Note: This was to have been chapter 9 in my sequence of postings. Due to (yet another) major rearrangement of chapters, this is now chapter 13.
The great and fruitful ideas which Darwin brought to the attention of the whole world have long since been incorporated into human thought. Not the least important among them is the new scientific concept of fitness, as it emerges from the discussion of natural selection…. [But] it has been the habit of biologists since Darwin to consider only the adaptations of the living organism to the environment. For them, in fact, the environment…has been an independent variable, and it has not entered into any of the modern speculations to consider if by chance the material universe also may be subjected to laws which are in the largest sense important in organic evolution. Yet fitness there must be, in environment as well as in the organism.
Lawrence Henderson,The Fitness of the Environment (1913)
Lawrence J. Henderson was a professor of biological chemistry at Harvard in the early 1900s. He entered Harvard College in 1894 at the tender age of sixteen (not particularly unusual in those days), graduated from Harvard Medical School with an M.D. eight years later, and spent his entire professional career there until his death in 1942. Henderson was one of the founders of Harvard Medical School’s Department of Physical Chemistry. Notably, he was responsible for establishing the history of science as a discipline—a first at any university in the United States—and taught the initial course starting in 1911. He was most known for his work in blood chemistry. Later in his career, Henderson published works on philosophical and sociological topics.
Henderson struck many as being cold and pompous. He was notorious for his manner of writing (and speaking) in a convoluted, circuitous fashion and was known, according to a friend, for making “passionate and intolerant assertions and suffered fools not at all.” While highly regarded by his students and peers, such qualities may account in part for Henderson’s not being known widely today. Be that as it may, he was decidedly forward-thinking in advocating a systems-level approach in his methodology. Everett Mendelsohn writes,
In spite of the several fields in which Henderson worked, a number of commentators, his contemporaries, and later analysts noted a markedly similar approach in many of his endeavors…. His focus was on organization and system: the organism, the universe, and society. John Parascandola, the author of a doctoral dissertation and several important articles on Henderson, put it succinctly: “The emphasis in his work was always on the need to examine whole systems and to avoid the error of assuming that the whole was merely the sum of its parts.”
In addition, Henderson is considered an early advocate of what is now known as the concept of cosmic fine-tuning—the notion that our entire universe, including physical constants at the heart of mathematical equations defining it, are biased to favor life. Accordingly, there is a renewed interest in Henderson’s ideas. His influential book The Fitness of the Environment, first published in 1913, ends with these words:
The properties of matter and the course of cosmic evolution are now seen to be intimately related to the structure of the living being and to its activities; they become, therefore, far more important in biology than has been previously suspected. For the whole evolutionary process, both cosmic and organic, is one, and the biologist may now rightly regard the universe in its very essence is biocentric.
By the early 1900s, life’s chemical nature was beginning to be well understood. Henderson’s early work, begun in 1905, centered around neutrality regulation—how an organism achieves and maintains a state of equilibrium between acids and bases within its body (that is, a neutral pH balance). Maintaining that balance in physiological fluids is imperative: when pH varies from some norm by even a small amount, enzymes cease functioning and proteins begin to break down. The results are catastrophic; in humans, for instance, death occurs in just minutes if blood pH exceeds certain limits.
In order to regulate acid levels in their bodies, all air-breathing terrestrial organisms employ a carbon dioxide–bicarbonate buffering system. A buffer consists of weak acids and their corresponding bases, which act together to minimize changes in pH by reversibly capturing or releasing ions. Here is a brief description of the process in the context of human physiology:
Recall that during cellular respiration, CO₂ and hydrogen ions (H⁺) are produced in abundance. As waste, CO₂ is excreted and excess hydrogen ions—which acidify their cellular environs—have to somehow be removed. Most of the CO₂ generated by respiration diffuses into the bloodstream where, along with H₂O, it is taken up by hemoglobin (the oxygen-carrying element in blood) in red blood cells and converted to carbonic acid (H₂CO₃). While this reaction occurs spontaneously, it is greatly accelerated by the enzyme carbonic anhydrase, a multipurpose catalyst that greatly speeds what is otherwise a slow reaction. Carbonic acid is unstable; most of it spontaneously dissociates into bicarbonate (HCO₃ˉ) and hydrogen ions. Hemoglobin also takes up much of the excess H⁺ (thereby preventing excessive acidification) and releases bicarbonate into the bloodstream, which carries it to the lungs. In the lungs, carbonic anhydrase reverses the reaction. The resulting H₂CO₃ again dissociates—this time into CO₂ and water—and the carbon dioxide is exhaled.
