Life's Ratchet: How Molecular Machines Extract Order from Chaos (8 page)

In his famous essay of 1847, “Über die Erhaltung der Kraft” (“About the conservation of force”), Helmholtz, then only twenty-six, followed much the same line of argument that Mayer had set forth. Helmholtz felt that postulating mysterious vital forces added nothing to the investigation of how life works. Moreover, the presence of vital forces that could generate mechanical force from nothing would make it possible to construct a perpetuum mobile, a machine that generates energy from nothing. It was widely accepted that this was impossible. Energy conservation had to be correct, and special vital forces could not exist. Helmholtz showed that the law of energy conservation could be mathematically proven. He only needed to make the assumption that matter was made of pointlike particles, interacting through forces depending only on the distance between the particles. This mathematical proof and the expansive view of the new law met with some resistance from his older colleagues. Soon, however, new experiments proved Helmholtz and his fellow energy conservers correct.

Helmholtz was a dedicated mechanist from the beginning. Although he had studied with the influential researcher and teacher Johannes Peter Müller (1801–1858), who was part of the teleomechanist faction, he despised the very idea of invoking a vital force to explain anything. He designed several experiments to prove that vital forces were unnecessary to explain irritability. Most of these experiments, ironically, were to be performed on frog legs, the favorite experimental object of Galvani. Galvani, of course, had presented his frog leg experiments as support for vitalism.

In the first set of experiments, Helmholtz set out to prove that motion in muscles is caused by chemical processes, that is, that animal motion is a physicochemical process and is not related to any mysterious vital force. To prove this, he irritated frog legs several hundred times by passing electrical currents through them, just as Galvani had done. He then made several chemical extracts of the irritated frog legs and compared the extracts with extracts from non-irritated frog legs. He found that if the muscles had been irritated, a water-based extract lost mass and an ethanol-based extract gained an equivalent amount of mass. Clearly, some chemical compound in the muscles had been changed from a water-soluble to an alcohol-soluble form through the action of the muscles. This proved that the motion of the muscles caused a chemical change in the muscles, and Helmholtz concluded that muscles were machines that converted chemical to mechanical energy.

To establish that this energy was purely chemical, he next compared the heat that can be released upon chemical breakdown of food, called the latent heat, with the latent heat of excreted substances in animals. This was Lavoisier’s experiment. However, since the time of Lavoisier, more-refined experiments had improved on Lavoisier’s guinea pig. Helmholtz reviewed these experiments and concluded that the difference in energy between food and excrement accounted well for the observed animal heat. He was able to correct an error introduced by Lavoisier and the famous German chemist Justus von Liebig (1803–1873): Liebig (like Lavoisier before him) believed that the energy expended by an animal was exactly the same as oxidizing (burning) the animal’s food in the oxygen the animal breathes. But the French physicist Pierre Louis Dulong (1785–1838) and the Belgian physicist César-Mansuète Despretz (1798–1863) had shown in their careful repeats of Lavoisier’s experiments that an animal generated about 10 percent more energy than could be accounted for by the oxidation from respiration alone. This left an opening to the vitalists, who could point to the missing 10 percent as the contribution of the vital force. Instead, Helmholtz showed, the missing 10 percent came from the oxygen already contained in food, especially in carbohydrates and sugars. If this additional oxygen was included, food energy perfectly matched animal heat plus energy of the excrements, and no vital force was needed.

One more experiment was needed to completely eliminate the need for the vital force: Helmholtz had to show unequivocally that the energy
to move muscles was contained in the chemical energy of the muscles (which they had received from food) and did not come from someplace else. At the time, it was known that a loss of the nervous system led to a cooldown of the body. Therefore, some biologists believed that the nervous system provided a source of vital force or animal heat. Helmholtz devised an ingenious setup to eliminate this last refuge of the vital force. By irritating three selections of tissue—a frog leg with its attached spinal nerve, a frog leg without the nerve, and the nerve without the leg—he ventured to show that any temperature increase due to the motion of the leg originated in the leg and was not due to any vital energies transferred to the leg from the nerves. To measure the minute temperature increase of a moving frog leg, he constructed a very sensitive device, comprising a thermocouple (a kind of electrical thermometer, which converts temperature into voltages), a magnetic coil to magnify the resulting voltage, and a dial that displayed the temperature after calibration of the device. His setup was accurate enough to record temperature changes as small as one-thousandth of a degree. Moreover, this setup was a physical representation of the law of energy conservation: Chemical energy in the frog leg (unleashed by electrical irritation) was converted into mechanical energy (motion of the leg), then into heat, electrical energy (thermocouple), magnetic energy (coil), and, finally, mechanical motion of the dial.

Helmholtz found that the presence of the nerves made no difference and that a nerve alone did not heat up at all. As long as the muscles moved the same amount, they heated up by the same amount, regardless of the presence of a nerve. In Helmholtz’s view, the idea of a vital force was now untenable.

It would be too easy to conclude that the physicists carried the day. Yes, Helmholtz and others had shown that anything happening in a body—all the hallmarks of being alive, from animal heat to irritability— had to occur within the energy budget prescribed by the physicochemical world. If there was such a thing as a vital force, it had to be a force without energy and thus without potency. After Helmholtz, vital forces quickly fell from favor and have not been resurrected since, at least not in serious science. Nevertheless, for all of his single-minded eradication of the vital force, Helmholtz and his fellow physicists could not explain how unformed matter could organize into a complex organism. He had shown
that it must happen within the law of energy conservation, but this did not explain how living beings formed. His research merely removed vital forces from the list of possible explanations.

