Cosmos (7 page)

If we plunged through a pore into the nucleus of the cell, we would find something that resembles an explosion in a spaghetti factory—a disorderly multitude of coils and strands, which are the two kinds of nucleic acids: DNA, which knows what to do, and RNA, which conveys the instructions issued by DNA to the rest of the cell. These are the best that four billion years of evolution could produce, containing the full complement of information on how to make a cell, a tree or a human work. The amount of information in human DNA, if written out in ordinary language, would occupy a hundred thick volumes. What is more, the DNA molecules know how to make, with only very rare exceptions, identical copies of themselves. They know extraordinarily much.

DNA is a double helix, the two intertwined strands resembling a “spiral” staircase. It is the sequence or ordering of the nucleotides along either of the constituent strands that is the language of life. During reproduction, the helices separate, assisted by a special unwinding protein, each synthesizing an identical copy of the other from nucleotide building blocks floating about nearby in the viscous liquid of the cell nucleus. Once the unwinding is underway, a remarkable enzyme called DNA polymerase helps ensure that the copying works almost perfectly. If a mistake is made, there are enzymes which snip the mistake out and replace the wrong nucleotide by the right one. These enzymes are a molecular machine with awesome powers.

In addition to making accurate copies of itself—which is what heredity is about—nuclear DNA directs the activities of the cell—which is what metabolism is about—by synthesizing another nucleic acid called messenger RNA, each of which passes to the extranuclear provinces and there controls the construction, at the right time, in the right place, of one enzyme. When all is done, a single enzyme molecule has been produced, which then goes about ordering one particular aspect of the chemistry of the cell.

Human DNA is a ladder a billion nucleotides long. Most possible combinations of nucleotides are nonsense: they would cause the synthesis of proteins that perform no useful function. Only an extremely limited number of nucleic acid molecules are any good for lifeforms as complicated as we. Even so, the number of useful ways of putting nucleic acids together is stupefyingly large—probably far greater than the total number of electrons and protons in the universe. Accordingly, the number of possible individual human beings is vastly greater than the number that have ever lived: the untapped potential of the human species is immense.
There must be ways of putting nucleic acids together that will function far better—by any criterion we choose—than any human being who has ever lived. Fortunately, we do not yet know how to assemble alternative sequences of nucleotides to make alternative kinds of human beings. In the future we may well be able to assemble nucleotides in any desired sequence, to produce whatever characteristics we think desirable—a sobering and disquieting prospect.

Evolution works through mutation and selection. Mutations might occur during replication if the enzyme DNA polymerase makes a mistake. But it rarely makes a mistake. Mutations also occur because of radioactivity or ultraviolet light from the Sun or cosmic rays or chemicals in the environment, all of which can change the nucleotides or tie the nucleic acids up in knots. If the mutation rate is too high, we lose the inheritance of four billion years of painstaking evolution. If it is too low, new varieties will not be available to adapt to some future change in the environment. The evolution of life requires a more or less precise balance between mutation and selection. When that balance is achieved, remarkable adaptations occur.

A change in a single DNA nucleotide causes a change in a single amino acid in the protein for which that DNA codes. The red blood cells of people of European descent look roughly globular. The red blood cells of some people of African descent look like sickles or crescent moons. Sickle cells carry less oxygen and consequently transmit a kind of anemia. They also provide major resistance against malaria. There is no question that it is better to be anemic than to be dead. This major influence on the function of the blood—so striking as to be readily apparent in photographs of red blood cells—is the result of a change in a single nucleotide out of the ten billion in the DNA of a typical human cell. We are still ignorant of the consequences of changes in most of the other nucleotides.

We humans look rather different from a tree. Without a doubt we perceive the world differently than a tree does. But down deep, at the molecular heart of life, the trees and we are essentially identical. We both use nucleic acids for heredity; we both use proteins as enzymes to control the chemistry of our cells. Most significantly, we both use precisely the same code book for translating nucleic acid information into protein information, as do virtually all the other creatures on the planet.
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The usual explanation
of this molecular unity is that we are, all of us—trees and people, angler fish and slime molds and paramecia—descended from a single and common instance of the origin of life in the early history of our planet. How did the critical molecules then arise?

In my laboratory at Cornell University we work on, among other things, prebiological organic chemistry, making some notes of the music of life. We mix together and spark the gases of the primitive Earth: hydrogen, water, ammonia, methane, hydrogen sulfide—all present, incidentally, on the planet Jupiter today and throughout the Cosmos. The sparks correspond to lightning—also present on the ancient Earth and on modern Jupiter. The reaction vessel is initially transparent: the precursor gases are entirely invisible. But after ten minutes of sparking, we see a strange brown pigment slowly streaking the sides of the vessel. The interior gradually becomes opaque, covered with a thick brown tar. If we had used ultraviolet light—simulating the early Sun—the results would have been more or less the same. The tar is an extremely rich collection of complex organic molecules, including the constituent parts of proteins and nucleic acids. The stuff of life, it turns out, can be very easily made.

Such experiments were first performed in the early 1950’s by Stanley Miller, then a graduate student of the chemist Harold Urey. Urey had argued compellingly that the early atmosphere of the Earth was hydrogen-rich, as is most of the Cosmos; that the hydrogen has since trickled away to space from Earth, but not from massive Jupiter; and that the origin of life occurred before the hydrogen was lost. After Urey suggested that such gases be sparked, someone asked him what he expected to make in such an experiment. Urey replied,
“Beilstein.” Beilstein
is the massive German compendium in 28 volumes, listing all the organic molecules known to chemists.

