>From a correspondent on another list:
>
>Prof. J.Y. Chen of the Chinese Academy of Sciences gave a lecture Tuesday
>at the University of California, Berkeley, under the auspices of the U.C.
>Museum of Paleontology. Prof. Chen's lecture was similar to the one he
>gave at the University of Washington in Seattle on Feb. 3 that was
>mentioned previously.
>
>Prof. Chen is one of the world's top experts on Cambrian animal fossils.
>His main point was that the major animal phyla arose suddenly and fully
>formed in the early Cambrian. Since the early Cambrian fauna from
>Chengjiang, China, includes soft-bodied animals, the absence of precursors
>presents a serious challenge to Darwinian evolution....
>
The challenge has been answered:
On Embryos and Ancestors
Fossils of tiny embryos 570 million years old may well be the greatest
paleontological discovery of our time.
By Stephen Jay Gould
ãEvery day, in every way, I'm getting better and better.ä I had always
regarded this famous phrase as a primary example of the intellectual vacuity
that often passes for profundity in our current era of laid-back, New Age
bliss--a verbal counterpart to the vapidity of the "have a nice day" smiley
face. But when I saw this phrase chiseled in stone on the pediment of a
French hospital built in the early years of our century, I knew that I must
have missed a longer and more interesting pedigree. This formula for
well-being, I then discovered, had been devised in 1920 by Ernile Coueâ
(1857-1926), a French pharmacist who made quite a stir in the pop-psych
circles of his day with a theory of self-improvement through autosuggestion
based on frequent repetition of this mantra--a treatment that received the
name of Coueism. (In a rare example of improvement in translation, this
phrase gains both a rhyme and better flow at least to my ears, when
converted to English from Coue's French original: Tous les jours, a tous
points de vue, je vais de mieux en mieux.)
I don't doubt the efficacy of Coue's mantra, for the placebo effect (its
only
possible mode of action) should not be dismissed as a delusion but cherished
as a useful strategy for certain forms of healing--a primary example of the
influence that mental attitudes can wield upon our physical sense of
well-being However, as a general description for the usual style and pacing
of human improvement, the constant and steady incrementalism of Coueâs
motto--a twentieth-century version of an ancient claim embodied in the
victory cry of Aesop's tortoise, "slow and steady wins the race"--strikes me
as only rarely applicable, and surely secondary, to the usual mode of human
enlightenment, either attitudinal or intellectual: that is, not by global
creep forward, inch by subsequent inch, but rather in rushes or whooshes,
usually following the removal of some impediment or the discovery of some
facilitating device, either ideological or technological.
The glory of science lies in such innovatory bursts. Centuries of vain
speculation dissolved in months before the resolving power of Galileo's
telescope, trained upon the full range of cosmic distances, from the Moon to
the Milky Way. About 350 years later, centuries of conjecture and indirect
data about the composition of lunar rocks melted before a few pounds of
actual samples brought back by Apollo 11 after Mr. Armstrong's small step
onto a new world.
In the physical sciences, such explosions of discovery usually follow the
invention of a device that can, for the first time, penetrate a previously
invisible realm--the "too far" by the telescope, the "too small" by the
microscope, the imperceptible by X rays, or the unreachable by spaceships.
In the humbler world of natural history, episodes of equal pith and moment
often follow a "eureka" triggered by continually available mental, rather
than expensively novel physical, equipment. In other words, great discovery
often requires a map to a hidden mine filled with gems then easily gathered
by conventional tools, not a shiny new space-age machine for penetrating
previously (and utterly) inaccessible worlds.
The uncovering of life's early history has featured several such cascades of
discovery following a key insight about proper places to look, and I
introduce this year's wonderful story by citing a previous episode of
remarkably similar character from the last generation of our science
(literally so, for this year's discoverer wrote his Ph.D. dissertation under
the guidance of one of the earlier two innovators).
