The Panda’s Thumb (21 page)

Often, the grouping of species reflects more subtle and pervasive aspects of culture. The Kalam of New Guinea, for example, divide their nonreptilian four-footed vertebrates into three classes:
kopyak
, or rats;
kmn
for an evolutionary heterogeneous collection of larger game mammals, mostly marsupials and rodents; and as for an even more heterogeneous collection of frogs and small rodents. (Under repeated questioning by Bulmer, the Kalam deny any subdivision between frogs and rodents within
as
, although they do acknowledge [and dismiss as unimportant] the morphological similarity between small furry as and rodents among
kmn
. They also recognize that some
kmn
have pouches and others do not.) The divisions reflect fundamental facts of Kalam culture.
Kopyak
, associated with excrement and unclean food around homesteads, are not eaten at all.
As
are collected primarily by women and children and, although eaten by most men and collected by some, are forbidden foods for boys during their rites of passage and for adult men who practice sorcery.
Kmn
are hunted primarily by men.

Likewise, birds and bats are all
yakt
, with the single exception of the large, flightless cassowary called
kobty
. The distinction is made for deeper and more complex reasons than mere appearance—for the Kalam do recognize avian characters in
kobty
. Cassowaries, Bulmer argues, are the prime game of the forest and the Kalam maintain an elaborate cultural antithesis between cultivation (represented by taro and pigs) and the forest (represented by pandanus nuts and cassowaries). Cassowaries are also the mythological sisters of men.

We maintain similar practices in our own folk taxonomy. Edible mollusks are “shellfish,” but Linnaean species all have common names. I well remember the reprimand I received from a New England shipmate when I applied the informal scientific term “clam” to all bivalved mollusks (to him a clam is only the steamer,
Mya arenaria
): “A quahog is a quahog, a clam is a clam, and a scallop is a scallop.”

The evidence of folk taxonomy is persuasive for the modern world. Unless the tendency to divide organisms into Linnaean species reflects a neurological style wired into all of us (an interesting proposition, but one that I doubt), the world of nature is, in some fundamental sense, really divided into reasonably discrete packages of creatures as a result of evolution. (I do not, of course, deny that our propensity for classifying in the first place reflects something about our brains, their inherited capacities, and the limited ways in which complexity may be ordered and made sensible. I merely doubt that such a definite procedure as classification into Linnaean species could reflect the constraints of our mind alone, and not of nature.)

But are these Linnaean species, recognized by independent cultures, merely temporary configurations of the moment, mere way stations on evolutionary lineages in continual flux? I argue in essays 17 and 18 that, contrary to popular belief, evolution does not work this way, and that species have a “reality” through time to match their distinctness at a moment. An average species of fossil invertebrates lives five to ten million years (terrestrial vertebrates have shorter average durations). During this time, they rarely change in any fundamental way. They become extinct, without issue, looking much as they did when they first appeared.

New species usually arise, not by the slow and steady transformation of entire ancestral populations, but by the splitting off of small isolates from an unaltered parental stock. The frequency and speed of such speciation is among the hottest topics in evolutionary theory today, but I think that most of my colleagues would advocate ranges of hundreds of thousands of years for the origin of most species by splitting. This may seem like a long time in the framework of our lives, but it is a geological instant, usually represented in the fossil record by a single bedding plane, not a long stratigraphic sequence. If species arise in hundreds or thousands of years and then persist, largely unchanged, for several million, the period of their origin is a tiny fraction of one percent of their total duration. Therefore, they may be treated as discrete entities even through time. Evolution at higher levels is fundamentally a story of the differential success of species not the slow transformation of lineages.

Of course, if we happen to encounter a species during the geological microsecond of its origin, we will not be able to make clear distinctions. But our chances of finding a species in this state are small indeed. Species are stable entities with very brief periods of fuzziness at their origin (although not at their demise because most species disappear cleanly without changing into anything else). As Edmund Burke said in another context: “Though no man can draw a stroke between the confines of day and night, yet light and darkness are upon the whole tolerably distinguishable.”

