A human embryo - or for that matter any kind of embryo - begins life as a single cell, which promptly starts dividing into progeny cells until a critical mass of apparently identical cells is formed. Then, as though a bell had sounded, it begins the process of differentiation, and cells with specialized functions for the future organism make their appearance and migrate to one region or another for the formation of tissues and organs. The current view is that the sorting out of cellular elements is organized and governed by a system of chemical signals exchanged among the cells. The nature of these signals and their specific receptors at each cell surface are presumably determined and controlled by that very first cell and passed along to its offspring.
It is not at all understood how this system works, even though we have a clear and detailed picture of the structural changes that take place at every stage of the process, and small bits of information about some of the chemical messages in the differentiation of certain cells. The phenomenon of embryologic development and differentiation is generally regarded as one of the two most profound unsolved problems in human biology - the other being the operation of the brain. In both cases, the core of the mystery is the cooperative and collaborative behavior of the cells themselves. The embryo develops from a single cell into an elaborately complex structure, a baby, made up of trillions of cells, each one specialized for doing what it is supposed to do and confining its activities to its designated anatomical area but kept in communication with all the rest by chemical advertisements. The brain is made up of billions of neurons arranged in wiring networks of a complexity beyond comprehension, but under the governance as well of chemical signals which regulate the firing and response of every cell.
The life of the earth resembles that of an embryo, and the life of our species within the life of the earth resembles that of a central nervous system. The earth itself is an organism, still developing and differentiating.
The planet formed as a solid oblate sphere and swung into its orbit roughly 4.7 billion years ago. Less than a billion years later, the first life appeared. We do not have an attested date for this, but we do have fossil evidence for the existence of chains of bacteria resembling streptococci some 3.7 billion years old.
Nobody knows how the first living thing was put together, although hypotheses abound. It is, however, a near certainty that it was a cell, and most likely a cell resembling one of today's bacteria. It might have been a virus, but if so it would have to have been a virus carrying genetic specifications for making a cell. The earth was still very hot in many places 4 billion years ago, and it is possible that life may have turned up first in a very hot place. Indeed, it makes it easier to account for so deeply improbable an event if we assume a high temperature. Most of our guesses about the origin of life postulate, necessarily, a series of random accidents - the presence of amino acids and precursors of nucleotides in the water covering most of the planet, the assembling of these building blocks into more complex nucleic acids and protein molecules under the influence of lightning or intense ultraviolet light, the formation of biological membranes to enclose reactants, and presto, life. One trouble with the scenario is the time required for the right sequence of events to occur at what has always been assumed to be today's optimal temperature for life. Each step by itself would be almost inconceivably improbable, but if the events were to have occurred at very high temperatures, with everything greatly speeded up, a billion years doesn't seem so short a time. It now becomes a possibility, for speculation anyway, that the original ancestors of all the planet's life were bacteria of this sort.
We have inherited many, perhaps all, of our systems for intercellular communication from our bacterial ancestors. Investigators at the National Institutes of Health have recently discovered that certain bacteria manufacture protein molecules indistinguishable from insulin in their properties. Other microorganisms elaborate peptide messengers identical to those used by specialized cells in our bodies for the regulation of brain function and for switching on the activity of our thyroid, adrenal, ovarian, and digestive cells. We did not invent our steroid hormones; molecules like these were probably being made for other reasons 2 billion years ago by our bacterial forebears. Biochemically speaking, there is nothing new under the sun.
The level of oxygen in the atmosphere has gradually increased because of the steady increase in the earth's population of photosynthetic organisms - some of them still in their original bacterial form, the blue-green algae living in the waters of the earth, others now in the more complex and various forms of higher plants. From a level close to zero 3.5 billion years ago, oxygen now stands at 20 percent of the earth's atmosphere, all of it the product of life itself. There is another feature of that level of oxygen that seems to be equally remarkable: it stabilized at its present level around 400 million years ago, and it seems to be fixed there. It is a lucky thing for us, and for the life of the earth, that it did stabilize at that concentration. If it were to increase by more than two percentage points, most of the planet would ignite. If it were to decrease a few points, most of the life would suffocate. It is nicely balanced, against all hazards, at an absolutely optimal level.
