Life's Ratchet: How Molecular Machines Extract Order from Chaos (4 page)

Thus, from the Greek philosophers to the early-nineteenth-century biologists, there were three possible solutions to the problem of explaining life: Assume an overarching, universal principle that determines the purpose of the entire universe (animism); assume a special life force that distinguishes life from matter, thus reserving purpose for life alone (vitalism); or deny purpose altogether (mechanism, atomism). All of these approaches had their problems. Animism erased the clear distinction of the inanimate and the alive, vitalism gratuitously introduced an unseen force and raised the additional question of how this force interacted with the body, and atomism seemed impotent to account for those of life’s activities that seemed to show clear purposefulness, such as growth and reproduction.

Atomism made a brief resurgence with the philosopher Epicurus (341–270 BCE) and was later revived by the Roman philosopher Lucretius (99–55 BCE) in his famous poem
De rerum natura
. Explaining the universe as the result of atomic motion, Epicurus invented the “swerve”—the sudden, random swerving of atoms that otherwise would move on straight, predictable paths. The swerve explained how atoms clumped together or bounced off each other. It explained creation, spontaneity, chance, and free will. While the idea of the swerve seems gratuitous, Epicurus understood that an atomistic explanation of the universe needed a mixture of necessity and chance.

However, the difficulty of reconciling the random motion of atoms with the obvious purposefulness of life doomed atomism for many centuries and has cast a long shadow on our understanding of life until today. The battle of soul versus the atom continued to be central to understanding life, even though the terms changed and biological knowledge became more refined.

For the most part, the ancients vacillated between vitalism and animism. Aristotle clearly understood that life was special and did not postulate souls for rocks and mountains. He did, however, think of motion as due to a purpose. A rock “wanted” to fall down, because it was made of the element earth and wanted to go back to the earth. On the other hand, the Stoics (a school of philosophy founded by Zeno of Citium circa 300 BCE) believed in a more animistic world, where a mysterious ordering force, the
pneuma
(“breath”) gave rise to all existence. The pneuma was like an ancient version of “the force” in
Star Wars
, the fictional energy field created by all living things.

With the rise of Christianity, both the atomism of Epicurus and the animism of the Stoics became discredited. For the early Christians, Plato’s philosophy, which was based on the transcendent world of ideas and not our material reality, was much more palatable. Plato’s universe was the result of reason, not chance. Steeped in Platonic philosophy, the evangelist John wrote: “In the beginning was the Word, and the Word was with God, and the Word was God.” Following John, the early Church, and especially St. Augustine, equated this “Word” or reason with God, and the world of ideas with heaven. The material world was relegated to a corrupted reflection of the spiritual world, which contained the real truth, the truth of God.

Many writings of Aristotle, lost in the West for nearly a thousand years, were saved by the Muslims. In the twelfth century, his works reentered Western philosophy. The early schools of theology in Paris, Oxford, Toledo, and Cologne, which later became universities, were stunned when they encountered the comprehensive knowledge contained in Aristotle’s numerous writings, from logic to physics, statecraft to biology. Unlike Plato, Aristotle saw the material world as primary, and ideas as mere generalizations of observed objects and phenomena. In this, he was quite close to what scientists believe today.

While Aristotle’s ideas threatened the established neoplatonic theology of the time, he could not be ignored. His philosophy was too comprehensive and too well reasoned to be dismissed. A new philosophy,
scholasticism
, was born to reconcile Aristotle’s philosophy and science with Christian theology. Not everything in Aristotle’s books was counter to Christian beliefs. For example, he clearly dismissed atomism and the accompanying idea of chance as an important player in the universe. For Aristotle, the most important force was purpose. Motion also required an explanation and could not be attributed to the unexplained random motion of atoms. Instead, Aristotle postulated a first mover—which St. Thomas Aquinas, the most famous of the twelfth-century scholastic philosophers, equated with God. In living beings, the soul was the prime mover. According to Aristotle, “[the soul] acts and [the body] is acted upon, and the [body] is moved and the [soul] moves.”

Aristotle’s concept of the soul has survived until today, and is evident in the catechism of the Catholic Church: “‘Soul’ also refers to the innermost aspect of man, that which is of greatest value in him, that by which he is most especially in God’s image: ‘soul’ signifies the spiritual principle in man. . . . The unity of soul and body is so profound that one has to consider the soul to be the ‘form’ of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.” This idea of the soul as the form of the body, which renders it alive, comes from Aristotle. The Catholic catechism contains two meanings to the word
soul
: Aristotle’s life-giving “form” and a “spiritual principle.” Even though these two meanings are often conflated, they are radically different concepts.

Modern physics, chemistry, and evolution can explain what makes a cell or an organism alive and what gives it “form.” Bodies are complex assemblies of interacting cells, operating according to an evolved program written in the cell’s DNA. A soul is not needed as the source of form, locomotion, nutrition, or reproduction (in contrast to what Aristotle thought). The concept of soul may make sense in the second meaning—as a noncorporeal, unique essence of a human being; as a shorthand to aspects of a living being that encompasses personality, dignity, intelligence, mind, and the connection to others.

The Christian adoption of Aristotelian ideas, while at first quite radical, put a straightjacket on science for many centuries. The Greek philosophers welcomed debate, and it is reasonable to assume that Aristotle would have been horrified to learn that his musings were now taken as gospel.

The vitalistic ideas of the Greek philosophers, as well as their early penchant for scientific observation, profoundly influenced the practical science of life: medicine. Originally based on magic and faith-healing, medicine was put on a more rational footing by Hippocrates and other Hippocratic thinkers around the time of Aristotle. Medicine became a rational science, based on an understanding of the universe. Greek (and, later, Roman) medicine culminated in the ideas of Galen (129–217 AD), whose books dominated Western medicine for fourteen hundred years. As late as 1559, a member of the London College of Physicians had to publicly rescind his comments when he dared to criticize Galen in front of his colleagues.