As mentioned, a small amount of carbonic acid remains in the bloodstream without becoming ionized, along with some H⁺ and significant amounts of bicarbonate. If, through some physiological change, blood becomes more acidic due to an increase of H⁺ it is neutralized by H⁺ bonding with HCO₃ˉ, reforming carbonic acid. Or, if blood pH rises, carbonic acid dissociates and in doing so releases H⁺ until blood pH returns to equilibrium. These reversible reactions happen automatically and, with impressive efficiency, maintain a vitally important state of chemical balance, or homeostasis, in the blood at all times.
While studying the bicarbonate buffering system, Henderson noticed that its effectiveness depended to a large extent on several critical substances’ natural chemical properties—particularly those of carbonic acid and CO₂. Carbon dioxide being a gas made its excretion as a waste product a simple matter. He wrote,
Needless to say…carbonic acid is also of great importance in many physiological processes, chiefly perhaps in excretion. In the course of a day a man of average size produces…nearly two pounds of carbon dioxide. All this must be rapidly removed from the body. It is difficult to imagine by what elaborate chemical and physical devices the body could rid itself of such enormous quantities of material were it not for the fact that, in the blood, the acid can circulate partly free and, in the lungs…[it] can escape into air which is charged with but little of the gas. Were carbon dioxide not gaseous, its excretion would be the greatest of physiological tasks; were it not freely soluble, a host of the most universal…physiological processes would be impossible.
Carbon dioxide is unusual in other ways: it is one of the few oxides that is a gas at ambient temperatures. It is exceptionally water soluble (having twenty times oxygen’s solubility) and whenever air is in contact with water, carbon dioxide will dissolve until its concentration is equal in both substrates. This quality makes CO₂ a consummate vehicle for spreading carbon around the globe via both the hydrosphere and atmosphere.
Over time, Henderson increasingly noted that the element carbon’s many fortuitous properties lent it the appearance of being tailor-made for a central role in all life processes—indeed, in making life possible. Carbon is unique in being the only element that can form multiple covalent bonds—not only with other elements but, crucially, with itself. Carbon bonds (importantly, neither too strong nor too weak) enable the formation of countless organic macromolecules such as fatty acids, proteins, sugars, and a wide array of hydrocarbons. Over 20 million organic compounds have been described thus far, and they continue to be discovered at a furious rate—a rate amounting to hundreds of new ones added daily, many of them through being isolated from plant tissues. There is virtually no limit to the number that could potentially exist, due to carbon’s ability to form long-chain molecules of great length.
Organic compounds are matchlessly suited for use by living things thanks to other equally fortuitous qualities. One important attribute of (most) carbon compounds is the relative mildness of their chemical properties. Organic acids—DNA for instance—are not violently reactive in the fashion of their inorganic counterparts (highly corrosive substances such as nitric acid). Nor are the organic bases highly corrosive. This mildness is due in part to carbon being a relatively inert element prone to sharing electrons rather than gaining or losing them in reactions (in the fashion of a highly reactive element like oxygen). Additionally, carbon compounds have a characteristic known as metastability. Metastable compounds readily release energy in the course of reactions but are durable enough to not break down over time unless subjected to heat, radiation, or the activity of enzymes. The metastability of organic substances happens to be best exploited at temperatures neatly bracketed within the range of ambient temperatures found on Earth.
No other element comes close to being as impeccably suited for life’s needs. Put another way: for living things to exist on other planets, an element with carbon’s qualities and capabilities would have to be readily available. As for the possibility of alien life-forms being based on the interaction of several non-carbon elements: there is no combination of elements could begin to match carbon’s accommodating versatility. Any attempt to envision a functional alternative to carbon-based chemistry encounters a succession of compounding problems. Similarly, efforts to design an alternative biochemical system—particularly if it includes being capable of self-replication—almost immediately run into insurmountable dilemmas. These are due in part to the necessity of satisfying a number of stringent (and often mutually incompatible) criteria. Carbon-based organic chemistry has serendipitously avoided all such problems.
