Bully for Brontosaurus

by Stephen Jay Gould

  Neurophysiologists can locate areas of the brain responsible for activating definite parts of the body and draw a “map” of the body upon the brain itself. (These experiments proceed from either direction. Either one stimulates a body part and records the pattern of activity in a set of electrodes implanted into the brain, or one pulses a spot on the brain and determines the resulting motion of body parts.) We have no finer demonstrations of evolutionary adaptation than numerous brain maps that record the importance of specially developed organs by their unusually enlarged areas of representation upon the cortex. Thus, a raccoon’s brain map displays an enormous domain for its fore-paws, a pig’s for its snout, a spider monkey’s for its tail. Bohringer and Rowe have added the platypus to this informative array. A map of the platypus’s cortex is mostly bill.

  We have come a long way from the first prominent evolutionary interpretation ever presented for the platypus bill. In 1844, in the major pre-Darwinian defense of evolution written in English, Robert Chambers tried to derive a mammal from a bird in two great leaps, via the intermediate link of a duckbilled platypus. One step, Chambers wrote,

  would suffice in a goose to give its progeny the body of a rat, and produce the ornithorhynchus, or might give the progeny of an ornithorhynchus the mouth and feet of a true rodent, and thus complete at two stages the passage from the aves to the mammalia.

  The platypus, having suffered such slings and arrows of outrageous fortune in imposed degradation by human hands, has cast its arms (and its bill) against a sea of troubles and vindicated itself. The whips and scorns of time shall heal. The oppressor’s wrong, the proud man’s contumely have been reversed by modern studies—enterprises of great pith and moment. The platypus is one honey of an adaptation.

  19 | Bligh’s Bounty

  IN 1789, a British naval officer discovered some islands near Australia and lamented his inability to provide a good description:

  Being constantly wet, it was with the utmost difficulty I could open a book to write, and I am sensible that what I have done can only serve to point out where these lands are to be found again, and give the idea of their extent.

  As he wrote these lines, Captain William Bligh was steering a longboat with eighteen loyal crew members into the annals of human heroism at sea—via his 4,000-mile journey to Timor, accomplished without loss of a single man, and following the seizure of his ship, The Bounty, in history’s most famous mutiny.

  Bligh may have been overbearing; he surely wins no awards for insight into human psychology. But history and Charles Laughton have not treated him fairly either. Bligh was committed, meticulous, and orderly to a fault—how else, in such peril, could he have bothered to describe some scattered pieces of new Pacific real estate.

  Bligh’s habit of close recording yielded other benefits, including one forgotten item to science. Obsessed by the failure of his Bounty mission to bring Tahitian breadfruit as food for West Indian slaves, Bligh returned to Tahiti aboard the Providence and successfully unloaded 1,200 trees at Port Royal, Jamaica, in 1793 (his ship was described as a floating forest). En route, he stopped in Australia and had an interesting meal.

  George Tobin, one of Bligh’s officers, described their quarry as

  a kind of sloth about the size of a roasting pig with a proboscis 2 or 3 inches in length…. On the back were short quills like those of the Porcupine…. The animal was roasted and found of a delicate flavor.

  Bligh himself made a drawing of his creature before the banquet. The officers of the Providence had eaten an echidna, one of Australia’s most unusual mammals—an egg-laying anteater closely related to the duckbilled platypus.

  Bligh brought his drawing back to England. In 1802, it appeared as a figure (reproduced here) accompanying the first technical description of the echidna’s anatomy by Everard Home in the Philosophical Transactions of the Royal Society (G. Shaw had published a preliminary and superficial description in 1792).

  Home discovered the strange mix of reptilian and mammalian features that has inspired interest and puzzlement among biologists ever since. He also imposed upon the echidna, for the first time, the distinctive burden of primitivity that has continually hampered proper zoological understanding of all monotremes, the egg-laying mammals of Australia. Home described the echidna as not quite all there in mammalian terms, a lesser form stamped with features of lower groups:

  These characters distinguish [the echidna] in a very remarkable manner, from all other quadrupeds, giving this new tribe a resemblance in some respects to birds, in others to the Amphibia; so that it may be considered as an intermediate link between the classes of Mammalia, Aves, and Amphibia.