When, as a boy in the early 1950s, I first became fascinated with
paleontology and evolution, the standard dogma proclaimed the origin of life
was inherently improbable but achieved on this planet only because the
immensity of geological time must convert the nearly impossible into the
virtually certain. (With no limit on the number of tries, you will
eventually flip fifty heads in row with an honest coin.) As evidence for
asserting the exquisite specialness of life in the face of overwhelmingly
contrary odds, these conventional sources cited the absence of any fossils
representing the first half of the earth's existence--a span of more than 2
billion years, often formally designated on geological charts as the Azoic
(literally, "lifeless") era. Although scientists do recognize the
limitations of such negative evidence (the first example of a previously
absent phenomenon may, after all, turn up tomorrow), this failure to find
any fossils for geology's first 2 billion years did seem fairly persuasive.
Paleontologists had been searching assiduously for more than a century and
had found nothing but ambiguous scraps and blobs. Negative results based on
such sustained effort over so many years do begin to inspire belief.
But the impasse broke in the 1950s, when Elso Barghoorn and Stanley Tyler
reported fossils of unicellular life in rocks more than 2 billion years old.
Paleontologists, to summarize a long and complex story with many exciting
turns and notable heroes, had been looking in the wrong place÷in
conventional sediments that rarely preserve the remains of single-celled
bacterial organisms without hard parts. They had not realized that life had
remained so simple for so long, or that the ordinary sites for good fossil
records could not preserve such organisms.
Barghoorn and colleagues dispelled a century of frustration by looking
in a different place, where cellular remains of bacteria might be
preserved--in chert beds. Chert has the same chemical formula (with a
different molecular arrangement) as quartz: silicon dioxide. Paleontologists
rarely think of looking for fossils in silicate rocks-for the perfectly
valid and utterly obvious reason that silicates form by the cooling of
volcanic magmas and therefore cannot contain organic remains. (Life, after
all, doesn't flourish in bubbling lavas, and anything falling in gets burnt
to a crisp.) But cherts can form at lower temperatures and be deposited amid
layers of ordinary sediments in oceanic waters. Bacterial cells, when
trapped in this equivalent of surrounding glass, can be preserved as
fossils.
This cardinal insight--that we had been searching in the wrong venue of
ordinary sediments rather than in fruitful cherts--created an entire field
of study: collecting data from the first two-thirds or so of life's full
history. Forty years later, we may look back with wonder at the flood of
achievement and the complete overturn of established wisdom. We now possess
a rich fossil record of early life, extending right back to the earliest
potential source for cellular evidence. (The oldest rocks on earth that
could preserve such data do contain abundant fossils of bacterial organisms.
These 3.5- to 3.6-billion-year-old rocks from Australia and South Africa are
the most ancient strata on earth that have not been sufficiently altered by
subsequent heat and pressure to destroy all anatomical evidence of life.)
Such ubiquity and abundance have forced a reversal of the old view. Life of
simplest bacterial grade now seems inevitable rather than improbable. As a
mantra for memory, may I suggest: "Life on earth is as old as it could be:'
I realize, of course, that an earliest possible appearance constitutes no
proof of inevitability. After all, even a highly improbable event might
occur, by good fortune, early in a series of trials. (You might flip those
fifty successive heads on your tenth attempt, but don't count--or bet--on
it.) Nonetheless, faced with the data we now possess--that life appears as
soon as it could and remains pervasive forever after÷our thoughts must move
to ideas about almost predictive inevitability. Given a planet of earthly
size, distance from a central star, and composition, life of simplest grade
may originate with virtual certainty as a consequence of principles of
organic chemistry and the physics of self-organizing systems.