Other gases in the atmosphere, including carbon dioxide, nitrogen, and methane, also seem to have been regulated and stabilized over long periods of time at optimal concentrations, despite the constant intervention of natural forces tending to push them up or down. At the moment, which is to say over the past century, the level of carbon dioxide has been rising slowly due to the heavy increase in the burning of fossil fuels, but the climatic consequences of this rise have not yet been observed.
Methane exists as a quantitatively minor constituent of the atmosphere, although it plays a crucial role in the interests of living things. Most of what is there is the product of life itself, the tremendous populations of methanogenic bacteria in the soil and water, in the intestinal tracts of ruminant vertebrates, and (a substantial source) in the hindgut of termites. How it is regulated so that the methane concentrations are everywhere fixed and stable is not known, but it is known that if the level were to decrease appreciably the concentration of oxygen in the atmosphere would begin to rise to hazardous levels. There is probably a feedback loop, in which methane serves as a regulator of oxygen and vice versa.
The mean temperature at the surface of the earth has also remained remarkably stable over stretches of geological time, perhaps due in large part to the relatively stable concentrations of carbon dioxide in the atmosphere. From time to time fluctuations have occurred, with the cyclical development of ice ages as the result, but over all time, the temperature stays much the same. This also suggests a regulatory mechanism of some sort, since the radiant heat coming from the sun has increased by approximately 30 percent since life began.
Lovelock and Margulis proposed in 1972 that life on the planet has been chiefly responsible for the regulation of that life's own environment. They postulated that the stability of the constituents of the earth's surface, and the pH and salinity of the oceans, are held more or less constant, and at optimal levels for life, by intricate loops of feedback reactions involving microbial, plant, and animal life. The concept is analogous to the phenomenon of homeostasis within a multicellular organism, as outlined by Claude Bernard and later elaborated by Walter Cannon to explain the stability of the internal environment of the human body. If one tissue component begins to change, other sets of components will respond by changing things back again to where they were.
The "Gaia Hypothesis," as Lovelock and Margulis termed their theory,plainly implies that the conjoined life of the earth behaves like a huge, coherent, self-regulating organism. It is a notion that has aroused both skepticism and antipathy within the biological community, especially among evolutionary biologists. They do not much like the name, for one thing, with its undertones of deity and deification. For another, they doubt its compatibility with the solid body of evolutionary theory. How could such a creature have evolved, they ask, in the absence of anything to be selected against? Moreover, they object to the idea that evolution can plan ahead for future contingencies - for example, the adjustment of atmospheric gases to provide optimal conditions for forms of life that have not yet come into existence. I will not deal with these objections here, except to remark that similar puzzles confront biologists who wish to explain the development of an embryo. I can find it in my imagination to suppose that once that first primordial cell came on the scene, equipped with a molecule of DNA for its replication but also, more importantly, for its progressive mutation into new cellular forms with new strategies for living, a system had come into existence. When a living system becomes sufficiently complex, it automatically provides a series of choices among strategies for future contingencies. When these turn up, it has the look of planning and purposiveness, but these are the wrong words. What we do not know enough about are the strategies never used, for contingencies that never turned up.
This is not to suggest that the earth's environment is perpetually benign, nor that accidents cannot happen. To the contrary, immense accidents are the rule, but they have been spaced far enough apart in geologic time so that whatever the damage, life itself has always had time to recover and reappear in a new abundance of more complex forms. The most devastating accident on the paleontological record was the mass extinction of the late Permian, 225 million years ago, when at least 50 percent of the marine fauna were lost. The catastrophe was the result of the coalescing of the world's continents into a supercontinent (now referred to geologically as Pangea), which eliminated most of the shallow sea habitats available to the marine creatures then alive. The second greatest extinction occurred around 65 million years ago, when half of the marine creatures and many terrestrial animals, including all the dinosaurs, simply vanished. Various explanations for this extinction have been proposed, including volcanic eruptions or an asteroid collision resulting in earthwide clouds of dust blotting out the sun for long enough time to bring most plant and animal life to an end.