  Unfortunately, Home could not study the organ that most clearly belies the myth of primitivity. “The brain,” he wrote, “was not in a state to admit of particular examination.” Home did have an opportunity to infer the echidna’s anomalously large brain from the internal form of its skull, well drawn on the plate just preceding Bligh’s figure (and also reproduced here). But Home said nothing about this potential challenge to his general interpretation.

  Original drawing of an echidna by none other than Captain Bligh of Bounty fame. NEG. NO. 337535. COURTESY DEPARTMENT OF LIBRARY SERVICES, AMERICAN MUSEUM OF NATURAL HISTORY.

  And so the burden of primitivity stuck tenaciously to echidnas, and continues to hold fast in our supposedly more sophisticated age. Some great zoologists have struggled against this convenient fallacy, most notably the early French evolutionist Etienne Geoffroy Saint-Hilaire, who coined the name Monotremata (see Essay 18) and labored unsuccessfully to establish the echidna and platypus as a new class of vertebrates, separate from both mammals and reptiles and not merely inferior to placentals. By his own manifesto, he chose his strategy explicitly to avoid the conceptual lock that assumptions of primitivity would clamp upon our understanding of monotremes. He wrote in 1827:

  What is defective, I repeat, is our manner of perception, our way of conceiving the organization of monotremes; that is, our determination, made a priori, to join them violently to mammals [by violemment, Geoffroy means, of course, “without any conceptual justification”], to place them in the same class and, after our disappointments and false judgments, then to make our unjust grievances heard, as when we speak of them as mammals essentially and necessarily outside the rules.

  But Geoffroy’s legitimate complaint, so eloquently expressed, did not prevail, and the myth of primitivity continues, despite its blatant flaw. As I argue in the preceding essay on platypuses, the myth of primitivity rests upon a logical confusion between early branching from the ancestors of placental mammals (the true meaning of reptilian characters retained by monotremes) and structural inferiority. Unless geological age of branching is a sure guide to level of anatomical organization—as it is not—egg laying and interclavicle bones do not brand platypuses and echidnas as inferior mammals.

  Everard Home’s 1802 figure of an echidna’s skull. The large size of the brain was apparent even then. NEG. NO. 337429. COURTESY DEPARTMENT OF LIBRARY SERVICES, AMERICAN MUSEUM OF NATURAL HISTORY.

  Beyond this general defense, echidnas can provide ample specific evidence of their adequacy. They are, first of all, a clear success in ecological terms. Echidnas live all over the Australian continent (and extend into Papua-New Guinea), the only native mammal with such a wide range. Moreover, the echidna, as a single struggling relict, ranks with the rat and the monkey (those meaningless synecdoches of the psychological literature) as an absurd abstraction of nature’s richness. Echidnas come as two species in two separate genera and with quite different habits. Tachyglossus aculeatus (the Australian form with Papuan extensions) rips apart ant and termite nests with its stout forelimbs and collects the inhabitants on its sticky tongue. The larger and longer-snouted Zaglossus bruijni of Papua-New Guinea lives on a nearly exclusive diet of earthworms. Moreover, three other species, including the “giant” echidna, Zaglossus hacketti, have been found as fossils in Australia. Echidnas are a succes
sful and at least modestly varied group.

  But echidnas hold a far more important ace in the hole as their ultimate defense against charges of primitivity. The same cultural biases that lead us to classify creatures as primitive or advanced have established the form and function of brains as our primary criterion of ranking. Echidnas have big and richly convoluted brains. Scientists have recognized this anomaly in the tale of primitivity for more than a century—and they have developed an array of arguments, indeed a set of traditions, for working around such an evident and disconcerting fact. Large brains undoubtedly serve echidnas well; but they also help to instruct us about an important issue in the practice of science—how do scientists treat factual anomalies? What do we do with evidence that challenges a comfortable view of nature’s order?

