Shadows of Forgotten Ancestors

by Carl Sagan

  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.”7

  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.8 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

  ——

  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 cr
emated. 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, O2, only on Earth. Fire was, much later, to have profound consequences for life and intelligence. One thing leads to another.

  ——

  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.

  Even when the incompleteness of the fossil record is taken into account, the diversity or “taxonomic richness” of life on Earth is found to have been steadily increasing, especially in the last 100 million years.9 Diversity seems to have peaked just as humans were really getting going, and has since declined markedly—in part because of the recent ice ages, but in larger part because of the depredations of humans, both intentional and inadvertent. We are destroying the diversity of beings and habitats out of which we emerged. Something like a hundred species become extinct each day. Their last remnants die out. They leave no descendants. They are gone. Unique messages, painstakingly preserved and refined over eons, messages that a vast succession of beings gave up their lives to pass on to the distant future are lost forever.

  More than a million species of animals are now known on Earth, and perhaps 400,000 species of eukaryotic plants. There are at least thousands of known species of other organisms, non-eukaryotes, including bacteria. Doubtless we have missed many, probably most. Some estimates of the number of species range beyond 10 million; if so, we have even glancing acquaintance with less than 10% of the species on Earth. Many are becoming extinct before we even know of their existence. Most of the billions of species of life that have ever lived are extinct. Extinction is the norm. Survival is the triumphant exception.

  We’ve sketched the changes on the Earth’s surface at the end of the Permian Period, some 245 million years ago; they resulted in the most devastating biological catastrophe so far displayed in the fossil record. Perhaps as many as 95% of all the species then living on Earth became extinct.* Many kinds of filter-feeding animals attached to the ocean floor, beings that had for hundreds of millions of years characterized life on Earth, disappeared. Ninety-eight percent of the families of crinoids became extinct. We don’t hear much about crinoids these days; sea lilies are their surviving remnant. Wholesale extinctions also occurred among the amphibians and reptiles that had settled the land. On the other hand, sponges and bivalves (like clams) did comparatively well in the late Permian extinction—one consequence of which is that they are still plentiful on Earth today.

  Following mass extinctions it typically takes 10 million years or more for the variety and abundance of life on Earth to recover—and then, of course, there are different organisms around, perhaps better adapted to the new environment, perhaps with better long-term prospects, or perhaps not. In the millions of years following the end of the Permian Period, volcanism subsided and the Earth warmed. This killed off many land plants and animals that had been adapted to the late Permian cold. Out of this set of cascading climatic consequences, conifers and ginkgoes emerged. The first mammals evolved from reptiles in the new ecologies established after the Permian extinctions.

  Of all the species of animals alive at the end of the Permian, only about twenty-five of them, it is estimated, have left any descendants at all; ten of which account for 98% of the contemporary families of vertebrates, which comprise about forty thousand species.10 The rate of evolutionary change is full of fits and starts, blind alleys and sweeping change—the latter driven often by the first filling of a previously untenanted ecological niche. New species appear quickly and then persist for millions of years. In only the last 2% or 3% of the history of life on Earth, the extravagant diversification of the placental mammals has produced

  shrews, whales, rabbits and mice, anteaters, sloths, armadillos, horses, pigs and antelopes, elephants, sea cows, wolves, bears, tigers, seals, bats, monkeys, apes, and men11

  For the vast bulk of Earth history, until just recently, not one of these beings had existed. They were present only potentially.

  Think of the genetic instructions of a given being, perhaps a billion ACGT nucleotide pairs long. Randomly change a few nucleotides. Perhaps these will be in structural or inactive sequences
and the organism is in no way altered. But if you change a meaningful DNA sequence, you change the organism. Most such changes, as we keep saying, are maladaptive; except in rare instances, the bigger the change, the more maladaptive it is. For all of mutation, gene recombination, and natural selection put together, the continuing experiment of evolution on Earth has brought into being only a minute fraction of the range of possible organisms whose manufacturing instructions could be specified by the genetic code. The vast bulk of those beings, of course, would be not merely maladapted, not just freaks, but wholly inviable. They could not be born alive. Nevertheless, the total number of possible functioning, living beings is still vastly greater than the total number of beings who have ever been. Some of those unrealized possibilities must be, by any standard we wish to adopt, better adapted and more capable than any Earthling who has ever lived.

  ——

  Sixty-five million years ago most of the species on Earth were snuffed out—probably because of a massive cometary or asteroidal collision. Among those killed off were all the dinosaurs, which had for nearly 200 million years—from before the breakup of Gondwanaland—been the dominant species, the ubiquitous masters of life on Earth. This extinction event removed the chief predators of a small, fearful, cowering nocturnal order of animals called the mammals. If not for that collision—a late step in the tidying up of interplanetary space of the remaining worlds on eccentric orbits—we humans and our primate ancestors would never have come to be. And yet, if that comet had been on a slightly different trajectory, it might have missed the Earth entirely. Perhaps, in its many relays around the Sun, its ices would all have melted and its rocky and organic contents slowly spewed as fine powder into interplanetary space. Then all it would have provided for life on Earth would have been a periodic shower of meteors, perhaps admired by some newly-evolved, curious, large-brained reptile.