The Panda’s Thumb (25 page)

Hanson documents an extensive set of similarities between the multinucleate ciliates and the acoeles. Acoeles are tiny marine flatworms. Some can swim, and a few live in water up to 250 meters in depth; but most crawl along the sea bottom in shallow water, living under rocks or in sand and mud. They are similar in size to the multinucleate ciliates. (It is not true that all metazoans are larger than all protists. The ciliates range in length from 1/100 to 3 millimeters, while some acoeles are less than 1 millimeter in length.) The internal similarities of ciliates and acoeles reside primarily in their shared simplicity; for acoeles, unlike conventional metazoans, lack both a body cavity and the organs associated with it. They have no permanent digestive, excretory, or respiratory system. Like the ciliate protists, they form temporary food vacuoles and perform digestion within them. Both ciliates and acoeles divide their bodies roughly into inner and outer layers. Ciliates maintain an ectoplasm (outer layer) and endoplasm (inner layer), and concentrate their nuclei in the endoplasm. Acoeles devote an inner region to digestion and reproduction, and an outer region to locomotion, protection, and capture of food.

The two groups also display some outstanding differences. Acoeles build a nerve net and reproductive organs that can become quite complex. Some have penises, for example, and impregnate each other hypodermically by penetrating through the body wall. They undergo embryonic development after fertilization. Ciliates, by contrast, have no organized nervous system. They divide by fission and have no embryology, although they do indulge in sex via a process called conjugation. (In conjugation, two ciliates come together and exchange genetic material. They then separate and each divides later to form two daughters. Sex and reproduction, combined in nearly all metazoa, are separate processes in ciliates.) Most prominently, of course, acoeles are cellularized, ciliates are not.

These differences should not debar a hypothesis of close genealogical relationship. After all, as I argued previously, contemporary ciliates and acoeles are more than half a billion years beyond their potential common ancestor. Neither represents a transitional form in the origin of multicellularity. The debate centers instead on the similarities, and on the oldest and most basic issue of all: are the similarities homologous or analogous?

Hanson argues for homology, claiming that acoele simplicity is an ancestral condition within the platyhelminths—and that similarities between ciliates and acoeles, largely a result of this simplicity, do record genealogical connection. His detractors reply that the simplicity of acoeles is a secondary result of their “regressive” evolution from more complex platyhelminths, a consequence of pronounced reduction in body size within acoeles. Larger turbellarians (the platyhelminth group including acoeles) have intestines and excretory organs. If acoele simplicity is a derived condition
within
the turbellarians, then it cannot reflect direct inheritance from a ciliate stock.

Unfortunately, the similarities that Hanson cites are of the sort that always produce unresolvable wrangling about homology vs. analogy. They are neither precise, nor numerous enough to guarantee homology. Many are based upon the
absence
of complexity in acoeles, and evolutionary loss is easy and repeatable, whereas separate development of precise and intricate structures may be unlikely. Moreover, acoele simplicity is a predictable result of their small body size—it may represent a functional convergence upon ciliate design by a group that secondarily entered their range of body size, not a connection by descent. Again, we invoke the principle of surfaces and volumes. Many physiological functions, including breathing, digestion, and excretion, must proceed through surfaces and serve the entire body's volume. Large animals have such a low ratio of external surface to internal volume that they must evolve internal organs to provide more surface. (Functionally, lungs are little more than bags of surface for exchange of gases, while intestines are sheets of surface for the passage of digested food.) But small animals maintain such a high ratio of external surface to internal volume that they often can breathe, feed, and excrete through the external surface alone. The smallest representatives of many phyla more complex than platyhelminths also lose internal organs.
Caecum
, for example, the smallest snail, has lost its internal respiratory system entirely and takes in oxygen through its external surface.

Other similarities, cited by Hanson, may be homologous, but so widespread among other creatures that they merely illustrate the broader affinity of all protists with all metazoans, not any specific pathway of descent. Meaningful homologies must be confined to characters that are both shared by descent
and
derived. (Derived characters evolve uniquely in the common ancestor of two groups that share them; they are marks of genealogy. A shared primitive character, on the other hand, cannot specify descent. The presence of DNA in both ciliates and acoeles tells us nothing about their affinity because all protists and metazoans have DNA.) Thus, Hanson mentions “complete ciliation” as a “permanent character significantly held in common by ciliates and acoeles.” But cilia, although homologous, are a shared primitive character; many other groups, including cnidarians, have them. The
completeness
of ciliation, on the other hand, represents an “easy” evolutionary event that may only be analogous in ciliates and acoeles. The external surface sets a limit to the maximal number of cilia that may be affixed. Small animals, with high surface/volume ratios, may indulge in ciliary locomotion; large animals cannot insert enough cilia on their relatively declining surface to propel their mass. The complete ciliation of acoeles may reflect a secondary, adaptive response to their small size. The tiny snail
Caecum
also moves by cilia; all its larger relatives use muscular contraction for locomotion.

