Thursday, March 26, 2009

Are We the Pinnacle of Evolution?

The term “evolution” conjures the picture of initially unicellular life marching triumphantly towards greater size and increasing complexity --and of humans as the undisputed pinnacle of evolutionary history. This smug view is compounded by the widespread notion that evolution and progress are synonyms. It is common in textbooks and popular accounts to depict evolutionary series as ladders --from hyracotherium, the “dawn horse”, to the modern horse; or from Australopithecus to exalted Homo sapiens.

       But, as Stephen Jay Gould has shown in his book Full House, ladders are misleading. Hyracotherium is indeed the ancestor of modern horses, and, yes, there is a continuous line from him to present-day Equus. But the line twists and turns in time, branching endlessly so that the “dawn horse” is also the grandfather of countless other species, some living, but most extinct. The same is true of the line of descent going from Australopithecus, of “Lucy” fame, to modern humans. The line is not a line --it’s a bush. Neanderthals, who can also claim Lucy as their grandmother, are not our direct ancestors.

       Evolutionary lineages in general are not linear. Today’s living species, which we might represent as the outer leaves of an evolutionary tree, are attached to twigs, which are attached to larger twigs, which shoot off from branches, which sprout from larger branches, which emerge from an ancient common trunk going way back into the past --some 3.6 billion years-- to the first living organisms, a kind of bacterium.

       The ladder representation conveys the false idea that evolution is going somewhere --that those first bacteria somehow knew they were to become us. But if ladders had any truth in them then the lower rungs ought to be extinct to open the way for the young, so to speak. Bacteria, however, thrive today. What’s more, by their diversity, by their presence in every nook and cranny of the earth, by their longevity, and by their sheer numbers, bacteria are and always have been the dominant organisms in this planet, as Gould argues in Full House. If we go back far enough, you and I have a common ancestor who was a reptile; go back even further and we will find we are related to a fish. Yet reptiles and fishes are alive and well today. Not the same species, to be sure, but modern ones which may be our cousins many times removed. Humans are not the end point of evolutionary history --all species living today are.

       Natural selection, the motor of evolution, does not have a plan. The only criterion for survival is adaptation to existing conditions. The dinosaurs didn’t die out because they were evolutionary failures or because they were less perfect animals than present-day animals. In fact, dinosaurs have been one of the most successful groups in the history of life. They dominated macroscopic life for over 200 million years. Mammals, in contrast, have only been conspicuous for some 60 million years. The dinosaurs died because their environment changed abruptly when a very large meteorite or comet collided with the earth, some 65 million years ago.

       Consider another example of the progress fallacy. Mammoths, the hairy ancestors of modern elephants, were well adapted to life in the latest Ice Age. Hairless elephants are well adapted to present-day conditions. But a hairless elephant, as Gould points out, is not a cosmically better elephant. When another ice age comes --and it will--, a hairy elephant will be more likely to survive.

       And this brings on my final point. Possible hairy elephants of the frigid future will NOT be mammoths. The mammoth is dead and gone. If elephants ever have hairy descendants, those descendants will be new adaptations to cold weather. They may conceivably look somewhat like mammoths --with all the hair and stuff--, but the resemblance will stem from the fact that both species are solutions to a similar problem --like bats and birds. Extinction, as diamonds, is forever.

Tuesday, March 17, 2009

A monk in his garden

Imagine a monastery in Moravia, and in the monastery a garden, and in the garden a monk. The monk is busy handling pea plants in pots, wrapping the flowers in paper bags after carefully dusting them with pollen from a different variety of the pea plant. He is no ordinary monk. He has studied mathematics and science. In a few years’ time he will be elected abbot of the monastery --which will force him to abandon his scientific work.

       He is Gregor Mendel, the father of the science of genetics, and his experiments with pea plants will provide the missing link to Charles Darwin’s theory of evolution by natural selection. But not before both Darwin and Mendel are dead and gone.

       Darwin will die in 1882, still plagued by the mystery of the mechanism of inheritance. The theory of evolution by natural selection requires that heredity work in such a way that mutations --or fortuitous variations in the hereditary makeup of an organism-- are passed on intact to offspring. This would guarantee the conservation of advantageous mutations (a longer neck in giraffes, a change in pigmentation in moths living in soot-covered trees in central England); whereas the alternative mechanism of blending inheritance --whereby offspring simply strike  an average between the characteristics of their parents-- would cut mutations in half with each generation, rapidly diluting their effect, advantageous or otherwise.