But whatever the predictability of life's origin, the subsequent pathways of
evolution have been mighty peculiar, at least with respect to our
conventional hopes and biases. The broadest pattern might seem to confirm
our usual view of generally increasing complexity, leading sensibly to human
consciousness; after all, the early earth sported only bacteria but now
features people, ant colonies, and oak trees. Fair enough, but any scrutiny
of general timings or particular details leaves little faith in any steady
pattern. If greater size and complexity bestow such Darwinian blessings, why
did life take so long to proceed ãonward,â and why do most of the supposed
steps occur so quirkily and so quickly? Consider the following epitome of
major events.
Fossils, as stated above, appear as soon as they possibly could in the
geological record. But life then remains exclusively at this simplest
so-called prokaryotic grade (unicells without any internal organelles--that
is, no nuclei, chromosomes, mitochondria, and so on) for about half its
subsequent history; the first unicells of the more complex eukaryotic grade
(with the conventional organelles of our high-school text figures of an
amoeba or paramecium) do not appear in the fossil record until about 2 billi
on years ago. The three great multicellular kingdoms of plants, fungi, and
animals arise subsequently (and, at least for algae within the plant
kingdom, more than once and independently) to eukaryotic unicells. Fossils
of simple multicellular algae extend back fairly reliably about 1 billion
years, and far more conjecturally to as many as 1.8 billion years.
But the real enigma--at least with respect to our parochial concerns about
the progressive inevitability of our own lineage--surrounds the origin and
early history of animals. If life had always been hankering to reach a
pinnacle of expression as the animal kingdom, then organic history seemed in
no hurry to initiate this ultimate phase. About five-sixths of life's
history had passed before animals made their first appearance in the fossil
record, some 600 million years ago. Moreover, as previous essays in this
series have explained in far greater detail, Earth's first community of
animals--which held nearly exclusive sway from the time of its appearance
right up to the dawn of the Cambrian period, 543 million years
ago--consisted of enigmatic species with no clear relation to modern forms.
These so-called Ediacaran animals (named for the locality of first discovery
in Australia, but now known from all continents) could grow quite large--up
to a few feet in length--but apparently contained neither complex internal
organs nor even any recognizable body openings of mouth, anus, and so on.
Many Ediacaran creatures were flattened forms, in a variety of shapes and
sizes, built of numerous tubelike sections complexly quilted together into a
single structure. Theories about the affinities of Ediacaran organisms span
the full gamut--from viewing them (most conventionally) as simple ancestors
for several modern phyla to interpreting them (most radically) as an
entirely separate, and ultimately failed, experiment in multicellular animal
life. An intermediate position now gaining favor (a situation that should
lead to no predictions about the ultimate outcome of this complex debate)
treats Ediacaran animals as a bountiful expression of the range of
possibilities for diploblastic animals (built of two body layers), a group
now so reduced in diversity (subsisting only as corals, jellyfishes, and
their allies) that living representatives provide little understanding of
full potentials.
Modern animals--except for sponges, corals, and a few other minor groups÷are
all triploblastic, or composed of three body layers: an ectoderm, forming
nervous tissue and other organs; mesoderm, forming reproductive structures
and other parts; and endoderm, building the gut and other internal organs.
(If you learned a conventional list of phyla back in high school biology,
all groups from the flatworms on ãup"--including the five "big" phyla of
annelids, arthropods, mollusks, echinoderms, and vertebrates--are
triploblasts.) This three-layered organization seems to act as a
prerequisite for the formation of conventional, complex, mobile, bilaterally
symmetrical organisms with body cavities, appendages, sensory organs, and
all other accoutrements setting our standard picture of a "proper" animal.
Thus, in our parochial manner (and ignoring such truly important groups as
corals and sponges), we tend to equate the problem of the beginning of
modern animals with the origin of triploblasts. If the Ediacaran animals are
all (or mostly) diploblasts, or something even more genealogically divergent
from triploblastic animals, then this first fauna does not resolve the
problem of the origin of animals (in our conventionally limited sense of
modern triploblasts).
