Eight Little Piggies

by Stephen Jay Gould

  Please don’t read this essay as a bloated effort in the soft tradition of, dare I say it, liberal academic apologies for human harshness, or wishy-washy, far-fetched attempts to make humans look good in a world of woe. This is not an essay about optimism; it is an essay about tragedy. If I felt that humans were nasty by nature, I would just say, the hell with it. We get what we deserve, or what evolution left us as a legacy. But the center of human nature is rooted in ten thousand ordinary acts of kindness that define our days. What can be more tragic than the structural paradox that this Everest of geniality stands upside down on its pointed summit and can be toppled so easily by rare events contrary to our everyday nature—and that these rare events make our history. In some deep sense, we do not get what we deserve.

  The solution to our woes lies not in overcoming our “nature” but in fracturing the “great asymmetry” and allowing our ordinary propensities to direct our lives. But how can we put the commonplace into the driver’s seat of history?

  20 | The Declining Empire of Apes

  GOD MUST HAVE created mistakes for their wonderful value in illuminating proper pathways. In all of evolutionary biology, I find no error more starkly instructive, or more frequently repeated, than a line of stunning misreason about apes and humans. I have been confronted by this argument in a dozen guises, from the taunts of fundamentalists to the plaints of the honorably puzzled. Consider this excerpt from a letter of April 1981: “If evolution is true, and we did come from apes, then why are there still apes living. It seems if we evolved from them they should not be here.”

  If we evolved from apes, why are apes still around? I label this error instructive because its correction is so transforming: If you accept a false notion of evolution, the statement is a deep puzzle; once you reject this fallacy, the statement is evident nonsense (in the literal sense of unintelligible, not the pejorative sense of foolish).

  The argument is nonsense because its unstated premise is false. If ancestors are groups of creatures that are bodily transformed, each and every one, into descendants, then human existence would preclude the survival of apes. But, plainly, we mean no such thing in designating groups as ancestors—lest no reptiles remain because birds and mammals evolved or no fishes survive because amphibians once crawled out upon the land.

  Ladders and bushes, the wrong and right metaphors respectively for the topology of evolution, resolve the persistent non-puzzle of why representatives of ancestral groups (apes, for example) can survive alongside their descendants (humans, for example). Since evolution is a copiously branching bush, the emergence of humans from apes only means that one branch within the bush of apes split off and eventually produced a twig called Homo sapiens, while other branches of the same bush evolved along their own dichotomizing pathways to yield the other descendants that share most recent common ancestry with us—gibbons, orangutans, chimps, and gorillas, collectively called apes. (These modern apes are, by genealogy, no closer than we are to the common ancestor that initiated the ape-monkey split more than 20 million years ago, but human hubris demands separation—so our vernacular saddles all modern twigs but us with the ancestral name ape. The figure and its caption should make this clear.)

  A Few Branches on the Bush of Apes and

  Old World Monkeys

  The genealogical sequence of branching in the evolution of apes and humans.

  The proper metaphor of the bush also helps us to understand why the search for a “missing link” between advanced ape and incipient human—that musty but persistent hope and chimera of popular writing—is so meaningless. A continuous chain may lack a crucial connection, but a branching bush bears no single link at a crucial threshold between no and yes. Rather, each branching point successively restricts the range of closest relatives—the ancestors of all apes separate from monkeys, then gibbon lineages from ancestors of other great apes and humans, then forebears of the orangutan from the chimp-gorilla-human complex, finally precursors of chimps from the ancestors of humans. No branch point can have special status as the missing link—and all represent lateral relationships of diversification, not vertical sequences of transformation.

  An even more powerful argument on behalf of the bush arises from the reanalysis of classical ladders in our textbooks, particularly the evolution of modern horses from little eohippus and the “ascent of man” from “the apes.” A precious irony—life’s little joke—pervades these warhorses of the ladder: The “best” examples must be based upon highly unsuccessful lineages, bushes so pruned of diversity that they survive as single twigs. (See my essay “Life’s Little Joke” in Bully for Brontosaurus for a fuller version of this argument.)

  Successful bushes never enter our texts as classical trends, because they boast too many related survivors, and we can draw no rising ladder for the evolution of antelopes, rodents, or bats—although these are the three great success stories of mammalian evolution. But if only one twig survives, we apply a conceptual steamroller and linearize its labyrinthine path of lateral branching back to the main stem of its depleted bush. Horses, rhinos, and tapirs are not glorious culminations of ascending series within the Perissodactyla (odd-toed hoofed mammals) but three little twigs, barely hanging on, the remnants of a bush that once dominated the diversity of large mammalian herbivores. Similarly, we can specify a ladder of human ascent only because the bush of apes has dwindled to a few surviving twigs, all clearly distinct. If the bush of apes were vigorous and maintained a hundred branchlets evenly spaced at an expanding periphery, we would have many cousins and no chain of unique ancestors. Our vaunted ladder of progress is really the record of declining diversity in an unsuccessful lineage that then happened upon a quirky invention called consciousness.