Thanks to Helmholtz and others, biology returned to mechanism by the end of the nineteenth century, but not to the primitive, naive mechanism of the seventeenth-century mechanical philosophers. It was now clear that all biological processes occurred within the framework of chemistry and physics. The two disciplines were key to explaining physiological processes, such as irritability or animal heat. But it was equally clear that biology was fundamentally different from physics and biology: Complexity and development remained to be explained. Purpose was not yet exorcised.

The essential debates surrounding the mysteries of life never really changed. New findings brought the controversy over purpose versus mechanism into sharper relief, but the dilemma of biology in the late nineteenth century was fundamentally the same as the ancient debates between Aristotle and Democritus. By the 1850s, nobody could deny that to explain life’s processes, physical, chemical, and mechanical forces had to be invoked. Yet mechanics seemed woefully insufficient to explain the extraordinary complexity and purposefulness of life. Mechanical explanations had become more powerful, and with the work of Helmholtz and his contemporaries, it was now clear that all forces or energies active in life were also present in inanimate matter. But this could not explain purpose. The dilemma of the physicists was the same dilemma Democritus had faced. How can complexity emerge from chaos?

The person who rescued mechanism from the bugbear of purpose was Charles Robert Darwin. Together with Alfred Russell Wallace (1823–1913), Darwin developed the theory of evolution based on natural selection. The idea of evolution was not entirely new. Ever since they noticed how different life forms were related to each other and to extinct forms, scientists had wondered if species could change over time. But without a satisfactory mechanism, the idea of evolution went nowhere.

Meticulously argued and written in a surprisingly accessible style, Darwin’s revolutionary
Origin of Species
presented a strong case for natural selection as the driving force of evolution. Despite its general acceptance in modern science, Darwin’s theory was absolutely earth-shattering and counter intuitive when it was published in 1859. When his contemporaries looked at marvelously designed plants and animals, Darwin’s claim that these forms could have emerged by a blind, step-by-step process seemed outrageous.

Yet the theory was persuasive: Darwin’s faithful supporter, the biologist Thomas Henry Huxley (1825–1895), exclaimed upon reading the book: “How extremely stupid not to have thought of that!” Indeed, like many brilliant ideas, Darwin’s is exceedingly simple: All species exist as a population of individuals, each one a little bit different from every other. How these variations arose was unknown to Darwin (today we have a pretty good idea), but that they did exist was obvious. Over time, the variations that led to more reproductive success would surely increase in the population. In other words, individuals with a variation that allowed them to mate more successfully would have more offspring, and soon there would be more individuals with this particular variation. As conditions change or as populations are cut off from other populations, different variations will be favored and new species can emerge. Because the process is extremely slow, there are few opportunities to observe the emergence of a new species in a human lifetime.

Darwin’s theory received serious opposition on religious grounds. In a popular book,
Natural Theology
, published in 1802, the theologian William Paley (1743–1805) had compared the complex mechanisms within living beings to the workings of a watch (as the rather less religious La Mettrie had done as well). Clearly, a watch assumed a watchmaker. It did not design and make itself. How could we even suggest that the infinitely more complex designs of living beings had made themselves? Paley’s book was very popular at the time and was one of Darwin’s favorites during his under graduate years. It was only very gradually, and through thousands of painstaking observations, that Darwin came to realize that Paley was wrong.

By removing the last vestiges of purpose, Darwin’s theory of evolution made God unnecessary for explaining the natural world. This, of course, was not the first time God had been made unnecessary. Physics
had already banned God from the heavens (“I have no need for that hypothesis” answered the physicist Pierre-Simon Laplace when Napoleon asked him about the role of God in the universe), and Helmholtz had banned vital forces. The last refuge of the supernatural seemed to be the wondrous multiplicity of life forms. Now even that became the result of blind forces.

While much has been made of the religious critics of Darwin’s ideas, his theory was similarly met with initial reservations by his fellow scientists. Here, the reasons were different. Darwin’s theory lacked crucial ingredients: How were variations transmitted to the next generation, and how were new variations generated? Certainly, Darwin made a strong case, based on the breeding of dogs, orchids, and fancy pigeons, that animals can be changed dramatically over relatively short time. But this seemed to require the guiding hand of a breeder. Where did variety in nature come from? How could this variety lead to the changes observed? How were variations inherited? Why was the offspring similar, but not identical, to the parents? Although Darwin hit on the crucial mechanism to explain evolution—variation and natural selection—there were many gaps.

Unbeknownst to Darwin, a Moravian monk, noting curious patterns among generations of pea plants in his garden, had already discovered some of the missing puzzle pieces. The research of this monk, Gregor Mendel (1822–1884), was published in a rather obscure journal, the
Proceedings of the Natural History Society of Brünn
, and remained virtually unknown until it was rediscovered in the early twentieth century. What Mendel had discovered in his research with his beloved pea plants were the laws of inheritance: the fact that traits are inherited whole, and that traits from each parent can be combined in various ways in the offspring. Before his work, it was not clear how inheritance worked, and
blending
inheritance could not be discounted. In blending inheritance, the traits of the offspring are a blend of the traits of the parents. Mendel found this to be wrong: If it were true, pea plants would soon assume some average color, the blend of the colors of parents and grandparents. This is not what happened. If, for example a red pea was crossed with a white pea, Mendel got pink peas in the next generation, but surprisingly, the “granddaughter” peas could again be pure white or red. The trait white had not been blended out, but was merely dormant (recessive) and reappeared in a
subsequent generation. The observation that the inheritance of traits was not blending, but rather was conservative, was extremely important for Darwin’s theory. Only traits that could be passed on whole to the next generation could spread through a population and explain the emergence of a new species. If traits were blending, any new traits would soon be blended back into mediocrity.