The story of modern animals then becomes even more curious. The inception of
the Cambrian period marks the extinction, perhaps quite rapid, of the
Ediacaran fauna and the beginning of a rich record for animals with
calcareous skeletons easily preserved as fossils. But the first phase of the
Cambrian, called Manakayan, lasting from 543 to 530 million years ago,
features primarily a confusing set of spines, plates, and other bits and
pieces called (even in our technical literature) the SSF, or "small shelly
fossils" (presumably the disarticulated fragments of skeletons that had not
yet evolved to large, discrete units covering the entire organism).
The next two phases of the Cambrian (called Tommotian and Atdabanian and
ranging from 530 to about 520 million years ago) mark the strangest, most
important, and most intriguing of all episodes in the fossil record of
animals--the short interval known as the Cambrian explosion and featuring
the first appearance of all animal phyla with skeletons subject to easy
preservation in the fossil record. (A single exception, a group of colonial
marine organisms called the Bryozoa, makes its appearance at the beginning
of the next, or Ordovician, period. Many intriguing "inventions,ä including
human consciousness and the dance language of bees, have arisen since then,
but no new phyla or animals of starkly divergent anatomical design.)
The Cambrian explosion ranks as such a definitive episode in the history of
animals that we cannot possibly grasp the basic tale of our own kingdom
until we achieve better resolution for both the antecedents and the
unfolding of this cardinal geological moment. The second discovery treated
in this essay, announced in February 1998 and also based on learning to look
in a previously unsuspected place, has thrilled the entire paleontological
community for its promise in unraveling the previously unknown history of
triploblast animals before the Cambrian explosion.
If the Cambrian explosion inspires frustration for its plethora of data--too
much, too confusing, and too fast--the Precambrian history of triploblast
animals engenders even more chagrin for its dearth. The complex animals of
the explosion, so clearly assignable to modern phyla, didn't arise ex nihilo
at their first moment of fossilization, so what (and where) are their
antecedents in Precambrian times? What were the forebears of modern animals
doing for 50 million prior years, when Ediacaran diploblasts (or stranger
creatures) ruled the animal world?
Up to now we have engaged in much speculation while possessing only a whiff
or two of data. Ediacaran strata also contain trails and feeding traces
presumably made by triploblast organisms of modern design (for the flattened
and mostly immobile Ediacaran animals could not crawl, burrow or feed in a
manner suggestive of activities now confined to triploblast organisms).
Thus, we do have evidence for the existence, and even the
activities, of precursors of modern animals before the Cambrian explosion,
but no data about their anatomy and appearance--a situation akin to the
frustration we might feel if we could hear birdsong but had never seen
birds.
A potential solution--or, at the very least, a firm and first source of
anatomical data--has just been discovered by applying the venerable motto
(so beloved by people, including yours truly, of shorter-than-average
stature): Good Things Often Come in Small Packages, or, to choose a more
literary and inspirational expression, Micah's statement (5:2), taken by the
later evangelists as a prophecy of things to come: "But thou,
Bethlehem·though thou be little among the thousands of Judah, yet out of
thee shall he come forth unto me that is to be ruler in Israel·.ä
In short, paleontologists had been looking for conventional fossils in the
usual (and visible) size ranges of adult organisms: fractions to a few
inches. But a solution had been lurking in the realm of smaller-sized
creatures just barely visible (in principle) but undetectable in
conventional practice--in the domain of embryos. But who would ever have
thought that delicate embryos might be preserved as fossils when presumably
hardier adults left no fragments of their existence? The story, a
fascinating lesson in the ways of science, has been developing for more than
a decade, but has only just found application to the problem of Precambrian
animals.
Fossils form in many modes and styles--as original hard parts preserved
within entombing sediments or as secondary structures formed by impressions
of bones or shells (molds) that may then become filled with later sediments
(casts). But original organic materials may also be replaced by percolating
minerals--a process called petrifaction, or, literally, "making into stone,ä
a phenomenon perhaps best represented in popular knowledge by gorgeous
specimens from the Petrified Forest in Arizona, where multicolored agate
(another form of silicon dioxide) has replaced original carbon so precisely
that the wood's cellular structure can still be discerned. (Petrifaction
enjoys sufficient public renown for many people to mistakenly regard such
replacement as the primary definition of a fossil. Not at all; any bit of an
ancient organism qualifies as a fossil, whatever its style of preservation.
