Shadows of Forgotten Ancestors (9 page)
Read Shadows of Forgotten Ancestors Online
Authors: Carl Sagan,Ann Druyan
A GOSPEL OF DIRTI detest all systems that depreciate human
nature. If it be a delusion that there is
something in the constitution of man that is
venerable and worthy of its author, let me live
and die in that delusion, rather than have my
eyes opened to see my species in a humiliating
and disgusting light. Every good man feels his
indignation rise against those who disparage his
kindred
or his
country;
why should it not rise
against those who disparage his
kind?
THOMAS REID
letter of 1775
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When I view all beings not as special creations,
but as the lineal descendants of some few
beings which lived long before the first bed of
the Cambrian [geological] system was
deposited, they seem to me to become
ennobled.
CHARLES DARWIN
The Origin of Species
,
Chapter XV
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M
ankind has conducted an experiment of gigantic proportions,” Darwin wrote in
The Origin of Species
. He was struck by the success of “husbandry,” as it is tellingly called, in generating new varieties of animals and plants useful for humans. Nature provides the varieties and we select who shall reproduce, which traits we want preferentially to propagate into future generations. By transferring pollen from flower to flower with a camel’s hair brush, or by letting the stallion in with the mare, humans take it upon themselves to determine who shall mate with whom. Indigestible crops, weakling horses, scrawny turkeys, sheep with knotty coats, and cows that are grudging with their milk are discouraged from reproducing. Generation after generation, by cumulative selection, humans impress their interests on the heredity of the plants and animals whose breeding they control. But Nature, too, selects those plants and animals which by its lights happen to be more favorably adapted than their fellows; such fortunate beings preferentially reproduce, leave more offspring and, as time goes on, supplant the competition. Artificial selection helps us to understand how natural selection works.
The ability of the environment to nurture and sustain large populations—the so-called carrying capacity—is of course finite. As the number of organisms increases, not all will be able to survive. There will be a stringent competition for scarce resources. Slight differences in ability, imperceptible to a casual observer, may spell life or death to the organism. Natural selection is a great sieve, straining out the vast majority and permitting only a tiny vanguard to pass its heredity on to the next generation. Natural selection is far more ruthless than the most callous and resolute animal breeder in determining the genetic makeup of future generations. And instead of the measly few thousand years since the domestication of animals began in earnest, natural selection has been working for billions.
Consider the diverse specializations that, through artificial selection,
we’ve generated in dogs—greyhounds and borzois for speed, to outrun the wolves; collies for herding sheep; beagles, pointers, and setters for hunting; Labrador retrievers for helping fishermen gather their nets; guide dogs for the blind; bloodhounds for tracking criminals; terriers for worrying prey out of burrows; mastiffs for guard duty; and the original Pekinese (of which only a dwarf remnant remains) for war. We did all that, in only a few thousand years, by meddling with the sex lives of dogs. We evolved cauliflower, rutabaga, broccoli, brussels sprouts, and the now common and luxuriant cabbage from the sorry wild cabbage (these vegetables, like the different breeds of dogs, remain interfertile). Now think of a much more rigorous, much more stringent selection operating on all of Nature over an expanse of time a million times longer—and established not by the conscious meddling of dog or plant breeders with some idea of what kind of dog or plant they’re aiming for, but by a blind, purposeless, and changing environment. If artificial selection represents an experiment of gigantic proportions, what must be the dimensions of the experiment that natural selection has performed? Isn’t it plausible that all the elegantly adaptive diversity of life on Earth could thereby be sifted and extracted? Indeed, it is the only known process that adapts organisms to their environments.
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Here are the passages from Darwin’s
Origin of Species
in which he first develops the point and counterpoint of artificial and natural selection:
One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy. Some variations useful to him have probably arisen suddenly, or by one step … But when we compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain pasture, with the wool of one breed good for one purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for man in different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little quarrelsome, with “everlasting layers” [of eggs, which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history. The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds.