Evolution is a theory of organic change, but it does not imply, as many people assume, that ceaseless flux is the irreducible state of nature and that structure is but a temporary incarnation of the moment. Change is more often a rapid transition between stable states than a continuous transformation at slow and steady rates. We live in a world of structure and legitimate distinction. Species are the units of nature's morphology.

P
OOH
-B
AH, THE
Lord High Everything Else of Titipu, boasted a family pride so strong as to be “something inconceivable.” “You will understand this,” he said to Nanki-Poo in suggesting that a bribe would be both appropriate and expensive, “when I tell you that I can trace my ancestry back to a protoplasmal primordial atomic globule.”

If human pride is nurtured by such vastly extended roots, then the end of 1977 was a bounteous time for self-esteem. Early in November, an announcement of the discovery of some fossil prokaryotes from South Africa pushed the antiquity of life back to 3.4 billion years. (Prokaryotes, including bacteria and blue green algae, form the kingdom Monera. Their cells contain no organelles—no nucleus, no mitochondria—and they are regarded as the simplest forms of life on earth.) Two weeks later, a research team from the University of Illinois announced that the so-called methane-producing bacteria are not closely related to other monerans after all, but form a separate kingdom of their own.

If true monerans were alive 3.4 billion years ago, then the common ancestor of monerans and these newly christened “methanogens” must be considerably more ancient. Since the oldest dated rocks, the Isua Supracrustals of West Greenland, are 3.8 billion years old, we are left with very little time between the development of suitable conditions for life on the earth's surface and the origin of life itself. Life is not a complex accident that required immense time to convert the vastly improbable into the nearly certain—to build laboriously, step by step, through a large chunk of time's vastness, the most elaborate machinery on earth from the simple constituents of our original atmosphere. Instead, life, for all its intricacy, probably arose rapidly about as soon as it could; perhaps it was as inevitable as quartz or feldspar. (The earth is some 4½ billion years old, but it passed through a molten or near-molten stage some time after its formation and probably did not form a solid crust much before the deposition of the West Greenland sequence.) No wonder these stories hit the front page of the
New York Times
, and even inspired an editorial for Veterans' Day musings.

Twenty years ago, I spent a summer at the University of Colorado, fortifying myself for the transition from high school to college. Amidst the various joys of snowcapped peaks and sore asses from trying to “set a trot,” I well remember the highlight of my stay—George Wald's lecture on the “Origin of Life.” He presented with infectious charm and enthusiasm the perspective that developed in the early 1950s and reigned as an orthodoxy until very recently.

In Wald's view, the spontaneous origin of life could be considered as a virtually inevitable consequence of the earth's atmosphere and crust, and of its favorable size and position in the solar system. Still, he argued, life is so staggeringly complex that its origin from simple chemicals must have consumed an immense amount of time—probably more time than its entire subsequent evolution from DNA molecule to advanced beetles (or whatever you choose to place atop the subjective ladder). Thousands of steps, each requiring the one before, each improbable in itself. Only the immensity of time guaranteed the result, for time converts the improbable to the inevitable—give me a million years and I'll flip a hundred heads in a row more than once. Wald wrote in 1954: “Time is in fact the hero of the plot. The time with which we have to deal is the order of two billion years…. Given so much time, the ‘impossible' becomes possible, the possible probable, and the probable virtually certain. One has only to wait: time itself performs the miracles.”

This orthodox view congealed without the benefit of any direct data from paleontology to test it, for the paucity of fossils before the great Cambrian “explosion” 600 million years ago is, perhaps, the outstanding fact and frustration of my profession. In fact, the first unambiguous evidence of Precambrian life appeared in the same year that Wald theorized about its origin. Harvard paleobotanist Elso Barghoorn and Wisconsin geologist S. A. Tyler described a series of prokaryotic organisms from cherts of the Gunflint Formation, rocks nearly two billion years old from the northern shore of Lake Superior. Still, the gap between the Gunflint and the earth's origin spanned 2½ billion years, more than enough time for Wald's slow and steady construction.

