neighbor against neighbor and throwing local school districts into chaos, it would seem criminal not to bring combatants to a common understanding. Then let them debate. This has plainly not happened and is a glaring fault of the American educational system. When students tell me (as they occasionally do in Introductory Psychology classes) that they do not accept Darwin, I usually ask them what they mean. I cannot recall a single occasion when such a forcefully opinionated student has gotten it right. The most frequent response is, "It means we come from monkeys." Who wouldn't reject that? Whether by incompetence or willful misrepresentation, natural selection is just not getting a good hearing.

Perhaps if people understood natural selection, they'd be more likely to accept it. The principle does not seem very threatening (although the wrongheaded version can be quite upsetting to many). Moreover, as Richard Dawkins argues, natural selection is inevitable once you accept a few basic premises. Susan Blackmore concludes in her book The Meme Machine, "If there is a replicator that makes imperfect copies of itself, only some of which survive, then evolution simply must occur.... The inevitabilty of evolution is part of what makes Darwin's insight so clever. All you need is the right starting conditions and evolution just has to happen."

So what conditions must be in place for this inevitable process to occur? For one thing, we are all replicators. Human beings replicate sexually. Some anti-Darwinians may be uncomfortable acknowledging that process, but it is nonetheless true. I have never heard an anti-Darwinian debate heredity. Most, if not all, accept that we pass along our genetic information, which is then combined with our partner's contribution to form a new organism. That is not contentious. The fact that our offspring are composed of a mixture of maternal and paternal DNA is also not contentious. Every gene you carry came from either your mother or your father. No one debates that.

So far, so good. We've got the "replicator" part into the acceptance column. What about that "makes imperfect copies" part? This simply means that copying errors occur. They don't occur very frequently (maybe about one in a million), but they do occur. These errors are called alternative alleles or, more commonly, mutations. They result in some change in the phenotype. Most of the time they are inconsequential. When they do matter, they are usually negative. In other words, the ancestral allele was a better deal than the mutated one. And so the mutation does not spread in the population. But the important point here-and it is indeed a central point to natural selection-is that mutations, those inevitable copying errors, provide variation in the human genome. Variation is good. You wouldn't want uniformity or perfection. Those errors are essential to the survival of the species. If there were no errors (i.e., if copying were perfect), there would be no variation among phenotypes and nothing for natural selection to work on.

So far, we have upset no one in the audience, regardless of whether seated on the extreme right or left side. What's next? Those slight variants go forth into their environments and some of them do better than others. You might not like the idea of competition, but it is an inevitable part of life on this planet. Some of those mutations result in greater reproductive success than others, as members of the species compete with each other for precious resources. The environment is a filter, and some traits lead to greater reproductive success than their alternatives.

As Darwin argued in 1859, occasionally animals undergo sufficient change to become reproductively isolated. Geography contributes to this isolation as well. When these noninbreeding populations become sufficiently differentiated, a new species has been formed. Rats and mice are good examples: they remain closely related, but as long as we have known them they have been separate species with a common ancestor. They share physical and behavioral traits, but they also have differences in size and behavior.

And there you have it. An absolutely blind and lawful process without values or agendas. It may not be the stuff of fairy tales or creation myths, but it grinds away in its relentless way, producing outcomes. My colleague Martin Daly has stated it eloquently: "Natural selection doesn't have goals, but it's the reason organisms do." That's a wonderful summary of life on this planet, although the idea is a bit difficult for some people to grasp. It also doesn't help that natural selection moves very slowly, making the concept an even more difficult sell.

True, there is no father figure/deity with agendas we can talk about in human terms. But that isn't Darwin's fault, any more than Newton was responsible for the law of gravity. In any case, natural selection is part of a world full of wonder, willing to reveal itself as we ask the right questions and pool our knowledge. The universe is no less thrilling as we gradually uncover its secrets and confront its mysteries. And we do so using the splendid intelligence our species has evolved. What could be more uplifting than that?