We are not finished with great extinctions. The current anxiety in some biological quarters is that the next one may be just ahead, and will be the handiwork of man.
At a national meeting of biologists and biogeographers held in Arizona in August 1983, the history and dynamics of extinction were the topics of discussion. The consensus was that the number and diversity of living species may be on the verge of plummeting to a level of extinction matching the catastrophe that took place 65 million years ago, and that this event will probably occur within the next hundred years and almost certainly before two hundred years. It will be caused, when it occurs, by the worldwide race for agricultural development, principally in the poorer countries, and by the appalling rate of deforestation. Although tropical forests cover only around 6 percent of the earth's land, they harbor at least 66 percent of the world's biota, animals, plants, birds, and insects. They are currently being destroyed at the rate of about 100,000 square kilometers per year. Elsewhere on the planet, urban development, chemical pollution (especially of waterways and shoreline ecosystems), and the steady increase in atmospheric carbon dioxide are posing new problems for a multitude of species. The animal species chiefly at risk for the near term is humankind. If there is to be a mass extinction just ahead, we will be the most conspicuous victims. Despite our vast numbers, we should now be classifying ourselves as an immediately endangered species, on grounds of our total dependence on other vulnerable species for our food, and our simultaneous dependence, as a social species, on each other.
But do not worry about the life of the earth itself. No extinction, no matter how huge the territory involved or how violent the damage, can possibly bring the earth's life to an end. Even if we were to superimpose on the more or less natural events now calculated to be heading toward a mass extinction the added violence and radioactivity of a full-scale, general nuclear war, we could never kill off everything. We might reduce the numbers of species of multicellular animals and higher plants to a mere handful, but the bacteria and their resident viruses would still be there, perhaps in greater abundance than ever because of the expanding ecosystems created for them by so much death. The planet would be back where things stood a billion years ago, with no way of predicting the future course of evolution beyond the high probability that, given the random nature of evolution, nothing quite like us would ever turn up again.
If the ecologists are right in their predictions, we are confronted by something new for humanity, a set of puzzles requiring close attention by everyone. It is something more than an international problem to be dealt with by the specialists in each nation who deal with matters of foreign policy. Human beings simply cannot go on as they are now going, exhausting the earth's resources, altering the composition of the earth's atmosphere, depleting the numbers and varieties of other species upon whose survival we, in the end, depend. It is not simply wrong, it is a piece of stupidity on the grandest scale for us to assume that we can simply take over the earth as though it were part farm, part park, part zoo, and domesticate it, and still survive as a species.
Up until quite recently we firmly believed that we could do just this, and we regarded the prospect as man's natural destiny. We thought, mistakenly, that that was how nature worked. The strongest species would take over. The weak would be destroyed and eaten or used in other ways, or pushed out of the way - nature red in tooth and claw. All that. We are about to learn better, and we will be lucky if we learn in time.
Getting along in nature is an art, not a combat by brute force. It is more like a great, complicated game of skill.
Altruism is one of the strange biological facts of life, puzzling the world of biology ever since Darwin. How can one explain the survival of any species in which certain members must, as a matter of routine, and under what appears to be genetic instructions, sacrifice their own lives in the interests of the group? At first glance, the theory of natural selection would seem to mandate the permanent elimination of any creature behaving this way.
Altruism is, on its face, a paradox, but it is by no means an exceptional form of behavior. It is extremely interesting to biologists, but not because it is freakish or anomalous. In most of the social species of animals altruism is essential for continuation of the species, and it exists as an everyday aspect of living. It is perhaps not so much an everyday aspect of human behavior, and there is no way of proving or disproving a genetic basis for its display when it does occur. Sociobiologists - E. O. Wilson, for example - believe that human altruism is genetically governed and exists throughout our species, whether or not in latent or suppressed form. Others, the antisociobiology faction, do not believe there is any evidence for altruistic genes at all, and attribute behavior of this kind solely to cultural influences. They do not, of course, deny the existence of human altruism; they simply deny that it is an inheritable characteristic. For all I know, either side of the argument could be right, but I would insert a footnote here with the reservation that human culture itself is not all that nonbiological a phenomenon. We may not be inheriting genes for individual items of cultural behavior, but surely we are dominated by genes for language, hence for culture itself, whatever its manifestations.