In almost any circumstance, a professional would much prefer to work with
unaltered hard parts than with petrified replacements.)
In any case, one poorly understood style of petrifaction leads to
replacement of soft tissues by calcium phosphate--a process called
phosphatization. This style of replacement can occur within days of death,
thus leading to the rare and precious phenomenon of petrifaction before
decay of soft anatomy. Phosphatization might provide a paleontologist's
Holy Grail if all soft tissues could thus be preserved at any size in any
kind of sediment. Alas, the process seems to work in detail only for tiny
objects up to about 2 millimeters in length. (Since 25.4 millimeters make
an inch, we are talking about barely visible dots, not even about bugs large
enough to be deemed "yucky" when found on our dinner plates or in our beds.)
Still, on the good old principle of not looking gift horses (or unexpected
bounties) in the mouth (by complaining about an unavailable better deal),
let us rejoice in the utterly unanticipated prospect that tiny
creatures--which are, after all, ever so abundant in nature, however much
they may generally pass beneath our exalted notice--might become petrified
in sufficient detail to preserve their bristles, hairs, or even their
cellular structure. The recognition that phosphatization may open up an
entire world of tiny creatures, previously never considered as candidates
for fossilization at all, may spark the greatest burst of paleontological
exploration since the discovery that 2 billion years of Precambrian life lay
hidden in chert.
The first hints that phosphatization of tiny creatures might resolve key
issues in the early evolution of animals dates to a discovery made in the
mid-1970s and then researched and reported in one of the most elegant (but
rather sadly under-appreciated) series of papers ever published in the
history of paleontology: the work of German scientists Klaus J. Muller and
Dieter Walossek on the fauna of distinctive Upper Cambrian rocks in Sweden
known as Orsten beds. In these layers of limestone concretions, tiny
arthropods (mostly larvae of crustaceans) have been preserved by
phosphatization in exquisite, three-dimensional detail. The photography and
drawings of Walossek and Muller have rarely been equaled in clarity and
aesthetic brilliance, and their papers are a delight both to read and see.
(For a good early summary, consult Muller and Walossek: "A remarkable
arthropod fauna from the Upper Cambrian 'Orsten' of Sweden," 1985,
Transactions of the Royal Society of Edinburgh, vol.76, pp.161-172; for a
recent review, see Walossek and Muller: "Cambrian 'Orsten'-type arthropods
and the phylogeny of Crustacea," in R. A. Fortey and R. H. Thomas [eds.],
Arthropod Relationships, London: Chapman and Hall, 1997.)
By dissolving the limestone in acetic acid, Walossek and Muller can recover
the tiny, phosphatized arthropods intact. They have collected more than
100,000 specimens following this procedure and have summarized their
findings in their paper of 1997 cited above:
The cuticular surface of these arthropods is still present in full detail,
revealing eyes and limbs, hairs and minute bristles·gland openings, and even
cellular patterns and grooves of muscle attachments underneath·The maximum
size of specimens recovered in this type of preservation does not exceed 2
mm.
and downward in growth from larvae to early embryonic stages containing just
a few cells. In 1994, Xi-guang Zhang and Brian R. Pratt found balls of
presumably embryonic cells measuring 0.30 to 0.35 millimeters in diameter
and representing, perhaps, the earliest stages of adult trilobites, which
are also found in the same Middle Cambrian strata (see Zhang and Pratt:
"Middle Cambrian arthropod embryos with biastomeres," 1994, Science, vol.