Hanson is, of course, well aware that he cannot prove his intriguing hypothesis with the classical evidence of morphology and function. “The best we can say,” he concludes, “is that many suggestive similarities are present [between ciliates and acoeles], but no rigorously definable homologies.” Is there another method that might resolve the issue, or are we permanently condemned to unresolvable wrangling? Homology might be established with confidence if we could generate a new set of characters sufficiently numerous, comparable, and complex—for analogy cannot be the explanation of detailed, part-by-part similarity in thousands of independent items. The laws of mathematical probability will not allow it.

Fortunately, we now have a potential source of such information—the DNA sequence of comparable proteins. All protists and metazoans share many homologous proteins. Each protein is built of a long chain of amino acids; each amino acid is coded by a sequence of three nucleotides in DNA. Thus, the DNA code for each protein may contain hundreds of thousands of nucleotides in a definite order.

Evolution proceeds by substitution of nucleotides. After two groups split from a common ancestor, their nucleotide sequences begin to accumulate changes. The number of changes seems to be at least roughly proportional to the amount of time since the split. Thus, overall similarity in nucleotide sequence for homologous proteins may measure the extent of genealogical separation. A nucleotide sequence is a homologizer's dream—for it represents thousands of potentially independent characters. Each nucleotide position is a site of possible change.

Techniques are just now becoming available for the routine sequencing of nucleotides. Within ten years, I believe, we will be able to take homologous proteins from all the ciliate and metazoan groups at issue, sequence them, measure the similarities between each pair of organisms and obtain greater insight (perhaps even resolution) for this old genealogical mystery. If acoeles are most similar to protist groups that might achieve multicellularity by evolving cell membranes within their bodies, then Hanson will be vindicated. But if they are closest to protists that can reach multicellularity by integration within a colony, then the classical view will prevail, and all metazoa will emerge as the products of amalgamation.

The study of genealogy has been unfairly eclipsed in our century by the analysis of adaptation, but it cannot lose its power to fascinate. Simply consider what Hanson's scenario implies about our relationship with other multicellular organisms. Few zoologists doubt that all higher animals achieved their multicellular status by whatever method the flatworms followed. If acoeles evolved by the cellularization of a ciliate, then our multicellular body is the homolog of a single protistan cell. If sponges, cnidarians, plants and fungi arose by amalgamation, then their bodies are the homologs of a protistan colony. Since each ciliate cell is the homolog of an individual cell in any protistan colony, we must conclude—and I do mean this literally—that the entire human body is the homolog of a single cell in a sponge, coral, or plant.

The curious paths of homology go further back. The protistan cell itself may have evolved from a symbiosis of several simpler prokaryotic (bacterial or blue green algal) cells. Mitochondria and chloroplasts seem to be the homologs of entire prokaryotic cells. Thus, each cell of any protist, and each cell in any metazoan body, may be, by genealogy, an integrated colony of prokaryotes. Shall we then view ourselves both as a congeries of bacterial colonies and as the homolog of a single cell in a sponge or onion skin? Think upon it next time you swallow a carrot or slice a mushroom.

WHEN
M
UHAMMAD
A
LI
flunked his army intelligence test, he quipped (with a wit that belied his performance on the exam): “I only said I was the greatest; I never said I was the smartest.” In our metaphors and fairy tales, size and power are almost always balanced by a want of intelligence. Cunning is the refuge of the little guy. Think of Br'er Rabbit and Br'er Bear; David smiting Goliath with a slingshot; Jack chopping down the beanstalk. Slow wit is the tragic flaw of a giant.

The discovery of dinosaurs in the nineteenth century provided, or so it appeared, a quintessential case for the negative correlation of size and smarts. With their pea brains and giant bodies, dinosaurs became a symbol of lumbering stupidity. Their extinction seemed only to confirm their flawed design.

Dinosaurs were not even granted the usual solace of a giant—great physical prowess. God maintained a discreet silence about the brains of behemoth, but he certainly marveled at its strength: “Lo, now, his strength is in his loins, and his force is in the navel of his belly. He moveth his tail like a cedar…. His bones are as strong pieces of brass; his bones are like bars of iron [Job 40:16–18].” Dinosaurs, on the other hand, have usually been reconstructed as slow and clumsy. In the standard illustration,
Brontosaurus
wades in a murky pond because he cannot hold up his own weight on land.