Altruism remains a puzzle, but an even deeper scientific quandary is posed by the pervasive existence of cooperative behavior, all through nature. To explain this we cannot fall back on totting up genes and doing arithmetic to estimate the evolutionary advantages to kinships. And yet it is there, and has been since the beginnings of life. The biosphere, for all its wild complexity, seems to rely more on symbiotic arrangements than we used to believe, and there is a generally amiable aspect to nature that needs more acknowledgment than we have tended to give it in the past.
We are not bound by our genes to behave as we do. Most other creatures - not all, surely, but most of them - do not have the option of introducing new programs for their survival, at will. They behave as they do, cooperate as they generally tend to cooperate, in accordance with rigid genetic specifications. It may be, probably in fact is, that we are similarly instructed, but only in very general terms, with options for changing our minds whenever we feel like it. Our options, and our risks of folly, are made more complicated by the possession of language. Using language makes it easy for us to talk ourselves out of cooperating, but the very changeability of our collective minds gives us a chance at survival. We can always, even at the last ditch, change the way we behave, to each other and to the rest of the living world. Since the time ahead cannot any longer be counted as infinite time, and since we tend to keep talking by our very nature, perhaps we still have time to mend our ways.
There are two immense threats hanging over the world ecosystem. Both of them are of our doing, and if they are to be removed we - humankind - will have to do the removing.
The first is damage to the earth we have already begun to inflict by our incessant demands for more and more energy. Although, as I said earlier, we have not yet changed the earth's climate, it is a certainty that we will do so sometime within the next two centuries, probably sooner rather than later. We are not only interfering with the balance of constituents in the atmosphere, placing more carbon dioxide there than has ever existed before by the way we burn fossil fuels and wood, risking several degrees of increase in the mean temperature of the whole planet. We are also risking a significant depletion of the thin layer of ozone in the outer atmosphere, principally by the nitrogen oxides associated with pollution. It is a telling example of the way we think about global problems that we always talk of the ozone layer as our own personal protection against human skin cancer, as if nothing else mattered. The ecological outcome of a significant depletion of the ozonosphere would matter considerably more. A 50 percent increase in the ultraviolet band would increase the amount of UV-B at the higher energy end of the band by a factor of about fifty times. The energy of these wavelengths would have highly destructive effects on plant leaves, oceanic plankton, and the immune systems of many mammals, and could ultimately blind most terrestrial animals.
We ought to be learning much more than we know about the day-to-day life of the earth, in order to catch a clearer glimpse of the hazards ahead. One way to begin learning would be to make better use of the technologies already at hand for the world's space programs. Somewhere on the list of NASA's projects for the future is the so-called Global Habitability program, a venture designed to make a close-up, detailed, deeply reductionist study of the anatomy, physiology, and pathology of the whole earth. The tools possessed by NASA for this kind of close, year-round scrutiny are flabbergasting, and better ones are still to come if the research program can be adequately funded. Already, instruments in space can make quantitative records of the concentrations of chlorophyll in the sea - and by inference the density of life; the acre-by-acre distribution of forests, fields, farms, deserts, and human living quarters everywhere on earth; the seasonal movements of icepacks at the poles and the distribution and depth of snowfalls; the chemical elements in the outer and inner atmosphere; and the upwelling and downwelling regions of waters of the earth. It is possible now to begin monitoring the planet, spotting early evidences of trouble ahead for all ecosystems and species, including ourselves.