266, pp. 637-38). Just last year, Stefan Bengston and Yue Zhao reported even
earlier phosphatized embryos from basal Cambrian strata in China and
Siberia. In an exciting addition to this growing literature, these authors
traced a probable growth series--from embryos to tiny near adults--for two
entirely different animals: a species from an enigmatic extinct group, the
conulariids; and a probable segmented worm (see Bengston and Zhao,
"Fossilized metazoan embryos from the earliest Cambrian," 1997, Science,
vol.277, pp. 1645-48).
When such novel techniques first encounter materials from a truly unknown or
unsuspected world, genuinely revolutionary conclusions often emerge. In what
may well go down in history as the greatest paleontological discovery of the
late twentieth century, Shuhai Xiao, a postdoctoral student in our
paleontological program; Yun Zhang, of Beijing University; and my colleague
(and Shuhai Xiao's mentor) Andrew H. Knoll, have just reported their
discovery of the oldest triploblastic animals, preserved as phosphatized
embryos in rocks from southern China estimated at 570 million years of age
(and thus even older than the richest Ediacaran faunas found in strata about
10 million years younger [see Xiao, Zhang, and Knoll, "Three-dimensional
preservation of algae and animal embryos in a Neoproterozoic phosphorite,"
1998, Nature, vol.391, pp.553-581). These phosphatized fossils include a
rich variety of multicellular algae, showing, according to the authors, that
"by the time large animals enter the fossil record, the three principal
groups of multicellular algae had not only diverged from other protistan
[uniceliular] stocks but had evolved a surprising degree of the
morphological complexity exhibited by living algae.ä
Given our understandably greater interest in our own animal kingdom,
however, most of the attention will be riveted upon some smaller and rarer
globular fossils, averaging half a millimeter in diameter and found
phosphatized in the same strata: an exquisite series of earliest embryonic
stages, beginning with a single fertilized egg and proceeding through
two-cell, four-cell, eight-cell, and sixteen-cell stages to small balls of
cells representing slightly later phases of early development. These embryos
cannot be assigned to any particular group (more distinctive, later stages
have not yet been found) but their identification as earliest stages of
triploblastic animals seems secure, both from characteristic features
(especially the overall size of the embryo during these earliest stages,
which remains unchanged as average cell size decreases to pack more cells
into a constant space) and from their uncanny resemblance to particular
traits of living groups (several embryologists have told Knoll and
colleagues that they would have identified these specimens as embryos of
living crustaceans had they not been informed of their truly ancient age).
Elso Barghoorn, Knoll's thesis advisor, opened up the world of earliest life
by discovering that bacteria could be preserved in chert. Now, a full
generation later, Knoll and colleagues have penetrated the world of the
earliest known ancestors of triploblast animals by accessing a new domain
where phosphatization preserves minute embryonic stages but no known process
of fossilization can reliably render potentially larger phases of growth.
When I consider the cascade of knowledge that proceeded from Barghoorn's
first report of Precambrian bacteria to our current record spanning three
billion Precambrian years and hundreds of recorded forms, I can only
conclude that the discovery by Xiao, Zhang, and Knoll places us at a gateway
of equal promise for reconstructing the earliest history of modern animals,
before their overt evolutionary burst to large size and greatly increased
anatomical variety in the subsequent Cambrian explosion. If we can, thereby,
gain any insight into the greatest of all mysteries surrounding the early
evolution of animals--the causes of both the anatomical explosion itself and
the "turning off" of evolutionary fecundity to generate new phyla
thereafter--then paleontology will shake hands with evolutionary theory in
the finest merger of talents ever applied to the resolution of a historical
enigma.
Two final comments might help to establish a context of both humility and
excitement at the threshold of this new quest. First, we might be able to
coordinate the growing direct evidence of fossils with a potentially
powerful indirect method for judging the times of origin and branching for
major animal groups: the measurement of relative degrees of detailed
genetic similarity among living representatives of diverse animal phyla.
