Shadows of Forgotten Ancestors (20 page)

Some 2 billion years ago, several different hereditary lines of bacteria seem to have begun stuttering—making full copies of parts of their hereditary instructions over and over again; this redundant information then gradually specialized, and, excruciatingly slowly, nonsense evolved into sense.
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Similar repetitions arose early in the eukaryotes. Over long periods of time, these redundant, repetitive sequences undergo their own mutations, and sooner or later there will be, by chance, rare short passages among them that begin to make sense, that are useful and adaptive. The process is much easier than the classic imaginary experiment of the monkeys poking at typewriter keys long enough that eventually the complete works of William Shakespeare emerge. Here, even the introduction of a very short new sequence—representing only a punctuation mark, say—may be able to increase the survival chances of the organism in a changing environment. And here, unlike the monkeys at their typewriters, the sieve of natural selection is working. Those sequences that are slightly more adaptive (to continue the metaphor, we might say those sequences that correspond even slightly to Shakespearean prose—“TO BE OR,” immersed in gibberish, would be a start) are preferentially replicated. Out of randomly changing nonsense, the accidental bits of sense are preserved and copied in large numbers. Eventually, a great deal of sense emerges. The secret is remembering what works. Just such a drawing forth of meaning from random sequences of nucleotides must have happened in the very earliest nucleic acids, around the time of the origin of life.

An illuminating computer experiment analogous to the evolution of a short DNA sequence was performed by the biologist Richard Dawkins. He starts with a random sequence of twenty-eight English-language letters (spaces are counted as letters):

WDLTMNLT DTJBKWIRZREZLMQCO P.

His computer then repeatedly copies this wholly nonsensical message. However, at each iteration there is a certain probability of a mutation,
of a random change in one of the letters. Selection is also simulated, because the computer is programmed to retain any mutations that move the sequence of letters even slightly toward a pre-selected goal, a particular, quite different sequence of twenty-eight letters. (Of course natural selection does not have some final ACGT sequence in mind, but—in preferentially replicating sequences that improve, even by a little, the fitness of the organism—it comes down to the same thing.) Dawkins’s arbitrarily chosen twenty-eight-letter sequence, toward which his selection was aiming, was

METHINKS IT IS LIKE A WEASEL.

(Hamlet, feigning madness, is teasing Polonius.)

In the first generation, one mutation in the random sequence occurs, changing the “K” (in DTJBKW …) to an “S.” Not much help yet. By the tenth generation, it reads

MDLDMNLS ITJISWHRZREZ MECS P,

and by the twentieth,

MELDINLS IT ISWPRKE Z WECSEL.

After thirty generations, we are at

METHINGS IT ISWLIKE B WECSEL,

and by forty-one generations, we’re there.

“There is a big difference,” Dawkins concludes, “between cumulative selection (in which each improvement, however slight, is used as a basis for future building), and single-step selection (in which each new ‘try’ is a fresh one). If evolutionary progress had had to rely on single-step selection, it would never have got anywhere.”
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Randomly varying the letters is an inefficient way to write a book, you might be thinking. But not if there are an enormous number of copies, each changing slightly generation upon generation, the new instructions constantly tested against the demands of the outside world. If human beings were devising the volumes of instruction contained in the DNA of the given species, we would, we might offhand
imagine, just sit down and write the thing out, front to back, and tell the species what to do. But in practice we are wholly unable to do this, as is DNA. We stress again, the DNA hasn’t the foggiest notion a priori about which sequences are adaptive and which are not. The evolutionary process is not omnicompetent, far-seeing, crisis-avoiding, top-down. It is instead trial-and-error, short-term, crisis-mitigating, bottom-up. No DNA molecule is wise enough to know what the consequences will be if one segment of a message is changed into another. The only way to be sure is to try it out, keep what works, and run with it.