  The echidna’s brain refutes the myth of primitivity with a double whammy—size and conformation. (I discuss only the Australian species, Tachyglossus aculeatus; its larger Papua-New Guinea relative, Zaglossus, remains virtually unknown to science—for basic information about echidnas, see the two books by M. Griffiths listed in the bibliography.) Since mammalian brains increase more slowly than body weight along the so-called mouse-to-elephant curve, we can use neither absolute nor relative brain weight as a criterion. (Big mammals have absolutely large brains as an uninteresting consequence of body size, while small mammals have relatively large brains because brains increase more slowly than bodies.) Biologists have therefore developed a standard criterion: measured brain weight relative to expected brain weight for an average mammal of the same body size. This ratio, dubbed EQ (or encephalization quotient) in amusing analogy with you know what, measures 1.0 for mammals right on the mouse-to-elephant curve, above 1.0 for brainier than average mammals, and less than 1.0 for brain weights below the norm.

  To provide some feel for the range of EQ’s, so-called basal insectivores—a selected stem group among the order traditionally ranked lowest among placental mammals—record a mean of 0.311. Adding advanced insectivores, the average rises to 0.443. Rodents, a perfectly respectable group (and dominant among mammals by sheer number), weigh in with a mean EQ of 0.652. (Primates and carnivores rank consistently above 1.0.) Monotremes are not, by this criterion, mental giants—their EQ’s range from 0.50 to 0.75—but they rank way above the traditional primitives among placentals and right up there with rodents and other “respected” groups. Monotremes continue to shine by other standards of size as well. Some neurologists regard the ratio of brain to spinal cord as a promising measure of mental advance. Fish generally dip below 1:1 (spinal cord heavier than brain). We top-heavy humans tip the scale at 50:1; cats score 4:1. The “lowly” echidna waddles in front of tabby at approximately 6:1.

  By conformation, rather than simple size alone, echidnas are even more impressive. The neocortex, the putative site of higher mental functions, occupies a larger percentage of total brain weight in supposedly advanced creatures. The neocortex of basal insectivores averages 13 percent of brain weight; the North American marsupial opossum records 22 percent. Echidnas score 43 percent (platypuses 48 percent), right up there with the prosimians (54 percent), basal group of the lordly primates. (All my figures for brain sizes come from H. J. Jerison, 1973, and P. Pirlot and J. Nelson, 1978.)

  The neocortex of echidnas is not only expanded and nearly spherical as in primates; its surface is also richly convoluted in a series of deep folds and bumps (sulci and gyri), a traditional criterion of mental advance in mammals. (Curiously, by comparison, the platypus neocortex, while equally expanded and spherical, is almost completely smooth.)

  Many famous nineteenth-century neuroanatomists studied monotreme brains, hoping to understand the basis of human mental triumph by examining its lowly origins. Echidnas provided an endless source of puzzlement and frustration. William Henry Flower dissected an echidna in 1865 and wrote of “this most remarkable brain, with its largely developed and richly convoluted hemispheres.” He admitted: “It is difficult to see in many of the peculiarities of their brain even an approach in the direction of that of the bird.” And Grafton Elliot Smith, the great Australian anatomist who later fell for Piltdown Man in such a big way, wrote with evident befuddlement in 1902:

  The most obtrusive feature of this brain is the relatively enormous development of the cerebral hemispheres…. In addition, the extent of the cortex is very considerably increased by numerous deep sulci. The meaning of this large neopallium is quite incomprehensible. The factors which the study of other mammalian brains has shown to be the determinants of the extent of the cortex fail completely to explain how it is that a small animal of the lowliest status in the mammalian series comes to possess this large cortical apparatus.

  One might have anticipated that scientists, so enlightened by monotreme mentality, would simply abandon the myth of primitivity. But prompt submission to items of contrary evidence is not, despite another prominent myth (this time about scientific procedure), the usual response of scientists to nature’s assaults upon traditional beliefs. Instead, most students of monotreme brains have recorded their surprise and then sought different criteria, again to affirm the myth of primitivity.