… [H]ardly any one is so careless as to breed from his worst animals …
If there exist savages so barbarous as never to think of the inherited character of the offspring of their domestic animals, yet any one animal particularly useful to them, for any special purpose, would be carefully preserved during famines and other accidents, to which savages are so liable, and such choice animals would thus generally leave more offspring than the inferior ones; so that in this case there would be a kind of unconscious selection going on …
Man … can never act by selection, excepting on variations which are first given to him in some slight degree by nature …
This preservation [in Nature] of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest Variations neither useful nor injurious would not be affected by natural selection …
When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger …
If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees …
There is no reason why the principles which have acted so efficiently under domestication should not have acted under nature. In the survival of favoured individuals and races, during the constantly-recurrent Struggle for Existence, we see a powerful and ever-acting form of Selection. The struggle for existence inevitably follows from
the high geometrical ratio of increase which is common to all organic beings. This high rate of increase is proved by calculation,—by the rapid increase of many animals and plants during a succession of peculiar seasons, and when naturalised in new countries. More individuals are born than can possibly survive. A grain in the balance may determine which individuals shall live and which shall die,—which variety or species shall increase in number, and which shall decrease, or finally become extinct … The slightest advantage in certain individuals, at any age or during any season, over those with which they come into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will, in the long run, turn the balance.
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In his 1858 paper in the Linnaean Society
Proceedings
, he asks us to imagine a being who could continue selecting, with unfailing attention, for a single desired characteristic over “millions of generations.” Natural selection implies—in effect, although not literally—that such a being exists. “We have almost unlimited time” for evolution, he wrote.
Darwin then went on to propose that, over such immense periods of time, continuing natural selection may generate such a divergence of an organism from its parental stock as to constitute a new species. Giraffes develop long necks because those whose necks are—by some spontaneous genetic variation—a little longer are able to browse on the topmost foliage, flourish when others are ill-fed, and leave more offspring than their shorter-necked fellows. He pictured a vast family tree, symbolic of the varied forms of life, slowly growing, branching, and anastomosing, organisms evolving to produce all the “exquisite adaptations” of the natural world.
There is “grandeur,” he thought, in the fact that “from so simple a beginning, endless forms most beautiful and most wonderful have been, and are being evolved.”
Analogy would lead me one step farther, namely, to the belief that all animals and plants are descended from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their cellular structure, their laws of growth, and their liability to injurious influences.… [O]n the principle of natural selection with divergence of
character, it does not seem incredible that, from such low and intermediate form, both animals and plants may have been developed; and, if we admit this, we must likewise admit that all the organic beings which have ever lived on this earth may be descended from some one primordial form.
And how did such a primordial form arise? In 1871, Darwin wistfully imagined, in a letter to his friend Joseph Hooker, “But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed, ready to undergo still more complex changes …”
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If such a thing were possible, why isn’t it happening today? Darwin immediately foresaw one reason: “At the present day, such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.” In addition, we now know that the absence of the oxygen molecule in the atmosphere of the primitive Earth made the formation and survival of organic molecules then much more likely. (And vastly more organic molecules were falling from the sky than do so today in our tidied-up and regularized Solar System.) That warm little pond—or something like it—laboratory experiments show, could have quickly produced the amino acids. Amino acids, energized a little, readily join up to make something like “a proteine compound.” In related experiments, simple nucleic acids are made. Darwin’s guess, as far as it went, is today pretty well confirmed. The building blocks of life were abundant on the early Earth, although we certainly cannot yet say we fully understand the origin of life. But we humans, starting with Darwin, have only just begun to look into the matter.
——
The publication of
The Origin of Species
met, as might have been expected, with a passionate response, both pro and con, including a stormy meeting of the British Association for the Advancement of Science shortly after publication. The larger debate can perhaps best be glimpsed by disinterring the literary reviews of the day. These magazines, generally published monthly, covered the widest range of topics—fiction and nonfiction, prose and poetry, politics, philosophy,
religion, and science. Reviews of twenty printed pages were not uncommon. Almost all articles were unsigned, although many were written by the leading figures in their fields. Comparable publications in the English language seem sparse today, although
The Times
of London’s
Literary Supplement
and
The New York Review of Books
perhaps come closest.
The Westminster Review
of January 1860 recognized that Darwin’s book might be of historic significance:
If the principle of Modification by Natural Selection should be admitted to anything like the extent to which Mr. Darwin would carry it … a grand and almost untrodden field of inquiry will be opened … Our classifications will come to be, as far as they can be so made, genealogies; and will then truly give what may be called the plan of creation.
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