The Precambrian encompasses nearly 90% of the history of the
Earth and around a third of the history of the Universe. The Precambrian
begins with the formation of the Solar System 4.57 billion years
ago (bya) and extends to the beginning of the Cambrian 540 million
years ago (Mya, or 0.54 bya). Over this immense time span the
Solar System condensed from a cloud of dust and gas, the Earth
was violently accreted from the collisions of smaller bodies,
organic molecules were formed, and life originated from the resulting
soup. In a very short period (geologically speaking) the progenitor
cell of all modern life evolved and then diverged into the three
great domains of modern life on Earth: the bacteria, the archea
(bacteria-like organisms often living in extreme environments),
and the eukarya (protists, fungi, plants and animals). Over the
next three billion years these three domains of life diversified
into the incredible variety of organisms appearing in the "Cambrian
explosion."
Essentially all of the great inventions of life were made during
the Precambrian, including the ability to capture the energy of
sunlight, the creation of an oxygen atmosphere, metabolism, the
vast repertoire of biosynthetic products, sexual reproduction,
the ability to detect light, communication by chemical signals,
the metazoan body plans, and vascular plants all appear during
this time. In fact the marvelous diversity of living organisms
seen in the past five hundred million years is essentially just
variations on themes already invented - refinements if you will
of the inventions of the Precambrian. For those who wish to learn
more about the Precambrian a bibliography is available via this
link.
Meteorites
Most meteorites were formed within about 100 million years
of the formation of the solar system. They are the oldest objects
on Earth and they tell us about the conditions in the solar system
when the Earth was just being condensed out of the solar nebula.
Most meteorites are fragments of asteroids resulting from collisions
in the asteroid belt. The most primitive meteorites are undifferentiated,
that is they have never been melted and their component materials
allowed to separate under gravitational attraction or recombine
chemically. Such meteorites may have formed on the surface of
an asteroid or been part of an asteroid too small to melt during
its accretion. Meteorites originating from the interiors of large
asteroids, on the other hand show varying degrees of differentiation,
depending on the size of the asteroid, and thus how long it may
have remained in a molten state.
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- Chondrite (Stony Meteorite) with
fusion crust (4.56 billion years old). Chondrites are characterized
by chondrules - small spherules of melted rock which condensed
out of the early solar nebula and then accreted into asteroids.
This specimen is made up of Olivine ([Mg,Fe]2SiO4)-hypersthene([Mg,Fe]SiO3]
and represents the primitive material of solar system with little
post-accretional differentiation. (Fell in Holbrook, Arizona
in 1912.)
- Stony Iron Meteorite (Pallasite
polished section; about 4.5 billion years old): Pallasites have
a continuous iron-nickel (Fe-Ni) alloy matrix with centimeter
sized olivine crystal inclusions. These meteorites appear to
come from a partially melted and differentiated asteroid (or
part of an asteroid), and are thus slightly younger than the
chondrites. (Found in Brenham, Kansas, 1882.)
- Iron Meteorite (octahedrite,
7% Ni Fe-Ni alloy; about 4.5 billion years old): These meteorites
appear to come from a differentiated asteroid (or part of an
asteroid), and are thus slightly younger than the chondrites.
(Canyon Diablo Meteor crater, Arizona.) Specimen on Please
Touch shelf.
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Precambrian Rocks and Minerals
One of the great stories of the Precambrian is the evolution
of an oxygen atmosphere. Oxygen in the atmosphere not only resulted
in one of the greatest ecological crises in Earth's history,
it also was a necessary precursor to the evolution of multicellular
animals (metazoans). Of course an oxygen atmosphere is a direct
consequence of living organisms - it could not exist with just
geologic/atmospheric origins. A direct result of the production
of oxygen is the conversion of iron pyrite (FeS2)
and oxygen (O2) into hematite (Fe2O3)
and gypsum (CaSO4.2H2O).
So when did life first begin producing oxygen in significant
amounts? The earliest records of cells are microfossils of cyanobacteria
from about 3.5 bya, with recognizable stromatolite fossils found
in Australia by 3.45 bya. Since all modern cyanobacteria produce
oxygen by photosynthesis it is widely thought that oxygen production
must date back to at least this time. Mysteriously, the geological
evidence (banded iron formation, sulfide vs. sulfate minerals,
cerium vs. cerium oxide deposition in paleosols etc.) indicate
that oxygen didn't rise above trace levels until over a billion
years later (2.3 bya). A variety of explanations have been offered,
for example, the oxygen may have been consumed by early geological
processes. But like much in the Precambrian, the true cause remains
unknown.
A variety of specimens in our collection document the change
to an oxygen atmosphere.
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- Iron pyrite can only be formed in an oxygen-free environment,
and is broken down to iron oxides and sulfate when oxygen is
present during weathering and transport by streams and rivers.
- Iron Pyrite crystals such as this
specimen occur in metamorphic and igneous source rocks.
- Iron pyrite and uraninite** are two minerals that decompose
so rapidly in the presence of free oxygen that they are rarely
if ever deposited in recent sediments. Both occur commonly in
Archean detrital sediments such as the:
- Witwatersrand conglomerate (Polished
slab, about 2.7 billion years old) in our collection. Note the
rounded pebbles showing a stream-detrital deposit, while the
presence of the pyrite flakes and uraninite indicate deposition
under anaerobic atmospheric conditions. These ancient placers
are mined for gold in South Africa to depths of 4 km. They have
produced almost half the world's gold and some uranium.
- Pyrite Pebbles (2.3 billion years
old, Transvaal, South Africa). Note the pyrite crystals have
been rounded by tumbling in an oxygen-free environment.
- Iron Pyrite Crystals in Oil Shale.:
Massive amounts of oil shale were deposited during the Precambrian.
Comparisons of the carbon/sulfur isotopes in this shale to modern
marine deposits indicate that this carbon is of biological origin.
Iron pyrite crystals are often found in Archean and Proterozoic
Precambrian shales, indicating the seas of this time were rich
in soluble ferrous ion (Fe2+) and a source of sulfide
ion. The only source of sulfide ion in shale would be from the
metabolism of sulfate to sulfide by sulfate-reducing bacteria.
Such bacteria must therefore have evolved and been active by
at least 2.7 bya, when such deposits become common.
- Banded Iron-Formation (BIF) constitutes the majority of the
world's iron deposits. Most commonly these deposits consist of
alternating layers of black hematite and chert. In our specimens
the chert is colored red with iron oxide impurity coated granules.
BIF deposits are a direct result of oxygen release by Precambrian
microbes. During much of the Precambrian the Earth's surface
waters and atmosphere were anoxic (oxygen-free) so that iron
would exist mostly in its reduced (ferrous or Fe2+)
form. Vast quantities of ferrous iron entered the ocean surface
through volcanic action, upwelling, and run-off. Photosynthesis
by cyanobacteria in the surface waters produced oxygen which
reacted with ferrous iron to give the much less soluble ferric
iron (Fe3+), precipitating out iron hydroxide (rust).
Seasonal and/or biological cycles resulted in intervening periods
when iron or oxygen were not as available resulting in the interlayered
chert (microcrystalline quartz precipitate).
- Cleavage fragment of gypsum. Gypsum
is soluble in water so is largely recycled into younger evaporite
sediments. Some survives, especially as anhydrite CaSO4,
in Precambrian rocks where its presence demonstrates the presence
of oxygen due to photosynthesis.
**Uraninite is rapidly oxidized to higher
uranium oxides. Though more complex in reality, the reaction
can be symbolized as:
2 UO2+ O2->
2 UO3
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Precambrian Microbial Life
The earliest evidence of life comes from chemical fossils
formed only 100 million years after the end of the Hadean period
of intense bombardment by meteorites and planetesimals (3.8 bya),
during which huge impacts by planetesimals would periodically
vaporize the oceans and sterilize the Earth. This earliest life
is inferred because the carbon 13:carbon 12 ratio (13C/12C)
of carbonaceous deposits is characteristic of biochemical processes
which preferentially use 12C. The earliest records
of cells are microfossils of cyanobacteria from about 3.5 bya,
with recognizable stromatolite fossils found in Australia by
3.45 bya.
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- Shallow sea bottoms appear to have been covered by mats of
cyanobacteria (engraving) due to a general lack of grazing animals
prior to the Cambrian. A cast of a bacterial
mat (sometimes called "Elephant skin") from the
White Sea region of Russia is displayed.
- Stromatolites are the dominant
fossil type for most of the Precambrian, with the oldest identified
examples going back at least 3450 million years. During the Precambrian
stromatolites formed reefs comparable in extent and magnitude
to the great coral reefs of recent and modern times. Stromatolites
became rare during the Cambrian with the advent of multicellular
grazing animals from which they have little natural protection.
Modern examples occur only in protected environments such as
high salt lagoons where animals don't exist (e.g. the famous
Shark's Bay examples in Australia), and in sand fields (such
as in the photograph to the left taken in the Bahamas) where
they are periodically covered in sand, denying access to grazing
animals.
Stromatolites are columnar or dome shaped finely layered structures
made up of mineral deposits resulting from bacterial community
growth. In modern stromatolites the living portion of the stromatolite
is a complex ecological community dominated by filamentous cyanobacteria
on the thin, surface, photosynthesizing, layer, with thicker,
anaerobic layers underneath. As the bacteria grow sand and other
particulate matter is trapped in the mat, along with calcium
carbonate deposits, to make up the mineralized structure. The
bacteria migrate upward to maintain access to light and food,
gradually building the mineralized stromatolite below. Ancient
stromatolites have fossil remains of cyanobacteria-like organisms
which appear identical to their modern descendants.