Such measurements can be made with great precision upon large masses of
data, but firm conclusions are hard to obtain because various genes evolve
at different rates that also maintain no constancy over time, and most
methods applied so far have made simplifying (and probably unjustified)
assumptions about relatively even ticking of supposed molecular clocks.
For example, in a paper that received much attention upon publication in
1996, G. A. Wray, J. S. Levinton, and L. H. Shapiro used differences in the
molecular sequences of seven genes in living representatives of major phyla
to derive an estimate of roughly 1.2 billion years for the divergence time
between chordates (our phylum) and the three great groups on the other major
branch of animals (arthropods, annelids, and mollusks) and 1 billion years
for the later divergence of chordates from the more closely related phylum
of echinoderms (see Wray, Levinton, and Shapiro, "Molecular evidence for
deep Precambrian divergences among metazoan phyla," 1996, Science, vol.274,
pp.568-73).
This paper sowed a great deal of unnecessary confusion when several
uncomprehending journalistic reports, and a few careless statements by the
authors, raised the old canard that such an early branching time for animal
phyla disproves the reality of the Cambrian explosion by rendering this
apparent burst of diversity as the artifact of an imperfect fossil record
(signifying, perhaps, only the invention of hard parts, rather than any
acceleration of anatomical innovation). For example, Wray et al. write: "Our
results cast doubt on the prevailing notion that the animal phyla diverged
explosively during the Cambrian or late Vendian [Ediacaran times], and
instead suggest that there was an extended period of divergence·commencing
about a billion years ago.ä
But such statements confuse the vital distinction, in both evolutionary
theory and actual results, between times of initial branching and subsequent
rates of anatomical innovation or evolutionary change in general. Even the
most vociferous advocates of a genuine Cambrian explosion have never argued
that this period of rapid anatomical diversification represents the moment
of origin for animal phyla--if only because we all acknowledged the evidence
for Precambrian tracks and trails of triploblasts even before the recent
discovery of embryos. Nor do these same vociferous advocates imagine that
only one wormlike species crawled across the great Cambrian divide to serve
as an immediate common ancestor for all modern phyla. In fact, I don't see
that it matters one whit (for the reality of the explosion--although it
matters a great deal for other evolutionary issues) whether one wormlike
species carrying the ancestry of all later animals, or ten similar wormlike
species already representing the lineages of ten subsequent phyla, crossed
this great divide from an earlier Precambrian history. The Cambrian
explosion embodies a claim for a rapid spurt of anatomical innovation within
the animal kingdom, not a statement about times of genealogical divergence.
The following example should clarify the fundamental distinction between
times of genealogical splitting and rates of change. Both rhinoceroses and
horses may have evolved from the genus Hyracotherium (formerly called
Eohippus). A visitor to the Eocene earth, about 50 million years ago, might
determine that the basic split had already occurred. He might be able to
identify one species of Hyracotherium is the ancestor of all later horses,
and another species of the same genus as the progenitor of all subsequent
rhinos. But such a visitor would be ridiculed with justified scorn if he
then argued that later divergences between horses and rhinos must be
illusory because the two lineages had already split. After all, the two
Eocene species looked like kissing cousins (as evidenced by their placement
in the same genus) and only gained their later status as progenitors of
highly distinct lineages by virtue of a subsequent history, utterly
unknowable at the time of splitting. Similarly, if ten nearly identical
wormlike forms (the analogs of the two Hyracotherium species) crossed the
Cambrian boundary but evolved the anatomical distinctions that would make
them great phyla only during the subsequent explosion, then the explosion
itself remains as real--and as vitally important for life's history--as any
advocate has ever averred.