  This argument against human arrogance can be grasped well enough as an abstraction but becomes impressive only with its primary documentation—the record of vigorous diversity among apes in former times of greater success. The theme of previous vigor has recently received a boost from a new discovery—one that I had the great good fortune to witness last year.

  The cercopithecoid, or Old World, monkeys are the closest relatives of the ape-human bush. Robert Jastrow, in his recent, popular book, The Enchanted Loom: Mind in the Universe, contrasts the evolutionary fate of these two sister groups:

  The monkey did not change very much from the time of his appearance, 30 million years ago, to the present day. His story was complete. But the evolution of the ape continued. He grew large and heavy, and descended from the trees.

  This statement, so preciously wrong, so perfectly arse-backward, shows just how far astray the metaphor of the ladder can lead. There is no such creature, not even as a useful abstraction, as the monkey or the ape. Evolution’s themes are diversity and branching. Most apes (gibbons and orangutans, and chimps and gorillas a good part of the time) are still living in trees. Old World monkeys have not stagnated; they represent the greatest success story among primates, a bush in vigorous radiation and including among its varied products baboons, colobins, rhesus and proboscis monkeys.

  In fact, precisely opposite to Jastrow’s claim, apes have been continuously losing and cercopithecoids gaining by the proper criteria of diversity and expansion of the bush. Let us go back to the early Miocene of Africa, some 20 million years ago, soon after the ape-monkey split, and trace the fate of these two sister groups. First of all, we would not find these Miocene ancestors as different from each other as their descendants are today—limbs of a bush usually diverge. Early Miocene apes were quite monkey-like in their modes of life. Compared with monkey forebears, early apes tended to be larger, more tree bound, more narrowly tied to fruit eating, and less likely to cope with a strongly seasonal or open environment.

  Second—and the crucial point for this essay—apes were more common in two important senses during the early Miocene: more common than cercopithecoid monkeys at this early stage in their mutual evolution, and absolutely more diverse (just in Africa) than apes are today (all over the w
orld). Taxonomic estimates vary, and this essay cannot treat such a highly technical and contentious literature, but early Miocene African apes have been placed in some three to five genera and perhaps twice as many species.

  The next snapshot of time, the African middle Miocene, already records fewer species, although apes now appear for the first time in the fossil records of Europe and Asia. Old World monkeys meanwhile begin an acceleration extending right to our own time. Apes continue to decline and hang on in restricted habitats—yielding isolated groups of gibbons and orangutans in Asia, and chimps, gorillas, and the descendants of a small African group called australopithecines. If the resident zoologist of Galaxy X had visited the earth 5 million years ago while making his inventory of inhabited planets in the universe, he would surely have corrected his earlier report that apes showed more promise than Old World monkeys and noted that monkeys had overcome an original disadvantage to gain domination among primates. (He will confirm this statement after his visit next year—but also add a footnote that one species from the ape bush has enjoyed an unusual and unexpected flowering, thus demanding closer monitoring.)

  We do not know why apes have declined and monkeys prevailed. We have no evidence for “superiority” of monkeys; that is, for direct struggles of Darwinian competition between apes and monkeys in the same habitat, with ape extinction and cercopithecoid prevalence as a result. Perhaps a greater flexibility in diet and environmental tolerance allowed monkeys to gain the edge, without any direct competition, in a world of changing climate and fewer stable habitats of trees and fruit. According to this interpretation, those few apes that could adapt to a more open, ground-living existence, had to develop some decidedly odd features, not in any way “prefigured” by their initial design—the knuckle walking of chimps and gorillas, and the upright gait of australopithecines and you know who.

  This striking reversal of Jastrow’s homily, and of all standard biases of the ladder, rests most forcefully upon the comparison of initial Miocene success with later restriction of the bush of apes. But how great was this first flowering, and how severe, therefore, the later pruning? Unfortunately, this most crucial of all empirical questions encounters the cardinal problem of our woefully imperfect fossil record. We know the extent of later pruning; it is not likely that any living species of ape remains undiscovered on our well-explored earth. But what was the true diversity of early Miocene apes? Did they live only in Africa? What fraction of the African fauna has been preserved? What have we collected and identified of the material that has been preserved?

  If our current collections contain most of what actually lived, then the pruning has been notable but modest. But suppose that we have only 10 percent or even only half the true diversity, then the story of decline and restriction among apes is far more pronounced. How can we know how much we have?