Using only the most abundant gases that were present on the early Earth and almost any energy source that breaks chemical bonds, we can produce the essential building blocks of life. But in our vessel are only the notes of the music of life—not the music
itself. The molecular building blocks must be put together in the correct sequence. Life is certainly more than the amino acids that make up its proteins and the nucleotides that make up its nucleic acids. But even in ordering these building blocks into long-chain molecules, there has been substantial laboratory progress. Amino acids have been assembled under primitive Earth conditions into molecules resembling proteins. Some of them feebly control useful chemical reactions, as enzymes do. Nucleotides have been put together into strands of nucleic acid a few dozen units long. Under the right circumstances in the test tube, short nucleic acids can synthesize identical copies of themselves.

No one has so far mixed together the gases and waters of the primitive Earth and at the end of the experiment had something crawl out of the test tube. The smallest living things known, the viroids, are composed of less than 10,000 atoms. They cause several different diseases in cultivated plants and have probably most recently evolved from more complex organisms rather than from simpler ones. Indeed, it is hard to imagine a still simpler organism that is in any sense alive. Viroids are composed exclusively of nucleic acid, unlike the viruses, which also have a protein coat. They are no more than a single strand of RNA with either a linear or a closed circular geometry. Viroids can be so small and still thrive because they are thoroughgoing, unremitting parasites. Like viruses, they simply take over the molecular machinery of a much larger, well-functioning cell and change it from a factory for making more cells into a factory for making more viroids.

The smallest known free-living organisms are the PPLO (pleuropneumonia-like organisms) and similar small beasts. They are composed of about fifty million atoms. Such organisms, having to be more self-reliant, are also more complicated than viroids and viruses. But the environment of the Earth today is not extremely favorable for simple forms of life. You have to work hard to make a living. You have to be careful about predators. In the early history of our planet, however, when enormous amounts of organic molecules were being produced by sunlight in a hydrogen-rich atmosphere, very simple, nonparasitic organisms had a fighting chance. The first living things may have been something like free-living viroids only a few hundred nucleotides long. Experimental work on making such creatures from scratch may begin by the end of the century. There is still much to be understood about the origin of life, including the origin of the genetic code. But we have been performing such experiments for only some
thirty years. Nature has had a four-billion-year head start. All in all, we have not done badly.

Nothing in such experiments is unique to the Earth. The initial gases, and the energy sources, are common throughout the Cosmos. Chemical reactions like those in our laboratory vessels may be responsible for the organic matter in interstellar space and the amino acids found in meteorites. Some similar chemistry must have occurred on a billion other worlds in the Milky Way Galaxy. The molecules of life fill the Cosmos.

But even if life on another planet has the same molecular chemistry as life here, there is no reason to expect it to resemble familiar organisms. Consider the enormous diversity of living things on Earth, all of which share the same planet and an identical molecular biology. Those other beasts and vegetables are probably radically different from any organism we know here. There may be some convergent evolution because there may be only one best solution to a certain environmental problem—something like two eyes, for example, for binocular vision at optical frequencies. But in general the random character of the evolutionary process should create extraterrestrial creatures very different from any that we know.

I cannot tell you what an extraterrestrial being would look like. I am terribly limited by the fact that I know only one kind of life, life on Earth. Some people—science fiction writers and artists, for instance—have speculated on what other beings might be like. I am skeptical about most of those extraterrestrial visions. They seem to me to rely too much on forms of life we already know. Any given organism is the way it is because of a long series of individually unlikely steps. I do not think life anywhere else would look very much like a reptile, or an insect or a human—even with such minor cosmetic adjustments as green skin, pointy ears and antennae. But if you pressed me, I could try to imagine something rather different:

On a giant gas planet like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water and ammonia, there is no accessible solid surface, but rather a dense, cloudy atmosphere in which organic molecules may be falling from the skies like manna from heaven, like the products of our laboratory experiments. However, there is a characteristic impediment to life on such a planet: the atmosphere is turbulent, and down deep it is very hot. An organism must be careful that it is not carried down and fried.

To show that life is not out of the question in such a very
different planet, my Cornell colleague E. E. Salpeter and I have made some calculations. Of course, we cannot know precisely what life would be like in such a place, but we wanted to see if, within the laws of physics and chemistry, a world of this sort could possibly be inhabited.

One way to make a living under these conditions is to reproduce before you are fried and hope that convection will carry some of your offspring to the higher and cooler layers of the atmosphere. Such organisms could be very little. We call them sinkers. But you could also be a floater, some vast hydrogen balloon pumping helium and heavier gases out of its interior and leaving only the lightest gas, hydrogen; or a hot-air balloon, staying buoyant by keeping your interior warm, using energy acquired from the food you eat. Like familiar terrestrial balloons, the deeper a floater is carried, the stronger is the buoyant force returning it to the higher, cooler, safer regions of the atmosphere. A floater might eat preformed organic molecules, or make its own from sunlight and air, somewhat as plants do on Earth. Up to a point, the bigger a floater is, the more efficient it will be. Salpeter and I imagined floaters kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.

The floaters may propel themselves through the planetary atmosphere with gusts of gas, like a ramjet or a rocket. We imagine them arranged in great lazy herds for as far as the eye can see, with patterns on their skin, an adaptive camouflage implying that they have problems, too. Because there is at least one other ecological niche in such an environment: hunting. Hunters are fast and maneuverable. They eat the floaters both for their organic molecules and for their store of pure hydrogen. Hollow sinkers could have evolved into the first floaters, and self-propelled floaters into the first hunters. There cannot be very many hunters, because if they consume all the floaters, the hunters themselves will perish.

Physics and chemistry permit such lifeforms. Art endows them with a certain charm. Nature, however, is not obliged to follow our speculations. But if there are billions of inhabited worlds in the Milky Way Galaxy, perhaps there will be a few populated by the sinkers, floaters and hunters which our imaginations, tempered by the laws of physics and chemistry, have generated.