Popularizations for grade school curricula provide a good illustration of prevailing orthodoxy. I still have my third grade copy (1948 edition) of Bertha Morris Parker's
Animals of Yesterday
, stolen, I am forced to suppose, from P.S. 26, Queens (sorry Mrs. McInerney). In it, boy (teleported back to the Jurassic) meets brontosaur:

Dinosaurs have been making a strong comeback of late, in this age of “I'm OK, you're OK.” Most paleontologists are now willing to view them as energetic, active, and capable animals. The
Brontosaurus
that wallowed in its pond a generation ago is now running on land, while pairs of males have been seen twining their necks about each other in elaborate sexual combat for access to females (much like the neck wrestling of giraffes). Modern anatomical reconstructions indicate strength and agility, and many paleontologists now believe that dinosaurs were warmblooded (see essay 26).

The idea of warmblooded dinosaurs has captured the public imagination and received a torrent of press coverage. Yet another vindication of dinosaurian capability has received very little attention, although I regard it as equally significant. I refer to the issue of stupidity and its correlation with size. The revisionist interpretation, which I support in this column, does not enshrine dinosaurs as paragons of intellect, but it does maintain that they were not small brained after all. They had the “right-sized” brains for reptiles of their body size.

Triceratops
GREGORY S. PAUL

I don't wish to deny that the flattened, minuscule head of largebodied
Stegosaurus
houses little brain from our subjective, top-heavy perspective, but I do wish to assert that we should not expect more of the beast. First of all, large animals have relatively smaller brains than related, small animals. The correlation of brain size with body size among kindred animals (all reptiles, all mammals, for example) is remarkably regular. As we move from small to large animals, from mice to elephants or small lizards to Komodo dragons, brain size increases, but not so fast as body size. In other words, bodies grow faster than brains, and large animals have low ratios of brain weight to body weight. In fact, brains grow only about two-thirds as fast as bodies. Since we have no reason to believe that large animals are consistently stupider than their smaller relatives, we must conclude that large animals require relatively less brain to do as well as smaller animals. If we do not recognize this relationship, we are likely to underestimate the mental power of very large animals, dinosaurs in particular.

Brachiosaurus
GREGORY S. PAUL

Second, the relationship between brain and body size is not identical in all groups of vertebrates. All share the same rate of relative decrease in brain size, but small mammals have much larger brains than small reptiles of the same body weight. This discrepancy is maintained at all larger body weights, since brain size increases at the same rate in both groups—two-thirds as fast as body size.

Put these two facts together—all large animals have relatively small brains, and reptiles have much smaller brains than mammals at any common body weight—and what should we expect from a normal, large reptile? The answer, of course, is a brain of very modest size. No living reptile even approaches a middle-sized dinosaur in bulk, so we have no modern standard to serve as a model for dinosaurs.

Fortunately, our imperfect fossil record has, for once, not severely disappointed us in providing data about fossil brains. Superbly preserved skulls have been found for many species of dinosaurs, and cranial capacities can be measured. (Since brains do not fill craniums in reptiles, some creative, although not unreasonable, manipulation must be applied to estimate brain size from the hole within a skull.) With these data, we have a clear test for the conventional hypothesis of dinosaurian stupidity. We should agree, at the outset, that a reptilian standard is the only proper one—it is surely irrelevant that dinosaurs had smaller brains than people or whales. We have abundant data on the relationship of brain and body size in modern reptiles. Since we know that brains increase two-thirds as fast as bodies as we move from small to large living species, we can extrapolate this rate to dinosaurian sizes and ask whether dinosaur brains match what we would expect of living reptiles if they grew so large.

Harry Jerison studied the brain sizes of ten dinosaurs and found that they fell right on the extrapolated reptilian curve. Dinosaurs did not have small brains; they maintained just the right-sized brains for reptiles of their dimensions. So much for Ms. Parker's explanation of their demise.

Jerison made no attempt to distinguish among various kinds of dinosaurs; ten species distributed over six major groups scarcely provide a proper basis for comparison. Recently, James A. Hopson of the University of Chicago gathered more data and made a remarkable and satisfying discovery.