The Global Habitability program could become an example of international science at its most useful and productive, if it could only be got under way. Right now, the chances of getting it set at the high priority it needs for funding seem slim. It has the disadvantage of offering only long-term benefits, which means political trouble at the outset. It is no quick fix. It is research for the decades ahead, not just the next few years. And it cannot be done on the cheap, which means wrangles over the budget in and out of Congress. And finally, it will require a full-time, steady collaborative effort by scientists from many different disciplines in science and engineering, and from virtually every country on the face of the earth, which means international politics at its most difficult. But it ought to be launched, and soon, no matter what the difficulties, for it would be a piece of science in aid of the most interesting object in the know universe, and the loveliest by far.
I said a moment ago that there were two great threats to the planet's viability as a coherent ecosystem. The second one is not a long-term one. It hangs over the earth today, and will worsen every day henceforth. It is thermonuclear warfare.
It is customary to estimate the danger of this new military technology in terms of the human lives that are placed at risk. We read that in the event of a full-scale exchange in the northern hemisphere, involving something like 5,000 megatons of explosives, perhaps 1 billion lives would be lost outright from blast and heat and another 1.5 billion would die in the early weeks or months of the aftermath. With more limited exchanges, say 500 megatons, the human deaths could be correspondingly reduced. We even hear arguments these days over the acceptable number of millions of deaths that either side could afford in a limited war without risking the loss of society itself, as though the only issue at stake were human survival.
But a lot of other things would happen in a thermonuclear war, more than the general public is aware of or informed about. What we call nature is itself intimately involved in the problem.
According to a study by a committee of biologists and climatologists for the Conference on the Long-Term Biological Consequences of Nuclear War, there are several probable events that will occur. Assuming that most or all of the detonations take place at ground level, the amount of dust and soot exploded into the atmosphere may darken the underlying earth over the entire Northern Hemisphere for a period of several months up to one year. The sunlight might be 99 percent excluded, and the surface temperatures in continental interiors would fall abruptly to below -40ūC, effectively killing most plants and all forests. In the tropical zones, the loss of forests could destroy a majority of the planet's species. The photosynthetic and other planktonic organisms in the upper layers of the oceans would be killed, and the foundation of most marine food chains eliminated.
The new and extensive temperature gradients between the oceans and land masses will bring about unprecedented storms at all coastal areas, with destruction of many shallow-water ecosystems.
Radioactive fallout in areas downwind from the fireballs is estimated to expose 5 million square kilometers to 1,000 rads or more, most of this within forty-eight hours. This exposure is much higher than in any previous scenario, and is enough to kill most vertebrates and almost all forms of plant life in the affected area, including the conifers that make up the forests in the cooler regions of the Northern Hemisphere.
Later on, months after the event, things will get worse. The ozonosphere will be gone, or nearly gone, and the planet will then be exposed to the full, lethal energy of ultraviolet radiation as soon as the dust and soot have cleared away. It was only because of the protective action of ozone that complex, multicellular organisms were able to gain a foothold in life a billion years ago, and most of these creatures are still as vulnerable as ever to ultraviolet light.
The Southern Hemisphere will be less affected, assuming that the nuclear exchange is confined to the north, but extensive damage is still inevitable throughout the globe, most of it due to chilling.
Bacterial species are less vulnerable to radioactivity and cold than are the higher organisms, but many species in the soil will be lost in the initial heat of fireballs or in the later firestorms and wildfires covering huge areas.
It is not known how many forms of life would be permanently lost. After a period of years, some of the surviving species might reestablish themselves and set up new ecosystems, but there is no way of predicting which ones, or what sorts of systems, beyond the certainty that everything would be changed.
In such an event, the question of the survival of human beings becomes almost a trivial one. To be sure, some might get through, even live on, but under conditions infinitely more hostile to humans than those that existed 1 or 2 million years ago when our species first made its appearance. Civilization, and the memory of culture, would be gone forever. Given the kinds of brains possessed by our species, and the gift of memory, all that might be left to the scattered survivors, staring around, would be the sense of guilt, for having done such damage to so lovely a creature, and a poor heritage for a poor beginning.
All material (except for some code, external links and Optional Readings) © Jeffery A. Schneider, 2003