  A favorite argument cites the absence in monotremes (and marsupials as well) of a corpus callosum—the bundle of fibers connecting the right and left hemispheres of “higher” mental processing in placental mammals. In a wonderful example of blatantly circular logic, A. A. Abbie, one of Australia’s finest natural historians, wrote in a famous article of 1941 (commissures, to a neuroanatomist, are connecting bands of neural tissue, like the corpus callosum):

  Since in mammals cerebral evolution and with it any progressive total evolution is reflected so closely in the state of the cerebral commissures it is clear that the taxonomic significance of these commissures far transcends that of any other physical character.

  In other words, since we know (a priori) that monotremes are primitive, search for the character that affirms a lowly status (lack of a corpus callosum) and proclaim this character, ipso facto, more important than any other (size of brain, convolutions, or any other indication of monotreme adequacy). (I shall have more to say about commissures later on, but let me just mention for now that lack of a corpus callosum does not preclude communication across the cerebral hemispheres. Monotremes possess at least two other commissures—the hippocampal and the anterior—capable of making connections, though by a route more circuitous than the pathway of the corpus callosum.)

  This tradition of switching to another criterion continues in modern studies. In their 1978 article on monotreme brain sizes, for example, Pirlot and Nelson admit, after recording volumes and convolutions for echidnas: “It is very difficult to isolate criteria that clearly establish the ‘primitiveness’ of monotreme brains.” But they seek and putatively find, though they honorably temper their good cheer with yet another admission of the puzzling size of the monotreme neocortex:

  This cortex could be considered to be among the most primitive mammalian cortices on the basis of the low number and low density of large, especially pyramidal neurons. It is surprising to find that a very high proportion of cortex is neocortex. This does not necessarily mean an advanced degree of progressiveness, although the two are usually related.

  The basic data on size and external conformation of echidna brains have been recorded (and viewed as troubling) for more than a century. More sophisticated information on neural fine structure and actual use of the cortical apparatus in learning has been gathered during the past twenty years—all affirming, again and again, the respectability of echidna intelligence.

  In 1964, R. A. Lende published the first extensive map of localized sensory and motor areas on the echidna’s cerebral cortex. (I discuss the general procedures of such study in the preceding essay on platypus brains. P. S. Ulinski, 1984, has recently confirmed and greatly extended Lende’s work in a series of elegant experiments.) Lende discovered a surprising pattern of localization, basically mammalian in character but different from pl
acental mappings. He identified separate areas for visual, auditory, and sensory control (the motor area overlapped the sensory region and extended forward to an additional section of the cortex), all demarcated one from the other by constant sulci (fissures of the cortex) and located together at the rear of the cortex.

  Most surprisingly, these areas abut one another without any so-called association cortex in between. (Association cortex includes areas of the cerebral surface that do not control any specific sensory or motor function and may play a role in coordinating and integrating the basic inputs. For this reason, amount and position of association cortex have sometimes been advanced as criteria of “higher” mental function. But such negative definitions are troubling and should not be pushed too hard or far.) In any case, Lende identified a relatively enormous area of unspecified (perhaps association) cortex in front of his mapped sensory and motor areas. Lende concluded, in a statement oft-quoted against those who maintain the myth of primitivity:

  Ahead of the posteriorly situated sensory and motor areas established in this study there is relatively more “frontal cortex” than in any other mammal, including man, the function of which remains unexplained.

  Other studies have tried to push the echidna brain to its practical limits by imposing upon these anteaters all the modern apparatus of mazes, levers, and food rewards so favored by the science of comparative psychology. Echidnas have performed remarkably well in all these studies, again confuting the persistent impression of stupidity still conveyed by textbooks, and even by the most “official” of all sources—the Australian Museum’s Complete Book of Australian Mammals, edited by R. Strahan (1983), which insists without evidence: