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Department of Earth and Space Sciences, Institute of Geophysics and
Planetary Physics (Center for the Study of Evolution and the Origin of
Life), and Molecular Biology Institute, University of California, Los
Angeles, CA 90095-1567
In 1859, in On the Origin of Species, Darwin
broached what he regarded to be the most vexing problem facing his
theory of evolution In 1950, when Ledyard
Stebbins' Variation and Evolution in Plants (1) first
appeared, the known history of life Darwin's Dilemma.
Like so many aspects of natural science, the beginnings of the search
for life's earliest history date from the mid-1800s and the writings
of Charles Darwin (1809-1882), who in On the Origin of
Species first focused attention on the missing Precambrian fossil
record and the problem it posed to his theory of evolution: "There
is another ... difficulty, which is much more serious. I allude to
the manner in which species belonging to several of the main divisions
of the animal kingdom suddenly appear in the lowest known
[Cambrian-age] fossiliferous rocks ... If the theory be true, it
is indisputable that before the lowest Cambrian stratum was deposited,
long periods elapsed ... and that during these vast periods, the
world swarmed with living creatures ... [But] to the question why
we do not find rich fossiliferous deposits belonging to these assumed
earliest periods before the Cambrian system, I can give no satisfactory
answer. The case at present must remain inexplicable; and may be truly
urged as a valid argument against the views here entertained" (ref.
2, Chapter X).
Colloquium Paper
Solution to Darwin's dilemma: Discovery of the missing
Precambrian record of life
Abstract
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Abstract
Introduction
Pioneering Pathfinders
Emergence of a New...
Lessons from the Hunt
References
the lack of a rich fossil record predating the
rise of shelly invertebrates that marks the beginning of the Cambrian Period of geologic time (
550 million years ago), an
"inexplicable" absence that could be "truly urged as a valid
argument" against his all embracing synthesis. For more than 100 years, the "missing Precambrian history of life" stood out as one
of the greatest unsolved mysteries in natural science. But in recent
decades, understanding of life's history has changed markedly as the
documented fossil record has been extended seven-fold to some 3,500 million years ago, an age more than three-quarters that of the planet itself. This long-sought solution to Darwin's dilemma was set in
motion by a small vanguard of workers who blazed the trail in the 1950s
and 1960s, just as their course was charted by a few pioneering
pathfinders of the previous century, a history of bold pronouncements,
dashed dreams, search, and final discovery.
Introduction
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Abstract
Introduction
Pioneering Pathfinders
Emergence of a New...
Lessons from the Hunt
References
the familiar progression from
spore-producing to seed-producing to flowering plants, from marine
invertebrates to fish, amphibians, then reptiles, birds, and
mammalsextended only to the beginning of the Cambrian Period of the
Phanerozoic Eon, roughly 550 million years ago. Now, after a
half-century of discoveries, life's history looks strikingly
differentan immense early fossil record, unknown and assumed
unknowable, has been uncovered to reveal an evolutionary progression
dominated by microbes that stretches seven times farther into the
geologic past than previously was known. This essay is an abbreviated
history of how and by whom the known antiquity of life has been
steadily extended, and of lessons learned in this still ongoing hunt
for life's beginnings.
Pioneering Pathfinders
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Abstract
Introduction
Pioneering Pathfinders
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Lessons from the Hunt
References
the case "inexplicable"for more than 100 years, the intervening century was not without bold pronouncements, dashed dreams, and more than little acid acrimony.
J. W. Dawson and the "Dawn Animal of Canada." Among the first to take up the challenge of Darwin's theory and its most vexing problem, the missing early fossil record, was John William Dawson (1820-1899), Principal of McGill University and a giant in the history of North American geology. Schooled chiefly in Edinburgh, Scotland, the son of strict Scottish Presbyterians, Dawson was a staunch Calvinist and devout antievolutionist (3).
In 1858, a year before publication of Darwin's opus, specimens of distinctively green- and white-layered limestone collected along the Ottawa River to the west of Montreal were brought to the attention of William E. Logan, Director of the Geological Survey of Canada. Because the samples were known to be ancient (from "Laurentian" strata, now dated at about 1,100 million years) and exhibited layering that Logan supposed too regular to be purely inorganic (Fig. 1), he displayed them as possible "pre-Cambrian fossils" at various scientific conferences, where they elicited spirited discussion but gained little acceptance as remnants of early life.
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C. D. Walcott: Founder of Precambrian Paleobiology. Fortunately, Dawson's debacle would ultimately prove to be little more than a distracting detour on the path to progress, a redirection spurred initially by the prescient contributions of the American paleontologist Charles Doolittle Walcott (1850-1927).
Like Dawson before him, Walcott was enormously energetic and highly influential (7, 8). He spent most of his adult life in Washington, DC, where he served as the CEO of powerful scientific organizationsfirst,
as Director of the U.S. Geological Survey (1894-1907), then Secretary
of the Smithsonian Institution (1907-1927) and President of the
National Academy of Sciences (1917-1923). Surprisingly, however,
Walcott had little formal education. As a youth in northern New York
State he received but 10 years of schooling, first in public schools
and, later, at Utica Academy (from which he did not graduate). He never
attended college and had no formally earned advanced degrees (a
deficiency more than made up for in later life when he was awarded
honorary doctorates by a dozen academic institutions).
In 1878, as a 28-year-old apprentice to James Hall, Chief Geologist of
the state of New York and acknowledged dean of American paleontology,
Walcott was first introduced to stromatoliteswavy layered
mound-shaped rock masses laid down by ancient communities of
mat-building microbesCambrian-age structures near the town of
Saratoga in eastern New York State. Named Cryptozoon
(meaning "hidden life"), these cabbagelike structures (Fig.
2) would in later years form the basis of
Walcott's side of a nasty argument known as the
"Cryptozoon controversy."
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small, millimeter-sized black coaly discs that
Walcott named Chuaria and interpreted to be "the remains of ... compressed conical shell[s]," possibly of primitive
brachiopods (10). Although Chuaria is now known to be a
large single-celled alga, rather than a shelly invertebrate, Walcott's
specimens were indeed authentic fossils, the first true cellularly
preserved Precambrian organisms ever recorded.
After the turn of the century, Walcott moved his field work northward
along the spine of the Rocky Mountains, focusing first in the Lewis
Range of northwestern Montana, from which he reported diverse
stromatolitelike structures (11) and, later, chains of minute cell-like
bodies he identified as fossil bacteria (12). His studies in the
Canadian Rockies, from 1907 to 1925, were even more rewarding,
resulting in discovery of an amazingly well preserved assemblage of
Cambrian algae and marine invertebratesthe famous Burgess Shale Fauna
that to this day remains among the finest and most complete samples of
Cambrian life known to science (13, 14).
Walcott's contributions are legendaryhe was the first discoverer in
Precambrian rocks of Cryptozoon stromatolites, of cellularly preserved algal plankton (Chuaria), and of possible fossil
bacteria, all capped by his pioneering investigations of the benchmark
Burgess Shale fossils. The acknowledged founder of Precambrian
paleobiology (15), Walcott was first to show, nearly a century ago and
contrary to accepted wisdom, that a substantial fossil record of
Precambrian life actually exists.
A. C. Seward and the Cryptozoon Controversy.
The rising tide in the development of the field brought on by
Walcott's discoveries was not yet ready to give way to a flood. Precambrian fossils continued to be regarded as suspect, a view no
doubt bolstered by Dawson's Eozoon debacle but justified
almost as easily by the scrappy nature of the available evidence.
Foremost among the critics was Albert Charles Seward (1863-1941),
Professor of Botany at the University of Cambridge and the most widely
known and influential paleobotanist of his generation. Because
practically all claimed Precambrian fossils fell within the purview of
paleobotanywhether supposed to be algal, like Cryptozoon
stromatolites, or even bacterialSeward's opinion had special impact.
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Emergence of a New Field of Science |
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In the mid-1960sa full century after Darwin broached the problem
of the missing early fossil recordthe hunt for early life began to
stir, and in the following two decades the flood-gates would finally
swing wide open. But this surge, too, had harbingers, now dating from
the 1950s.
A Benchmark Discovery by an Unsung Hero.
The worker who above all others set the course for modern studies
of ancient life was Stanley A. Tyler (1906-1963) of the University of
Wisconsin, the geologist who in 1953 discovered the now famous
mid-Precambrian (2,100-million-year-old) microbial assemblage petrified
in carbonaceous cherts of the Gunflint Formation of Ontario, Canada. A
year later, together with Harvard paleobotanist Elso S. Barghoorn
(1915-1984), Tyler published a short note announcing the discovery
(17), a rather sketchy report that on the basis of study of
petrographic thin sections documents that the fossils are
indigenousness to the deposit but fails to note either the exact
provenance of the find or that the fossils are present within, and were
actually the microbial builders of, large Cryptozoon-like stromatolites (an association that, once recognized, would prove key to
the development of the field). Substantive, full-fledged reports would
come lateralthough not until after Tyler's untimely death, an event
that cheated him from receiving the great credit he deservedbut this
initial short article on "the oldest structurally preserved
organisms that clearly exhibit cellular differentiation and original
carbon complexes which have yet been discovered in pre-Cambrian
sediments" (17) was a benchmark, a monumental "first."
Contributions of Soviet Science.
At about the same time, in the mid-1950s, a series of articles by
Boris Vasil'evich Timofeev (1916-1982) and his colleagues at the
Institute of Precambrian Geochronology in Leningrad (St. Petersburg)
reported discovery of microscopic fossil spores in Precambrian
siltstones of the Soviet Union. In thin sections, like those studied by
Tyler and Barghoorn, fossils are detected within the rock, entombed in
the mineral matrix, so the possibility of laboratory contamination can
be ruled out. But preparation of thin sections requires special
equipment, and their microscopic study is tedious and time-consuming. A
faster technique, pioneered for Precambrian studies in Timofeev's lab,
is to dissolve a rock in mineral acid and concentrate the
organic-walled microfossils in the resulting sludgelike residue. This
maceration technique, however, is notoriously subject to error-causing
contaminationand because during these years there was as yet no
established early fossil record with which to compare new finds,
mistakes were easy to make. Although Timofeev's laboratory was
not immune, much of his work has since proved sound (18), and the
technique he pioneered to ferret-out microfossils in Precambrian shaley
rocks is now in use worldwide.
Famous Figures Enter the Field. Early in the 1960s, the fledgling field was joined by two geologic heavyweights, an American, Preston Cloud (1912-1991), and an Australian, Martin Glaessner (1906-1989), both attracted by questions posed by the abrupt appearance and explosive evolution of shelly invertebrate animals that marks the start of the Phanerozoic Eon.
A feisty leader in the development of Precambrian paleobiology, Cloud was full of energy, ideas, opinions, and good hard work. His Precambrian interests were first evident in the late-1940s, when he argued in print that although the known Early Cambrian fossil record is woefully incomplete it is the court of last resort and, ultimately, the only court that matters (19). By the 1960s, he had become active in the field, authoring a paper that to many certified the authenticity of the Tyler-Barghoorn Gunflint microfossils (20) and, later, a series of reports adding new knowledge of the early microbial fossil record (21-23). But above all, he was a gifted synthesist, showing his mettle in a masterful article of 1972 that set the stage for modern understanding of the interrelated atmospheric-geologic-biologic history of the Precambrian planet (24). In the early 1960s, a second prime player entered this now fast-unfolding field, Martin Glaessner (1906-1989), of the University of Adelaide in South Australia. A scholarly, courtly, old-school professor, Glaessner was the first to make major inroads toward understanding the (very latest) Precambrian record of multicelled animal life (25, 26). In 1947, three years before Glaessner joined the faculty at Adelaide, Reginald C. Sprigg announced his discovery of fossils of primitive soft-bodied animals, chiefly imprints of saucer-sized jellyfish, at Ediacara, South Australia (27). Although Sprigg thought the fossil-bearing beds were Cambrian in age, Glaessner showed them to be Precambrian (albeit marginally so), making the Ediacaran fossils the oldest animals known. Together with his colleague, Mary Wade, Glaessner spent much of the rest of his life working on this benchmark fauna (see, e.g., refs. 28 and 29), bringing it to international attention in the early 1960s (30) and, later, in a splendid monograph (31). With Glaessner in the fold, the stage was set. Like a small jazz bandTyler and Barghoorn trumpeting microfossils in cherts, Timofeev
beating on fossils in siltstones, Cloud strumming the early
environment, Glaessner the earliest animalsgreat music was about to
be played. At long last, the curtain was to rise on the missing record
of Precambrian life.
Breakthrough to the Present. My own involvement dates from 1960, when as a sophomore in college I became enamored with the problem of the missing Precambrian fossil record, an interest that was to become firmly rooted during the following few years, when I was the first of Barghoorn's graduate students to focus on early life. I have recently recounted in some detail my recollections of those heady days (32) and need not reiterate the story here. Suffice it to note that virtually nothing had been published on the now-famous Gunflint fossils (Fig. 3) in the nearly 10 years that had passed between the Tyler-Barghoorn 1954 announcement of the find and my entry into graduate school in June, 1963. Then, quite unexpectedly, in October of that year, Stanley Tyler passed away at the age of 57, never to see the ripened fruits of his long-term labor reach the published page. Within a year thereafter, a series of events that would shape the field began to unfold, set off first by a squabble between Barghoorn and Cloud as to who would scoop whom in a battle for credit over the Gunflint fossils (32). By late 1964 this spat had been settled, with Cloud electing to hold off publication of his paper "illustrating some conspicuous Gunflint nannofossils and discussing their implications until Barghoorn could complete his part of a descriptive paper with the by-then deceased Tyler" (ref. 33, p. 23). The two articles appeared in Science in 1965, first Barghoorn and Tyler's "Microorganisms from the Gunflint chert" (34), followed a few weeks later by Cloud's contribution, "Significance of the Gunflint (Precambrian) microflora" (20). Landmark papers they were!
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backed by Cloud's affirmation of its
significancegenerated enormous interest. Yet it soon became apparent
that acceptance of ancient life would come only grudgingly. The well
had been poisoned by Dawson's debacle, the Cryptozoon
controversy, Seward's criticismobject lessons that had been handed
down from professor to student, generation to generation, to become
part of accepted academic lore. Moreover, it was all too obvious that
the Gunflint organisms stood alone. Marooned in the remote Precambrian,
they were isolated by nearly a billion and half years from all other fossils known to science, a gap in the known fossil record nearly three
times longer than the entire previously documented history of life.
Skepticism abounded. Conventional wisdom was not to be easily
dissuaded. The question was asked repeatedly: "Couldn't this whole
business be some sort of fluke, some hugely embarrassing awful mistake?"
As luck would have it, the doubts soon could be laid to rest. During
field work the previous year (and stemming from a chance conversation
with a local oil company geologist by the name of Helmut Wopfner),
Barghoorn had collected a few hand-sized specimens of Precambrian
stromatolitic black chert in the vicinity of Alice Springs, deep in the
Australian outback. Once the Gunflint paper had been completed, I was
assigned to work on the samples, which quite fortunately contained a
remarkable cache of new microscopic fossils, most nearly
indistinguishable from extant cyanobacteria and almost all decidedly
better preserved than the Gunflint microbes (Fig.
4). Although the age of the deposit (the
Bitter Springs Formation) was known only approximately, it seemed
likely to be about 1,000 million years, roughly half as old as the
Gunflint chert.
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viewed in light of the earlier articles on the
Gunflint organismsnot only served to dispell lingering doubts about
whether Precambrian fossils might be some sort of fluke, but seemed to
show that the early fossil record was surprisingly richer and easier to
unearth than anyone had dared imagine. Indeed, it now appears that the
only truly odd thing about the Gunflint and Bitter Springs fossils is
that similar finds had not been made even earlier. Walcott had started
the train down the right track only for it to be derailed by the
conventional wisdom that the early history of life was unknown and
evidently unknowable, a view founded on the assumption that the tried
and true techniques of the Phanerozoic hunt for large fossils would
prove equally rewarding in the Precambrian. Plainly put, this was wrong.
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The Gunflint and Bitter Springs articles of 1965 charted a new course, showing for the first time that a search strategy centered on the peculiarities of the Precambrian fossil record would pay off. The four keys of the strategy, as valid today as they were three decades ago, are to search for (i) microscopic fossils in (ii) black cherts that are (iii) fine-grained and (iv) associated with Cryptozoon-like structures. Each part plays a role.
(i) Megascopic eukaryotes, the large organisms of the
Phanerozoic, are now known not to have appeared until shortly before the beginning of the Cambrianexcept in immediately sub-Cambrian strata, the hunt for large body fossils in Precambrian rocks was doomed
from the outset.
(ii) The blackness of a chert commonly gives a good
indication of its organic carbon contentlike fossil-bearing
coal deposits, cherts rich in petrified organic-walled microfossils are
usually a deep jet black color.
(iii) The fineness of the quartz grains making up a chert
provides another hint of its fossil-bearing potentialcherts subjected to the heat and pressure of geologic metamorphism are often composed of
recrystallized large grains that give them a sugary appearance whereas
cherts that have escaped fossil-destroying processes are made up of
cryptocrystalline quartz and have a waxy glasslike luster.
(iv) Cryptozoon-like structures (stromatolites) are now known to have been produced by flourishing microbial communities, layer upon layer of microscopic organisms that make up localized biocoenoses. Stromatolites permineralized by fine-grained chert early during diagenesis represent promising hunting grounds for the fossilized remnants of the microorganisms that built them.
Measured by virtually any criterion one might propose (Fig.
5), studies of Precambrian life have
burst forth since the mid-1960s to culminate in recent years in
discovery of the oldest fossils known, petrified cellular microbes
nearly 3,500 million years old, more than three-quarters the age of the
Earth (36). Precambrian paleobiology is thrivingthe vast majority of
all scientists who have ever investigated the early fossil record are
alive and working today; new discoveries are being made at an ever
quickening clipprogress set in motion by the few bold scientists who
blazed this trail in the 1950s and 1960s, just as their course was
charted by the Dawsons, Walcotts, and Sewards, the pioneering
pathfinders of the field. And the collective legacy of all who have
played a role dates to Darwin and the dilemma of the missing
Precambrian fossil record he first posed. After more than a century of
trial and error, of search and final discovery, those of us who wonder about life's early history can be thankful that what was once "inexplicable" to Darwin is no longer so to us.
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Footnotes |
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* E-mail: schopf{at}ess.ucla.edu.
This paper was presented at the National Academy of Sciences colloquium "Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins," held January 27-29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.
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References |
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References
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| 1. | Stebbins, G. L. (1950) Variation and Evolution in Plants (Columbia Univ. Press, New York). |
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