What are the options? A standard response is the “life-as-we-know-it” argument: life on other worlds may be in some form we can barely even imagine. Still, other-life will have to possess the means to store and process large amounts of information. Science fiction writers with solid backgrounds in astrophysics have envisioned life forms flourishing in what would normally be considered unlikely environments (for instance, as electromagnetic circuit-life whizzing about inside neutron stars or floating in the atmosphere of a gas giant planet). While such possibilities can never be categorically ruled out, in all likelihood otherworld life forms will be made of atoms. If not carbon, what else might do? Silicon is a sci fi favorite as an alternative basis for alien life. This is no random choice; silicon and carbon share certain chemical properties, each having the same configuration of electrons in their outermost orbital shells—a feature that lends seemingly unrelated elements a tendency to exhibit similar chemical behavior. Both carbon and silicon readily form compounds, bonding with many other elements. But unlike carbon, silicon cannot form the host of complex, metastable compounds likely required by any organism, regardless of type. Silicon is incapable of bonding with itself and thus form long chain molecules. (It is for this reason that silicon chemistry is much less diverse). There can be no equivalent to proteins based on silicon, nor any of the other essential biomolecules. Silicon’s oxide form, SiO₂ (otherwise known as the mineral quartz), is one of Earth’s least soluble but most abundant substances. A frequently used quip: “Silicon is good for making rocks, not life.”
Then there are those other elements that form compounds crucial to life—the key players being hydrogen, oxygen, nitrogen, sulfur, phosphorus, calcium, sodium, and ions of several other metals. These elements all prove to be ideally suited for their roles and, it is interesting to note, their relative abundance in the cosmos closely mirrors (in most cases) their abundance within living things. Whether or not this curious fact has any significance is impossible to say.
Though found in living things in typically minute quantities relative to non-metal-bearing substances, a number of metallic ions are of crucial importance and it is no exaggeration to say that life would be impossible without them. On the far left side of the periodic table are the alkali metals sodium and potassium. Located to their right are the alkaline earth metals calcium and magnesium. The remainder of the left half of the table consists of the rare earth metals and, farther right, the transitional metals. (The members of each group share correspondences in properties such as conductivity, reactivity, and what sort of compounds they form.)
The alkali metals, sodium and potassium are essential ingredients in a host of basic life processes.Representing the alkaline earth metals are calcium, with its ability to convey chemical information at great speed (as in the triggering of muscle contractions and transmitting nerve impulses) and magnesium, whose unrivaled light-absorption capacity makes it ideal for photosynthesis.
Among the transitional metals there is iron—a key component of hemoglobin, that all-important carrier of oxygen in the bloodstream; copper, with its oxygen-binding capabilities exploited in the electron transport chain; and cobalt, an ingredient of vitamin B₁₂. Proteins involved with switching genes off and on contain zinc, as does that absolutely vital catalyst, carbonic anhydrase. Another transitional metal essential to life is molybdenum, a component of enzymes involved in nitrogen fixation. Given their importance, it may be surprising to learn that many enzymes containing metal ions are built around a single atom. In Lawrence’s time, the crucial role of metals was not yet fully understood or appreciated. But we now know that, as is the case with carbon, they are ideally fit for their roles.
Similarly, as was made clear in the discussion of carbon chemistry, many types of molecular compounds are seen to be supremely fit for their chemical tasks: the sugars and polysaccharides, lipids, and phosphates. The nucleotides, ideal vehicles for storing information. And proteins, with their myriad forms and capabilities, including their all-important role as catalysts. (This, as well as the dual function of being able to carry out their own tasks while simultaneously self-regulating them without assistance from intermediaries—recall the discussion of allostery, Chapter 7).