       Mendel will die in 1884 in total obscurity. The revolutionary nature of his experiments will only be recognized in 1904, when three European botanists will independently rediscover his work.

       Ironically, Darwin’s would-be savior was already working with his pea plants when the great scientist published The Origin of Species, in 1859. Mendel’s method of “hybridization” was straightforward. First he would open a pea flower before it was fully developed, removing the anthers (or male sexual organs) with tweezers to avoid self-pollinization. Then he would dust the flower’s stigma with pollen from the selected variety, immediately wrappping the flower in a paper bag to keep away other pollen. Finally, he would wait patiently for the plant to produce seeds and for the seeds to produce the next generation of plants. Mendel then recorded the results.

       The monk chose pea plants because they have traits (such as blossom color and plant height) that are easily distinguishable and that breed true. Thus, he crossed six-foot plants with one-foot plants, and plants with purple blossoms with plants with white blossoms. Would the result be three-foot plants with mauve-colored blossoms, as the popular theory of blending inheritance dictated?

       In contrast with other botanists who had performed hybridization experiments before him, Mendel had studied mathematics and was an able statistician. He found that when he crossed six-footers with the short variety the first-generation hybrids were all six-footers. No intermediate-sized plants were produced. However, when these first-generation hybrids were allowed to self-pollinize, the result was astonishing --the second generation included both tall plants and short plants, and in an approximately three-to-one ratio. A similar result was obtained for six other contrasting traits.

       Mendel’s conclusion was that, contrary to popular belief, the parent’s traits are not blended in the offspring. Inheritable characteristics are determined by units of inheritance that are segregated rather than blended in the offspring, with certain traits dominating over their “recessive” opposites (i.e. long stem versus short stem). Today we call these units “genes.” Mendelian genetics meshed perfectly well with natural selection. In the first decades of this century, genetics and evolution became integrated in what is known as the synthetic theory of evolution.

Monday, March 9, 2009

Literary Theme with Biological Variations

In his short story "Pierre Menard, Author of Don Quixote" the Argentinian writer Jorge Luis Borges recounts the story of a symbolist author in turn-of-the-(last)century France who endeavors to rewrite Miguel de Cervantes’ celebrated work. Pierre Menard, however, is no mere parasite intending to copy or paraphrase Cervantes. His intent is to write a verbally identical book based on his own experience. Menard, alas, dies after completing only two chapters. But how fascinating those two chapters can be! Read as the work of a twentieth-century writer, Menard’s Don Quixote is a completely different book.

This, of course, is only possible in Borges’ brilliant fantasy world. In real life, if you hold two books in your hand --for example, Cervantes’ Don Quijote de la Mancha and Menard’s Don Quijote de la Mancha--, and the books correspond word by word, or almost, you immediately smell a rat. The books must --to say the very least-- have a common ancestor. They can’t really be independent.

Odoriferous rodents of the same kind assail the discerning noses of biologists when they compare organisms from the present and from the past using the tools of old and new biological disciplines such as embryology, anatomy, genetics, and biochemistry. Charles Darwin’s original treatise was a steamroller of evidence for “descent with modification.” Today, evolutionists possess further detailed and consistent proof of the fact of biological evolution.

Consider the backbone in humans. Humans, as you probably know, walk upright most of the time. Our backbones are placed in the back (duh). But look at the famous roof at the Museo de Antropología, in Mexico City. Here it is: 

A hypothetical Cosmic Engineer designing humans from scratch would have endowed us with sturdier “backbones” passing through the center of the torso, not along the back. As things are, we are well adapted to an upright posture, but not perfectly adapted, because we have only recently evolved from ancestors that went about on all fours. Imperfections such as are manifest in anatomical studies argue for evolution and against design.

Anatomy, physiology, embryology and other tools that were already available in Darwin’s time can probe only so deep into the similarities of organisms, and go only so far back in time. It is the more recently developed field of molecular biology that provides the most detailed and convincing evidence that we are all, from human to bacterium, ultimately related by descent from common ancestors.