  One rough indication—about the best we can do at this early stage of knowledge about Miocene primates in Africa—comes from the composition of new collections. Suppose that every time we find new early Miocene apes in Africa, they belong to species already in our collections. After several repetitions (particularly if our collections span a good range of geographies and environments), we might conclude that we have probably sampled a substantial amount of the true bush. But suppose that new sites yield new species most of the time—and that we can mark no real decline in the number of novelties. Then we might conclude that we have sampled only a small part of a much more copious bush—and that the story of decline and shortfall in the empire of apes has been more profound than we realized. Quite an effective antidote to the bias of the ladder and its attendant invitation to human arrogance!

  In other words, we are seeking, as my colleague David Pilbeam, our leading student of fossil apes, said to me, “an asymptote” in the discovery of new apes. An asymptote is a limiting value approached by one variable of a curve as the other variable (often time or number of trials) increases towards infinity. When further collecting of fossils only yields more specimens of the same species, we have probably reached the asymptote in recoverable kinds of apes. We also reach asymptotes fairly quickly in training cats or cajoling children and should learn to recognize both the subtle point of diminishing returns and the actual asymptote not much further down the line.

  An exciting discovery about the history of Miocene apes has recently furnished our best evidence that we have not yet come near the asymptote of the early bush of apes. This discovery provides the strongest possible evidence for an even greater intensity of life’s little joke in our own evolution. The bush was bushier, the later decline in diversity more profound. We do not yet know the true extent of the initial success of apery.

  In January 1986, I spent a week with Richard Leakey at his field camp on early Miocene sediments near the western shore of Lake Turkana in Africa’s Great Rift Valley. Little vegetation obscures the geology of this arid region, and naked sediments stretch for miles, their eroding fossils littering the surface.

  The data on genetic differences between chimps and humans suggest that our twig on the bush of apes last shared a common ancestor with chimps some 5 to 8 million years ago; in other words, the human lineage has been entirely on its own only for this short stretch of geological time. The oldest human fossils are less than 4 million years old, and we do not know which branch on the copious bush of apes budded off the twig that led to our lineage. (In fact, except for the link of Asian Sivapithecus to the modern orangutan, we cannot trace any fossil ape to any living species. Paleontologists have abandoned the once popular notion that Ramapithecus might be a source of human ancestry.) Thus, sediments between 4 and 10 million years in age are potential guardians of the Holy Grail of human evolution—the period when our lineage began its separate end run to later domination and a time for which no fossil evidence exists at all.

  Richard Leakey almost surely has many square miles of good sediment from this crucial time in his field area at West Turkana. But he is not yet searching these beds. He is concentrating his efforts on older rocks of the early Miocene (15 to 20 million years ago) when the bush of apes had its great initial flowering in Africa. He is working before the time of maximal intrigue for several reasons. In part, he may be saving the best for later, perfecting his techniques and “feel” for the region before zeroing in on the potential prize. He also has the fine intuition and horse sense of any good historian—it may be best to begin at the beginning and work forward. But, most importantly, he has a professional’s understanding that problems of maximal public acclaim are not always the issues of greatest scientific importance.

  The public may yearn, above all, to know the status of our common ancestor with chimpanzees, but Richard Leakey recognizes that the early Miocene is also a time of mystery, promise, and conceptual importance: mystery because we know so little about the actual diversity of apes at this time of their greatest success; promise because he has sediments that can deliver many of the missing goods; conceptual importance because we have as much to learn from documenting the base of our ancestral bush as in searching for the little branchlet that led directly to us later on. The early Miocene is a good place to explore.

  The ground of West Turkana glistens with crystals of quartz and calcite. The local Turkana children, passing time during long hours of tending goats under the relentless sun, collect geodes into piles and smash them to reveal the crystals inside. We are looking for duller fragments of bone.

  There are no great secrets to success, no unusual basis for “Leakey’s luck,” beyond hard work and experience. In some areas, fossil-bearing strata are rare and must be traced through geological complexities of folding and faulting to assure that fieldworkers search only in profitable places. But here, the entire sequence is fair game (although some strata, as always, are richer than others), and all exposures of rock must be scrutinized. The key to success becomes patience and a trained workforce.

  Leakey maintains a staff of trained Kenyan observers. He provides a long course in pra
ctical mammalian osteology (study of bones)—until they can distinguish the major groups of mammals from small scraps. The main ingredient of Leakey’s luck is unleashing these people in the right place.

  Kamoya Kimeu supervises this exploration. He has found more important fossils than any one else now alive. One night in camp, he told me his story. As a boy, he tended goats, sheep, and cattle for his father. He attended school for six years and then went to work for a farmer. His employer urged him to return to school and study to become a veterinary paramedic. Kamoya then walked for several days back to Nairobi, where his uncle told him that Louis Leakey, Richard’s father, was recruiting people to “dig bones.” His mother gave him only cautious approval, telling him to quit and come home if the task involved (as he then suspected) digging up human graves. But when he saw so many bones from so many kinds of creatures, he knew that nature had strewn these burial grounds. The sediments of West Turkana are, if anything, even more profuse.