The more you know how to do, the more advanced you are—and, you might think, the better your chances for survival. But the DNA instructions for making a human being comprise some 4 billion nucleotide pairs, while those for a common one-celled amoeba contain 300 billion nucleotide pairs. There is little evidence that amoebae are almost a hundred times more “advanced” than humans, although the proponents of only one side of this question have been heard from to date. Again, some, maybe even most, of the genetic instructions must be redundancies, stutters, untranscribable nonsense. Again we glimpse deep imperfections at the heart of life.

Sometimes another organism inconspicuously slips through the defenses of the eukaryotic cell and steals into the heavily guarded inner sanctum, the nucleus. It attaches itself to the monarch, perhaps to the end of a time-tested and highly reliable DNA sequence. Now messages of a very different sort are dispatched out of the nucleus, messages that order the manufacture of a different nucleic acid, that of the infiltrator. The cell has been subverted.

Besides mutation, there are other ways (including infection and sex, to which we turn shortly) whereby new hereditary sequences arise. The net result is that a huge number of natural experiments are performed in every generation to test the laws, doctrine, and dogma encoded in the DNA. Each eukaryotic cell is such an experiment. Competition among DNA sequences is fierce; those whose commands work even slightly better become fashionable, and everyone has to have one.

The earliest known eukaryotic plankton floating on the surface of the oceans date to about 1.8 billion years ago; the earliest eukaryotes with a sex life to 1.1 billion years ago; the great burst of eukaryote
evolution (that would lead to algae, fungi, land plants and animals, among others) to about the same epoch; the earliest protozoa to about 850 million years ago; and the origin of the major animal groups and the colonization of the land to about 550 million years ago.
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Many of these epochal events may be tied to the increasing atmospheric oxygen. Since the oxygen is generated by plants, we see life forcing its own evolution on a massive scale. Of course, we can’t be sure of the dates; next week paleontologists may discover examples still more ancient. The sophistication of life has increased greatly over the last 2 billion years, and the eukaryotes have done extremely well—as we have only to look around us to verify.

But the eukaryotic kind of life, very different from the rough-and-ready first organisms, is exquisitely dependent on the near-perfect functioning of an elaborate molecular bureaucracy, whose responsibilities include covering up the fits of incompetence in the DNA. Some DNA sequences are too fundamental to the central processes of life to be able safely to change. Those key instructions simply stay fixed, precisely replicated, generation after generation, for aeons. Any significant alteration is simply too costly in the short term, whatever its ostensible virtues may be in the long, and the carriers of such change are wiped out by selection. The DNA of eukaryotic cells reveals segments that clearly and specifically come from the bacteria and archaebacteria of long ago. The DNA inside us is a chimera, long ACGT sequences having been adopted wholesale from quite different and extremely ancient beings, and faithfully copied for billions of years. Some of us—much of us—is
old

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Eventually, there got to be many beings whose cells had specialized functions, just as, for example, the chloroplasts or mitochondria within a given cell have specialized functions. Some cells were in charge of, say, disabling and removing poisons; others were the conduits of electrical impulses, part of a slowly evolving neural apparatus in charge of locomotion, breathing, feelings, and—much later—thoughts. Cells with quite diverse functions interacted harmoniously. Still larger beings evolved separate internal organ systems, and again survival depended on the cooperation of very different constituent parts. Your brain, heart, liver, kidneys, pituitary, and sexual organs
generally work together well. They are not in competition. They make a whole that is much more than the sum of the parts.

Our ancestors and collateral relatives were restricted to the seas until about 500 million years ago, when the first amphibian crawled out onto the land. A significant ozone layer may not have developed until about then. These two facts are probably related. Earlier, deadly ultraviolet light from the Sun reached the surface of the land, frying any intrepid pioneer attempting to homestead there. Ozone, as we’ve mentioned, is produced from the oxygen in the upper air by the Sun’s radiation. So that reckless oxygen pollution of the ancient atmosphere, generated by the green plants, seems to have had another accidental and this time salutary consequence: It made the land habitable. Who would have figured?