It's worth remembering that part about copying errors. If all individuals were identical (e.g., in a cloned population), selection would be forced to stand still. All it would take is a single parasite or predator to detect a weakness and a whole population would be at risk. The Irish potato famine of the mid-nineteenth century is a widely cited example of such vulnerability. Historians often point to Ireland's dependence on a single crop as the key to the tragedy, but it was more than that. Genetic variation could have saved the Irish potato crop. Without it, a single form of water mold (Phytophthora infestans) was able to destroy the crop almost overnight. Genetic variation is an essential building block for the design of well-adapted individuals, and it is also a source of resistance when a species comes into contact with predators, be they four footed or single celled. As many as 1 million people were killed by the Irish potato famine by 1850 and perhaps twice that number were forced to immigrate to the United States, Canada, Australia, and other parts of Great Britain. The expression "Celebrate Diversity" takes on new meaning when you consider how natural selection works.

THROUGH A MICROSCOPE DARKLY

Although molecular biology is well beyond the scope of this book, there are several basic things we need to cover. The collection of genes in an organism, called its genotype, is largely responsible for creating (with some help from the environment) the organism's observable traits, collectively called its phenotype. Genes are composed of deoxyribonucleic acid (DNA). DNA contains the instructions that specify which proteins should be made. By doing this, the DNA is responsible for shaping the phenotype.

If stretched out for display, every cell in your body contains about six feet of DNA. Every nine hundred cells therefore produce over a mile of DNA. Because the human body contains as many as 100 trillion cells, our bodies house over a billion miles of DNA. That's a pretty large number. The distance to the sun is only 93 million miles, in comparison.

We are still learning about DNA. It seems that every time we look at the human and chimpanzee genomes side by side, the degree of similarity has grown. Other than making glib statements like "Humans and chimps are more related than mice and rats," what do we really know? As I write this in 2008, it seems the less I say about this area of research, the better. Almost anything is likely to become obsolete in short order. Craig Venter has decoded the human genome and made his own diploid genome available on the Web (www.jcvi.org).

We have watched estimates of the "degree of overlap" between human and chimpanzee increase from 97 percent to nearly 99 percent in the span of less than ten years. At the same time, estimates of the number of base pairs in the human genome seem to be dropping at a steady rate. Prior to mapping the human genome in 2000, estimates of the number of human genes were fixed at about 100,000. Shortly thereafter, that number fell to about 25,000, a dramatic drop by any reckoning. Had I written this book a year ago, that number would have stood at about 22,000. By the time you read this, it may lie below 20,000. The estimate keeps shrinking, yet the human genome remains firmly in place. Plainly, the number of genes is not the whole story of what makes us distinctly human.

Somewhere, somehow, there are "switches" in the genome that tell our genes when to turn on and express themselves, thus turning a genotype into a phenotype. The switches that turn genes on and off are not well understood. Some of them lie within the genes themselves. Others are thought to lie within the so-called "junk DNA," poorly understood matter that comprises the large majority of the genome. The picture is even more complicated. In 2006, it was reported that individual differences may result from previously undetected variations in the number of times that certain key genes are copied in the genome. Preliminary findings suggest that entire sequences may be repeated as many as ten times, differing widely between individuals and yielding differences in the overall "copy number" of genes. Perhaps we have ignored material such as this or "junk DNA" for too long, focusing instead on the more obvious proteins that were easier to measure and investigate. In any case, high school genetics books (often called molecular biology texts in universities) are losing their resale value almost as quickly as they are published. It has become that difficult to stay current.

BACK TO THE PLEISTOCENE

Our own ancestors (don't visualize your grandfather here; try to imagine someone who lived two hundred thousand years ago) lived in small nomadic bands. A lot of inbreeding occurred and most of the persons within the band were genetically related. They made a living by hunting together and gathering nuts and fruit that grew locally. These activities were fraught with peril. Animals that were being hunted for food had no interest in being eaten. They had their own concerns about survival and reproduction and vigorously resisted our attempts to turn them into table scraps. Even nuts and berries that were available locally presented difficulties for our ancestors. "Nuts and fruit" may sound like a healthy diet today, but much of that locally grown produce had an agenda of its own that probably did not include traveling through a human digestive system.

(Continues...)

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Excerpted from CAVEMAN LOGIC by HANK DAVIS Copyright © 2009 by Hank Davis. Excerpted by permission.
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