This crucial distinction has been recognized by most commentators on the
work of Wray et al. Geerat J. Vermeij, in his direct evaluation (Science,
1996, page 526), wrote that "this new work in no way diminishes the
significance of the Vendian-Cambrian revolution.ä Fortey, Briggs, and Wills
added that "there is, of course, no necessary correspondence between
morphology and genomic change." (See BioEssays, 1997, vol.19, p.433.) In any
case, a recent publication by Ayala, Rzhetsky and Ayala (Proceedings of the
National Academy of Sciences, vol. 95, 1998, pp. 606-11) presents a powerful
rebuttal to Wray et al.'s conclusions. By correcting statistical errors and
unwarranted assumptions, and by adding data for twelve additional genes,
these authors provide a very different estimate for initial diversification
in late Precambrian times: about 670 million years ago for the split of
chordates from the line of arthropods, annelids, and mollusks; and 600
million years ago for the later divergence of chordates from echinoderms.
We are left, of course, with a key mystery (among many others): where are
Precambrian adult triploblasts "hiding" now that we have discovered their
embryos? An old suggestion, dating from the 1870s and devised by the
bombastic German theorist Ernst Haeckel (who was, nonetheless, outstandingly
right far more often than random guesswork would allow) held that
Precambrian animals had evolved as tiny forms not much larger than, or very
different from, modern embryos--and would therefore be very hard to find as
fossils. (The similarity between Haeckel's speculative ancestors and Xiao,
Zhang, and Knoll's actual embryos is almost eerie.) Recently, in a brilliant
paper, E. H. Davidson, K. J. Peterson, and K. A. Cameron (Science, 1995,
vol. 270, pp. 1319-25) have made a powerful case, based on genetic and
developmental arguments, that Precambrian animals did originate at tiny
sizes, and that the subsequent Cambrian explosion depended upon the
evolution of novel embryological mechanisms for greatly increasing cell
number and body size, accompanied by consequent potential for greatly
enhanced anatomical innovation. If Haeckel's old argument, buttressed by
Davidson's new concepts and data, has validity, we then gain genuine hope,
even realistic expectation, that Precambrian adult triploblasts may soon be
discovered, for such animals will be small enough to be preserved by
phosphatization.
As a final point, this developing scenario for the early history of animals
might foster humility and generate respect for the complexity of
evolutionary pathways. To make the obvious analogy, we used to regard the
triumph of "superiorä mammals over "antediluvian" dinosaurs as an inevitable
consequence of progressive evolution. We now realize that mammals originated
at the same time as dinosaurs and then lived for more than 100 million years
as marginal, small-bodied creatures in the nooks and crannies of a
dinosaur's world. Moreover, mammals would never have expanded to dominate
terrestrial ecosystems (and humans would surely never have evolved) without
the supreme good fortune (for us) of a catastrophic extraterrestrial impact
that, for some set of unknown reasons, eliminated dinosaurs and gave mammals
an unanticipated opportunity
Does the earlier story of Ediacaran "primitives" versus contemporary
Precambrian ancestors of modern animals differ in any substantial way? We
now know (from the evidence of Xiao, Zhang, and Knoll's embryos) that
animals of modern design had already originated before the Ediacaran fauna
evolved into full bloom. Yet "primitive" Ediacara dominated the world of
animal life for at least 50 million years, while modern triploblasts waited
in the proverbial wings, perhaps as tiny animals of embryonic size, living
in nooks and crannies permitted by much larger Ediacaran dominants. Only a
mass extinction of unknown cause, which wiped out Ediacara and initiated the
Cambrian transition 543 million years ago, gave modern triploblasts an
opportunity to shine--and so we have.
In evolution, as well as in politics, incumbency offers such powerful
advantages that even a putatively more competent group may be forced into a
long period of watchful waiting, hoping for an external stroke of good luck
to pick up the reins of power. If fortune continues to smile, the new regime
may even gain enough confidence to invent a comforting and commanding
mythology about the inevitability of its necessary rise to power by
gradually growing better and better--every day and in every way.
Stephen Jay Gould teaches biology geology, and the history of science at
Harvard University. He is also the Frederick P. Rose Honorary Curator in
Invertebrates at the American Museum of Natural History.