And lastly: that tasteless, odorless, and colorless naturally occurring fluid, dihydrogen monoxide (otherwise known as water), the most abundant chemical compound in the universe and sole earthly substance occurring in liquid, solid and gaseous phases. Henderson meticulously demonstrated how each of water’s physical properties make it ideally fit for life’s needs and requirements. He provided abundant evidence showing that water is supremely fit, not only for maintaining global climatic stability, but also in its role as a veritable matrix for all living matter. And that water is not only fit in many of its highly unusual (even unique) qualities but in virtually all of them.
Perhaps water’s most striking trait is that it is one of the few substances that expands instead of contracting as it freezes (becoming less dense). This is why ice floats. Crucially, if ice were denser than liquid H₂O, lakes and oceans would freeze from the bottom up and most of our planet’s bodies of water would remain permanently frozen.
An important property of water is its thermal capacity or specific heat. Water’s is higher than that of most fluids. One obvious affect of water’s specific heat is the tendency of large bodies of water to maintain nearly constant temperatures; were it less, seasonal differences would be far more extreme. Also, the effectiveness of ocean currents (such as the Gulf Stream) to transfer vast amounts of warm water from the tropics to polar regions would be far less. This would in turn exacerbate low latitude and high latitude temperature variances, resulting in more extreme weather patterns. Ocean currents are responsible for the generation of winds and their subsequent conveyance of water vapor—a major factor in the global distribution of water as rainfall. Changing the value of water’s specific heat, up or down, would dramatically affect Earth’s climate.
Working to help moderate temperature fluctuations in tandem with specific heat is the quality of latent heat. When steam condenses (or when ice freezes), heat is released. Upon evaporation (or melting), liquid water absorbs heat from the environment. Water’s latent heat of freezing and latent heat of vaporization are among the extremes of any fluid at temperatures encountered on Earth. This means that an unusually large amount of energy is involved in either raising or lowering the temperature of a body of water. This quality makes it easier for bigger organisms to maintain ideal body temperatures, since animal cells are predominantly water by weight. Mammals sweat to take advantage of water’s latent heat of vaporization as a means to remove excess heat, a serious physiological challenge. (The same is true for plants, achieving essentially the same result via the process of transpiration.) Another benefit of water’s high latent heat of vaporization is that it lends sea ice and glaciers a resistance to melting, a factor that helps balance equatorial heating and slows the warming of seawater by direct sunlight.
Then there is water’s thermal conductivity—its capacity to “conduct,” or transfer heat. While far lower than most metals and many other solid materials, the thermal conductivity of H₂O is highest among known liquids (and far higher than most). On the other hand, the thermal conductivity of both snow and ice are low. The insulative properties of ice help prevent further heat loss from oceans and lakes. Snow, an even better insulator, helps limit the melting of sea ice and glaciers. In addition, snow’s pristine whiteness makes it a first-rate reflector of sunlight, further slowing ice melt.
Taken together, these unusual properties lend water its ability to act as a buffer—moderating Earth’s overall climate, shielding the planet from rapid or intense temperature fluctuations. Without the unique thermal properties of H₂O, Earth’s surface would be subject to far more violent and severe weather events. Regional climates would be radically different, with extremes of dryness or wetness and humidity, heat and cold.
Water is an extremely versatile solvent. With the important exception of lipids and other hydrocarbons, most chemicals, if only to a slight extent, will dissolve in it. At the same time, water is not too chemically reactive. Individual molecules have a weak magnetic polarity giving water molecules a boomerang-like shape with the slightly negatively charged oxygen nuclei at the apex. This bi-polarity results from oxygen having a somewhat stronger pull on electrons than hydrogen, such that the shared electrons tend to spend more time around the oxygen nucleus. As a consequence, the slightly positive net charge in the region of the hydrogen nuclei, being attracted to other oxygens, forms ever-shifting short chains of water molecules bound by supple hydrogen bonds. (This is what accounts for water’s fluidity in its liquid state.) Water’s being electrically polar is but one more feature that makes water an ideal liquid medium for all essential activities within cells. As previously discussed, water’s chemical aversion to lipids is crucial in the formation and maintenance of biological membranes and the activity of catalysts.