The organic compounds known as aminoacids can be numbered in the hundreds, yet all bacteria, plants, animals and fungi synthesize all their proteins based on just 20 aminoacids, the same 20 for all living beings. Further, for all its staggering diversity, all life on Earth depends on the same few chemical pathways (fermentation, photosynthesis, respiration) to produce energy and build cell components. The molecular and chemical uniformity of life can only be accounted for by evolution.
Molecular biology is unique as a tool for comparative analysis of species in that it allows scientists to precisely quantify the degree of similarity of different organisms. The protein cytochrome c of humans is identical to that of chimpanzees. It differs by one aminoacid from that of rhesus monkeys, by 12 from that of horses, and by 21 aminoacids from that of tuna fish. Comparing the DNA of two species, molecular biologists can now even determine approximately how far back in time the species’ most recent common ancestor lived in the same way that linguists can tell how recently two languages diverged from a parent language by analyzing their similarities.

This is only a paltry sample of facts that can only be explained by evolution. Darwin himself provides many more in The Origin of Species. Today all scientists agree that, as Theodosius Dobzhansky, a leading evolutionist, once said: “Nothing in biology makes sense except in the light of evolution.”

Thursday, March 5, 2009

Quality Control

Lawyers, politicians and scientists love a good argument. As a scientist, however, I wouldn’t want to argue with neither a lawyer nor a politician. For, you see, while we all may love debate, a lawyer’s, a politician’s, and a scientist’s aim in debating are entirely different.

        Lawyers argue to win. That’s what they get paid for. Whether they are right or not is immaterial. Even when he knows his client is guilty a lawyer must defend the client’s innocence. Truth is not the lawyer’s main concern.

        A politician’s job --whatever Plato, Aristotle, and others may have said in the past-- is to attain office and to remain in office. Sad to say, but that’s the way it is, as you know. A politician argues to win or, failing that, to make people believe he has won. Politicians employ every trick in the rhetorician’s repertoire to defend even the wobbliest ideas.

        Scientists don’t argue to win. They enjoy victory  as much as the next guy, but winning is not so important. What’s important is the clash of ideas. In scientific debate only the fittest ideas survive. Flimsy notions perish. What you want as a scientist is not to be proven right, but to be proven, period. Sir Karl Popper, a contemporary philosopher of science, wrote: “The wrong view of science betrays itself in the craving to be right, for it is not his possession of knowledge, of irrefutable truth, that makes the man of science, but his persistent and recklessly critical quest for truth.”

        Popper also wrote: “Those among us who are unwilling to expose their ideas to the hazard of refutation do not take part in the scientific game.”  Wolfgang Pauli, one of the founders of quantum mechanics, once hired an assistant whose job it was to constantly refute his employer’s ideas with the strongest arguments he could muster. Like the warriors of yore, scientists value a worthy opponent.

        An earthquake that leaves one building standing among others in ruins proves the sturdiness of that building. It is in the interest of science to constantly submit its constructs to conceptual earthquakes in order to test their sturdiness. Here is Popper again: “Once put forward, none of our hypotheses are dogmatically upheld. Our method of research is not to defend them in order to prove how right we are. On the contrary, we try to overthrow them.”

        Karl Popper is the creator of the idea of “falsability” of scientific hypotheses. He contends that, in order to be considered scientific, a hypothesis must be formulated in such a way that, if false, it can easily be proven false. This contrasts with the old idea of verfiability of scientific theories, but it makes for more solid foundations to the scientific edifice.

        For example: “Energy is conserved” is a valid scientific statement in Popper’s sense because it is easily refutable --finding one single instance of its not being true would suffice to topple it. The principle of conservation of energy was first formulated more than one hundred years ago. So far, scientists have not found a single case in which it is violated. You see, then, how Popperian “falsability” can yield sturdy scientific principles: energy conservation is easy to disprove, yet it hasn’t been disproven. The more tests it survives, the more confident we are that energy is conserved even in situations in which we have not explicitly shown this to be the case.

        When you buy a car you kick the tires and slam the doors to guarantee that you are making a sound investment. A scientist invests much more than money in the ideas he accepts as true. What’s on the line is his ability to do useful work in the future, his worldview, and his inner equilibrium. So when it comes to selecting our truths --our cars and buildings-- we scientists are extremely picky. It is painstaking work but in reward we, more than lawyers or politicians, can feel truly safe in the cars we choose to drive and the buildings we decide to inhabit.