Galen’s medicine, based on Aristotle’s philosophy and Stoic ideas, was heavily vitalistic: It was loosely based on the four Greek elements, which he called the body’s vital fluids (or “humors”). Galen believed that the pneuma, the “life spirit” that circulated in the air, entered the body through the lungs. In the heart, the pneuma mixed with blood (one of the four humors) and produced the “vital spirits,” which were responsible for movement. As part of the soul, these vital spirits were associated with heat, which, according to Galen, was generated in the heart when blood mixed with air. The connections between air, heat, and soul were a recurring theme from ancient Greece to the dawn of the scientific age. And as we will see, thinking of heat as the “living power” is not so far from reality as you might think.

Ancient medicine combined observations (in Galen’s case, mostly by dissecting animals) with a philosophical understanding of the universe. For the ancient physicians, life was associated with heat, and heat was generated by fire (one of the four traditional elements), which must be nourished by air (another of the elements). This kind of reasoning sometimes came close to the right answers, but ultimately, the ancients were victims of their own philosophical predilections. Without the methods of modern
science—controlled experiments, the testing of hypotheses and quantitative arguments—medicine remained in a rut for more than fourteen hundred years.

In the Renaissance, when the study of human nature took center stage, medicine, astronomy, and physics finally broke out of the cage of Aristotelian and Galenic thought. The Renaissance was a time of rediscovery and reassessment, during which scholars combed the globe for ancient manuscripts. It was not the discovery of modern scientific methods that allowed Renaissance physicians to break with Galen and Aristotle; it was the discovery of ancient
magical
manuscripts. The arguments of the ancient magicians were based on the correspondence between the human body and the universe as a whole, and they led to the development of alchemical medicine. For example, in his book
Of Natural and Supernatural Things
, the Benedictine monk and alchemist Basilius Vesalius provides a recipe for “Spirit of Mercury . . . which cures all diseases, be it dropsie, consumption, gout, stone, falling sickness, apoplexy, leprosy, or howsoever called in general”—a recipe, surely, that would be more likely to cause apoplexy than to cure it.

The main proponent of this new medicine was the Swiss physician Paracelsus (1493–1541), born Phillippus Aureolus Theophrastus Bombastus von Hohenheim. (While apparently the word
bombastic
is not based on Hohenheim’s middle name, he certainly was that: “Let me tell you this: every little hair on my neck knows more than you and all your scribes, and my shoe buckles are more learned than your Galen and Avicenna, and my beard has more experience than all your high colleges.”) Even though Paracelsus’s alchemical medicine was often no better than the old Greek medicine, his decisive break with the ancient Greek tradition and his emphasis on using chemistry were an important step forward. At times, Paracelsus sounded amazingly modern: “Medicine is not only a science; it is also an art. It does not consist of compounding pills and plasters; it deals with the very processes of life, which must be understood before they may be guided
.
”

The various approaches to medicine—traditional and herbal medicines, Galenic medicine based on the balance of humors, and the new alchemical medicine of Paracelsus—coexisted and were vigorously debated, often on what appears to us today as dubious grounds. Yet, that they
were
debated ushered in the era of modern critical science.

The battle between ancient and alchemical medicine was raging when a new idea emerged: the idea that human bodies were merely machines and that the body’s function could be understood as the workings of discrete parts. An inadvertent hero of the mechanical view of life was the English physician William Harvey (1578–1657), a thoroughgoing vitalist who still believed in Galen’s vital spirits. Yet Harvey was the first to understand the true function of the heart. According to Galenic medicine, the heart was the source of heat in the blood and the place where blood mixed with air to create “vital spirits.” Galen believed that the arteries originated in the heart, and the veins in the liver. The arteries and the veins were separate systems, connected only through the porous septum in the heart.

Although little evidence supported the porous nature of the septum, few physicians were brave enough to criticize Galen’s theories. The famous physician Andreas Vesalius (1514–1564), who published some of the most influential and detailed books on human anatomy, could not find any porosity in the heart; nevertheless, in the first edition of his
De fabrica
he accommodated Galen: “The septum is formed from the very densest substance of the heart. It abounds on both sides with pits. Of these none, as the senses can perceive, penetrate from the right to the left ventricle. We wonder at the art of the Creator which causes blood to pass . . . through invisible pores.” But by the second edition of his book, Vesalius had to admit, with some regret, that there was simply no way that Galen’s theory could be correct.

The path was now clear for Harvey, who, like Vesalius before him, had studied at the University of Padua. The way Harvey disproved Galen was enormously influential: Going beyond dissections and observations, Harvey used a simple
quantitative
argument, which was unprecedented and powerful. If the heart was the source of blood, the total amount of blood generated in the heart could easily be estimated by multiplying the volume of the heart with the rate of pumping. This would result in 540 pounds of blood every hour—a giant amount. Where would it all go? The only reasonable explanation was that a limited amount of blood circulated through
the body; and whatever the heart pumped out, came back to the heart a short time later. Harvey’s mathematical reasoning had an enormous impact on the subsequent history of the life sciences: Life, like the rest of nature, could yield to quantitative analysis and, with it, careful experimentation.

Despite Harvey’s modern scientific methods, his work drew praise from the Paracelsian physician Robert Fludd (1574–1637). Fludd saw in the circulation of blood a confirmation of his alchemist views that the macroscopic world of the stars was reflected in the microscopic world of the human body: As the planets go around the sun, so the blood circulates around the heart. But Harvey’s findings also received nods from more modern scientists and philosophers, especially René Descartes (1596–1650), who in
The Description of the Human Body
vigorously championed Harvey and argued that the body was a machine.