Another property: surface tension, a measure of a liquid surface’s resistance to being stretched or broken. This quality is responsible for drawing water up through the soil to reach plants’ roots and assisting its rise to the tops of tall trees. Water is crucial for soil development; its high surface tension draws water into cracks and fissures in rocks, helping leach out chemicals. In addition, when surface water freezes and expands, the weathering process is accelerated through enlarging cracks and crevices and breaking down rock, which increases surface area, thus allowing more chemicals vital to life to be freed up. And because water has low viscosity compared to most fluids, rain and melting snow further the rapid transport and distribution of these materials via streams and rivers. In this way, nutrients essential to sea-life are made available.
In fact, water’s viscosity is an ideal balance. Were it lower, cells would be unable to withstand forces and loads they are regularly subjected to and delicate cellular structures would not likely survive normal buffeting. Were water’s viscosity significantly higher, many aquatic organisms would likely not exist. The movement of mobile organelles and large molecules through a cell’s cytoplasm would be hindered to the point that physiological processes, normally carried out at fantastic speeds, would no longer proceed. On the other hand, water’s low viscosity is ideal for the diffusion of critical materials on a cellular scale. (The rapid diffusion of small molecules over short distances is a fundamental process for all cells and microorganisms.)
At the end of his lengthy treatment of this wondrous substance, Henderson, in ponderous style, wrote:
In no case do the advantages which [water’s] properties confer seem to be trivial; commonly they are of the greatest moment; and I cannot doubt, even after allowances have been made for the probability of occasional fallacies in the development of an argument which, though simple, is beset with many pitfalls, that they are decisive. Water, of its very nature…is fit, with a fitness no less marvelous and varied than that fitness of the organism which has been won by the process of adaptation in the course of organic evolution…. If doubts remain, let a search be made for any other substance which, however slightly, can claim to rival water as the milieu of simple organisms, as the milieu intérior of all living things, or in any other of the countless physiological functions which it performs either automatically or as a result of adaptation.
The innate capabilities of the elements, the host of bio-friendly substances they form in combination, the suitability of Earth’s many distinctive features—indeed, the universe’s—all work together harmoniously to make life both possible and actual. (Later chapters will take up these matters in detail.) The significance of these facts—and their implications—were not fully appreciated until surprisingly recent times. A major reason for this can be traced to Darwin’s original conception of the basis for his theory, with its emphasis on nature’s unrelenting brutality as opposed to its “gentler” aspects. This point of view is changing, though the process has been a slow one.
©2018 by Tim Forsell draft 14 Apr 2018
The pH scale measures the relative acid/base (H⁺/OHˉ ion) concentration in a solution. It is a logarithmic scale (based on powers of 10) ranging from 0 to 14, with 7 being neutral; anything under 7 is an acidic solution, and anything over 7 is basic. A fluid with pH 3 is thus ten times, not twice, as acidic as one with a pH of 4. Representative examples: stomach acids are pH 2, household bleach about pH 12. Pure water is pH 7 (neutral) and human blood is maintained at a steady 7.4.
Homeostasis refers collectively to a self-regulating process (by way of feedback loops, for instance) used by living things to adjust to conditions and maintain physiological stability.
Oxides are two-element compounds containing oxygen. Other gaseous oxides include carbon monoxide (CO) and several forms of nitrogenic oxides including “laughing gas” (NO) and several components of air pollution.
Ambient temperatures, as used here, refers to the range of environmental temperatures typically encountered on Earth.
Covalent bonds are the result of two substances sharing electrons in order to achieve chemical stability through having their outermost orbital shells filled.
For all substances, specific heat expressed in terms of calories, the amount of heat required to raise the temperature of some quantity of water by one degree celsius. (A calorie is defined as the amount of heat rquired to raise the temperature of one gram of water by 1°C.) Among ordinary liquids, only ammonia has a higher specific heat than water.
At Earth’s ambient temperatures, water’s latent heat of evaporation is greater than any liquid. Its latent heat of freezing is exceeded only by ammonia.
Hydrogen bonds between individual H₂O molecules are around one twentieth as strong as the molecule’s covalent bonds.
Excluding mercury, water has the highest surface tension of all common fluids at ambient temperatures.