But our knowledge of life continued its trek backward. Laminated carbonate deposits, called stromatolites, had been known for some time from rocks of the Bulawayan Series, 2.6 to 2.8 billion years old, in Southern Rhodesia. The laminations resemble patterns formed by modern blue green algal mats that trap and bind sediment. The organic interpretation of stromatolites won many converts after Barghoorn and Tyler's Gunflint discoveries removed the odor of heresy from belief in Precambrian fossils. Then, ten years ago in 1967, Barghoorn and J. W. Schopf reported “algalike” and “bacteriumlike” organisms from the Fig Tree Series of South Africa. Now the orthodox idea of slow construction spanning most of the earth's history began to crumble, for the Fig Tree rocks, based on dates available in 1967, seemed to be more than 3.1 billion years old. Schopf and Barghoorn dignified their discoveries with formal Latin names, but their own characterizations—algalike and bacteriumlike—reflected their doubts. In fact, Schopf later decided that the balance of evidence stood against the biological nature of these structures.

The recent announcement of 3.4-billion-year-old life is not a startlingly new discovery, but a satisfactory culmination of a decade's debate about the status of life in the Fig Tree. The new evidence, gathered by Andrew H. Knoll and Barghoorn, also comes from cherts of the Fig Tree Series. But now the evidence is close to conclusive; moreover, recent dates indicate a greater age of 3.4 billion years for the series. In fact, the Fig Tree cherts may be the oldest appropriate rocks on earth for the discovery of ancient life. Older Greenland rocks have been too altered by heat and pressure to preserve organic remains. Knoll tells me that some unstudied cherts in Rhodesia may range back to 3.6 billion years, but eager scientists will have to await a political denouement before their arcane concerns attract sympathy or ensure safety. Still, the notion that life has been found in the oldest rocks that could contain evidence of it forces us, I think, to abandon the view of life's slow, steady, and improbable development. Life arose rapidly, perhaps as soon as the earth cooled down sufficiently to support it.

The new fossils from the Fig Tree Series are far more convincing than the previous discoveries. “In younger rocks [they] would without hesitation be called algal microfossils,” Knoll and Barghoorn claim. This interpretation rests upon five arguments:

1. The new structures are within the size range of modern prokaryotes. The earlier structures described by Schopf and Barghoorn were disturbingly large; Schopf later rejected them as biological, primarily on the basis of their large size. The new fossils, averaging 2.5 micrometers in diameter (a micrometer is a millionth of a meter), have a mean volume only 0.2 percent as large as the earlier structures now considered inorganic.

2. Populations of modern prokaryotes have a characteristic distribution of size. They can be arranged in a typical bell-shaped curve, with the average diameter most frequent and a continual decrease in number towards larger or smaller sizes. Thus, prokaryotic populations not only have a diagnostic average size (point 1 above), they also have a characteristic pattern of variation about this average. The new microfossils form a beautiful bell-shaped distribution with limited spread (range from 1 to 4 micrometers). The previous, larger structures exhibited much greater variation and no strong mean.

3. The new structures are “variously elongated, flattened, wrinkled, or folded” in a manner strikingly similar to Gunflint and later Precambrian prokaryotes. Such shapes are characteristic of postmortem degradation in modern prokaryotes. The larger, earlier structures were distressingly spherical; spheres, as a standard configuration of minimal surface area, can be easily produced by a host of inorganic processes—consider bubbles.

4. Most convincingly, about one quarter of the new microfossils have been found in various stages of cell division. Lest such a proportion caught
in flagrante delicto
sems unreasonably high, I point out that prokaryotes can divide every twenty minutes or so and take several minutes to complete the process. A single cell might well spend one-fourth of its life making two daughters.

5. These four arguments based on morphology are persuasive enough for me, but Knoll and Barghoorn add some biochemical evidence as well. Atoms of a single element often exist in several alternate forms of different weight. These forms, called isotopes, have the same number of protons but different numbers of neutrons. Some isotopes are radioactive and break down spontaneously to other elements; others are stable and persist unchanged throughout geologic time. Carbon has two major stable isotopes, C
¹²
with 6 protons and 6 neutrons, and C
¹³
with 6 protons and 7 neutrons. When organisms fix carbon in photosynthesis, they use preferentially the lighter isotope C
¹²
. Hence, the C
¹²
/C
¹³
ratio of carbon fixed by photosynthesis is higher than the ratio in inorganic carbon (in a diamond, for example). Moreover, since both isotopes are stable, their ratio will not alter through time. The C
¹²
/C
¹³
ratios for Fig Tree carbon are too high for an inorganic origin; they are in the range for fixation by photosynthesis. This, in itself, would not establish the case for life in the Fig Tree; light carbon can be fixed preferentially in other ways. But combined with the evidence of size, distribution, shape, and cellular division, this additional support from biochemistry completes a convincing case.