Hopson needed a common scale for all dinosaurs. He therefore compared each dinosaur brain with the average reptilian brain we would expect at its body weight. If the dinosaur falls on the standard reptilian curve, its brain receives a value of 1.0 (called an encephalization quotient, or EQ—the ratio of actual brain to expected brain for a standard reptile of the same body weight). Dinosaurs lying above the curve (more brain than expected in a standard reptile of the same body weight) receive values in excess of 1.0, while those below the curve measure less than 1.0.

Hopson found that the major groups of dinosaurs can be ranked by increasing values of average EQ. This ranking corresponds perfectly with inferred speed, agility and behavioral complexity in feeding (or avoiding the prospect of becoming a meal). The giant sauropods,
Brontosaurus
and its allies, have the lowest EQ's—0.20 to 0.35. They must have moved fairly slowly and without great maneuverability. They probably escaped predation by virtue of their bulk alone, much as elephants do today. The armored ankylosaurs and stegosaurs come next with EQ's of 0.52 to 0.56. These animals, with their heavy armor, probably relied largely upon passive defense, but the clubbed tail of ankylosaurs and the spiked tail of stegosaurs imply some active fighting and increased behavioral complexity.

The ceratopsians rank next at about 0.7 to 0.9. Hopson remarks: “The larger ceratopsians, with their great horned heads, relied on active defensive strategies and presumably required somewhat greater agility than the tail-weaponed forms, both in fending off predators and in intraspecific combat bouts. The smaller ceratopsians, lacking true horns, would have relied on sensory acuity and speed to escape from predators.” The ornithopods (duckbills and their allies) were the brainiest herbivores, with EQ's from 0.85 to 1.5. They relied upon “acute senses and relatively fast speeds” to elude carnivores. Flight seems to require more acuity and agility than standing defense. Among ceratopsians, small, hornless, and presumably fleeing
Protoceratops
had a higher EQ than great three-horned
Triceratops
.

Carnivores have higher EQ's than herbivores, as in modern vertebrates. Catching a rapidly moving or stoutly fighting prey demands a good deal more upstairs than plucking the right kind of plant. The giant theropods (
Tyrannosaurus
and its allies) vary from 1.0 to nearly 2.0. Atop the heap, quite appropriately at its small size, rests the little coelurosaur
Stenonychosaurus
with an EQ well above 5.0. Its actively moving quarry, small mammals and birds perhaps, probably posed a greater challenge in discovery and capture than
Triceratops
afforded
Tyrannosaurus
.

I do not wish to make a naive claim that brain size equals intelligence or, in this case, behavioral range and agility (I don't know what intelligence means in humans, much less in a group of extinct reptiles). Variation in brain size within a species has precious little to do with brain power (humans do equally well with 900 or 2,500 cubic centimeters of brain). But comparison across species, when the differences are large, seems reasonable. I do not regard it as irrelevant to our achievements that we so greatly exceed koala bears—much as I love them—in EQ. The sensible ordering among dinosaurs also indicates that even so coarse a measure as brain size counts for something.

If behavioral complexity is one consequence of mental power, then we might expect to uncover among dinosaurs some signs of social behavior that demand coordination, cohesiveness, and recognition. Indeed we do, and it cannot be accidental that these signs were overlooked when dinosaurs labored under the burden of a falsely imposed obtuseness. Multiple trackways have been uncovered, with evidence for more than twenty animals traveling together in parallel movement. Did some dinosaurs live in herds? At the Davenport Ranch sauropod trackway, small footprints lie in the center and larger ones at the periphery. Could it be that some dinosaurs traveled much as some advanced herbivorous mammals do today, with large adults at the borders sheltering juveniles in the center?

In addition, the very structures that seemed most bizarre and useless to older paleontologists—the elaborate crests of hadrosaurs, the frills and horns of ceratopsians, and the nine inches of solid bone above the brain of
Pachycephalosaurus—
now appear to gain a coordinated explanation as devices for sexual display and combat. Pachycephalosaurs may have engaged in head-butting contests much as mountain sheep do today. The crests of some hadrosaurs are well designed as resonating chambers; did they engage in bellowing matches? The ceratopsian horn and frill may have acted as sword and shield in the battle for mates. Since such behavior is not only intrinsically complex, but also implies an elaborate social system, we would scarcely expect to find it in a group of animals barely muddling through at a moronic level.

But the best illustration of dinosaurian capability may well be the fact most often cited against them—their demise. Extinction, for most people, carries many of the connotations attributed to sex not so long ago—a rather disreputable business, frequent in occurrence, but not to anyone's credit, and certainly not to be discussed in proper circles. But, like sex, extinction is an ineluctable part of life. It is the ultimate fate of all species, not the lot of unfortunate and ill-designed creatures. It is no sign of failure.