Hundreds of millions of years later, a rich biology filled almost every nook and cranny of the land. The moving continental plates now carried with them cargoes of plants and animals and microbes. When new continental crust appeared, it was quickly colonized by life. When old continental crust was carried down into the Earth’s interior, we might be worried that its living cargo would be carried down with it. But the conveyor belt of plate tectonics moves only an inch a year. Life is quicker. Ancient fossils, though, can’t jump off the conveyer belt. They are destroyed by plate tectonics. The precious records and remains of our ancestors are swept down into the semi-fluid mantle and cremated. We are left with the odd remnants that by accident escaped.

Before there was enough oxygen, or anything combustible, fire was impossible, an unrealized potential, latent in matter (just as the release of nuclear energy was unrealized during the tenure of humans on Earth until 1942–1945). There must, therefore, have been an age of the first flame, a time when fire was new. Perhaps it was a dead fern, ignited by a flash of lightning. Since plants colonized the land long before animals, there was no one to notice: Smoke rises; suddenly, a tongue of red flickers upward. Perhaps a little thicket of vegetation has caught fire. The flame isn’t a gas, or a liquid, or a solid.
It’s some other, some fourth state of matter that physicists call plasma. Never before had Earth been touched by fire.

Long before humans made use of fire, plants did. When the population density is high and plants of different species are closely packed together, they fight—for access to nutrients and underground water, but especially for sunlight. Some plants have invented hardy, fire-resistant seeds, along with stems and leaves that readily burst into flames. Lightning strikes, an intense fire burns out of control, the seeds of the favored plant survive, and the competition—seeds and all—has been burned to a crisp. Many species of pines are the beneficiaries of this evolutionary strategy. Green plants make oxygen, oxygen permits fire, and fire is then used by some green plants to attack and kill their neighbors. There is hardly any aspect of the environment that has not been used, one way or another, in the struggle for existence.

A flame looks unearthly, but in this neck of the Cosmos it’s unique to Earth. Of all the planets, moons, asteroids, and comets in our Solar System, there is fire only on Earth—because there are large amounts of oxygen gas, O
₂
, only on Earth. Fire was, much later, to have profound consequences for life and intelligence. One thing leads to another.

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The human pedigree wends its tortuous way back to the beginning of life 4 billion years ago. Every being on Earth is our relative, since we all come from that same point of origin. And yet, precisely because of evolution, no lifeform on Earth today is an ancestor of ours. Other beings did not stop evolving because a pathway that would someday lead to humans had just been generated. No one knew what branch in the evolutionary tree was going where, and no one before humans could even raise the question. The beings from whom our ancestral line deviated continued to evolve, inside and out, or became extinct. Almost all became extinct. We know from the fossil record something of who our predecessors were, but we cannot bring them into the laboratory for interrogation. They are no more.

Luckily, though, there are organisms alive today that are similar—in some cases, very similar—to our ancestors. The beings that left stromatolite fossils probably performed photosynthesis and in other
respects behaved as contemporary stromatolitic bacteria do. We learn about them by examining their surviving close relatives. But we cannot be absolutely sure. For example, ancient organisms were not necessarily and in all respects simpler than modern ones. Viruses and parasites, in general, show signs of having evolved by loss of function from some more self-sufficient forebear.

Many features in the biological landscape arrived late. Sex, for example, doesn’t seem to have evolved until three quarters of the history of life till now had passed. Animals big enough for us to see—had we been there—animals made of many different kinds of cells, also do not seem to have emerged until almost three quarters of the way between the origin of life and our time. Except for microbes, there were no beings on the land until something like 90%, and no creatures with big brains for their body sizes until about 99% of the history of life thus far was over.

Enormpus gaps yawn through the fossil record, although less so now than in Darwin’s time. (If there were more paleontologists in the world, we’d doubtless be a little further along.) From the comparatively low rate of discovery of new fossils, we know that huge numbers of ancient organisms have not been preserved. There’s something poignant about all those species—some ancestral to humans, on some sturdy trunk of our family tree, most not—about whom we know nothing, not a single example of them having survived, even in fossil form, to our own time.