If prokaryotes were well established 3.4 billion years ago, how much further back shall we seek the origin of life? I have already pointed out that no suitable (or at least accessible) older rocks are known on earth, so for now we can proceed no further from the direct evidence of fossils. We turn instead to the second front-page item, the claim of Carl Woese and his associates that methanogens are not bacteria at all, but may represent a new kingdom of prokaryotic life, distinct from the Monera (bacteria and blue green algae). Their report has been widely distorted, most notably in the
New York Times
editorial of November 11, 1977. The
Times
proclaimed that the great dichotomy of plants and animals had finally been broken: “Every child learns about things being vegetable or animal—a division as universal as the partition of mammals into male and female. Yet…[we now have] a ‘third kingdom' of life on earth, organisms that are neither animal nor vegetable, but of another category altogether.” But biologists abandoned “the great dichotomy” long ago, and no one now tries to cram all single-celled creatures into the two great groups traditionally recognized for complex life. Most popular these days is a system of five kingdoms: plants, animals, fungi, protists (single-celled eukaryotes, including amoebas and paramecia, with nucleus, mitochondria, and other organelles), and the prokaryotic monerans. If methanogens are promoted, they will form a sixth kingdom, joining the monerans in a superkingdom, Prokaryota. Most biologists regard the division between prokaryotes and eukaryotes, not between plants and animals, as the fundamental partition of life.

Woese's research group (see Fox,
et al
., 1977 in the bibliography) isolated a common RNA from ten methanogens and from three monerans for comparison (DNA makes RNA, and RNA serves as the template upon which proteins are synthesized). A single strand of RNA, like DNA, consists of a sequence of nucleotides. Any one of four nucleotides can occupy each position, and each group of three nucleotides specifies an amino acid; proteins are built of amino acids arranged in folded chains. This, in a compressed phrase, is the “genetic code.” Biochemists can now “sequence” RNA, that is, they can read the entire sequence of nucleotides in order down the RNA strand.

The prokaryotes (methanogens, bacteria, and blue-green algae) must have had a common ancestor at some time near the origin of life. Thus, all prokaryotes had the same RNA sequence at one point in their past; any current differences arose by divergence from this common ancestral sequence, after the trunk of the prokaryotic tree split up into its several branches. If molecular evolution proceeded at a constant rate, then the extent of current difference between any two forms would directly record the amount of time since their lineages split from a common ancestor—that is, the last time they shared the same RNA sequence. Perhaps, for example, a different nucleotide in the two forms at 10 percent of all common positions would indicate a time of divergence a billion years ago; 20 percent, two billion years, and so on.

Woese and his group measured the RNA differences for all pairs of species among the ten methanogens and three monerans and used the results to construct an evolutionary tree. This tree contains two major limbs—all the methanogens on one, all the monerans on the other. They chose their three monerans to represent the greatest differences within the group—enteric (gut) bacteria versus free-living blue-green algae, for example. Nonetheless, each moneran is more similar to all other monerans than any moneran is to any methanogen.

The simplest interpretation of these results holds that methanogens and monerans are separate evolutionary groups, with a common ancestry preceding the appearance of either. (Previously, methanogens had been classified among the bacteria; in fact, they had not been recognized as a coherent entity at all, but had been regarded as a set of independent evolutionary events—convergent evolution for the ability to make methane). This interpretation underlies Woese's claim that methanogens are separate from monerans and should be recognized as a sixth kingdom. Since good monerans had already evolved by Fig Tree times, 3.4 billion or more years ago, the common ancestry of methanogens and monerans must have been even earlier, thus pushing the origin of life even further back toward the beginning of the earth itself.