- Stromatolite (Proterozoic, 1200
Mya, Mount Nelson Formation, British Columbia)
- Stromatolite (Proterozoic, ca. 1800
Mya, Biwabik Formation, Minnesota). The typical layered structure of stromatolites
is obvious in this specimen. The structure of this specimen shows something of the growth and ecology of these biostructures. Photosynthetic cyanobacteria on the surface released oxygen that then precipitated red iron oxide during the day, adding layer after layer (see the closeup image showing the fine structure of these layers). As the layers build up the buried layers the oxygen was consumed and anerobic bacteria thrive, some metabolizing iron oxide and sulfur to create crystals of iron pyrite seen as golden flecks deeper in the stromatolite. Two crystals are also visible in the closeup. The white quartz layer is a later intrusion as the rock was buried and fractured over time.
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Multicellular Animals - the Ediacaran Fauna
Multicellular animals first become significant life-forms,
leaving the earliest clear fossil evidence, at the beginning
of the Vendian, about 0.6 bya (600 Mya). The Ediacaran fauna
of the Vendian Period is represented by the casts in our collection
from the famous fossil beds of the Flinders Ranges of South Australia,
the White Sea beds of Northern Russia, and the beds in Charnwood
Forest of Leicestershire, England. The Ediacaran fauna probably
represent the first ecosystem in which multicellular organisms
play a significant role.
The Ediacaran fauna represent a mystery however - with the
exception of coelenterates (medusa and anemones), these organisms
are not clear ancestors of those which followed them in the Cambrian.
In addition, modern DNA-based sequence information predicts that
today's phyla split much earlier than the Vendian, perhaps as
much as a billion years ago. So where did the organisms of the
Cambrian, and our modern phyla, come from?
As with many later Periods the Vendian probably ended with
a mass extinction, in this case associated with an ice age. The
extent of the Vendian extinction event may have even exceeded
later events, such as the Permo-Triassic crisis, when 90% of
marine species vanished. In this case the loss of the Ediacaran
fauna allowed the rapid diversification of bilaterian animals
to fill the empty niches. Recent work has shown that the Cambrian
Explosion was not only a phenomena of bilaterian animals, it
was an ecosystem-wide diversification of organisms including
bacteria, protists, algae, and sponges as well.
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Radiata: The Radiata (coelenterata) include today's
Cnidaria (hydras, jellies, sea anemones, and corals) and Ctenophora
(comb jellies) and are characterized by radial symmetry. They
probably represent the earliest multicellular animals with true
tissues, and strongly dominate the Vendian fauna around the world.
Fossils of Radiata, impressions since they have no hard tissues,
are common in the Vendian Period.
The Radiata are represented by two body-plans: polyps (e.g.
anemones and corals) and medusa (jellies).
- Polyps: The burrowing sea anemone (Niemiana), is
thought to represent the earliest and most primitive Radiata.
Note that they have no tentacles, as seen the the group in our
cast from the White Sea deposits of Russia, and unlike modern
examples as in the engraving. These organisms also had an undifferentiated
body cavity and reproduced asexually.
- Medusa: Medusa-like fossils have been found as old
as 1200 Mya in Australia. As with the polyps they appear to be
much simpler than modern medusa, such as the jelly in the engraving.
The museum has a number of representative fossil casts of White
Sea specimens courtesy of M. A. Fedonkin (Paleontological Institute,
Moscow) and the University of California Museum of Paleontology:
- Cyclomedusa represents
one of the earliest forms of medusa. It was a sedentary form,
which may have evolved by a flattening of a polyp resulting in
a concentric body plan. This simple organism reproduced asexually
by fission. (White Sea deposits of Russia)
- Protodipleurosoma rugulosum
another sedentary form of medusa, it appears to have reproduced
asexually by budding. (White Sea deposits of Russia)
- Eoportita appears to be
a more advanced form of medusa, incorporating both concentric
and radial symmetry in its body plan. In the cast the radially
arranged tentacles can be seen to be concentrically arranged
in series around the oral cavity. As with Cyclomedusa, reproduction
was asexual. (White Sea deposits of Russia)
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Colonial Forms: Some authors also consider a variety
of other pinnate-shaped, fan-shaped, and comb-shaped organisms
to be colonial representatives of the Radiata. Others consider
them to be failed early experiments in animal biology, with no
modern descendants. These organisms have no apparent mouths or
anuses, thus they must have absorbed food, or harbored photosynthetic
symbionts to provide them with energy. Examples of both modes
of feeding exist in modern organisms. A variety of corals have
never been observed to feed, rather existing on the photosynthetic
products of symbiotic cyanobacteria. And the giant worms found
around hydrothermal vents in the ocean have no mouths or anuses.
Rather they harbor symbiotic bacteria which use the methane,
hydrogen sulfide and other reduced substances to generate energy
for themselves and their hosts. The original ink drawing by Rachel
Rogge at the left is an artists representation of a living Charnia
masoni commissioned by the museum (©2002 HSU NHM).
Casts of Charnia masoni fossils from two localities
are on display:
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Trilateria: A variety of Vendian organisms exhibit
trilateral radial symmetry. The relationship of these animals
to modern organisms is controversial. Many paleontologists feel
they represent early experiments in animal biology which failed,
becoming completely extinct and leaving no modern representatives.
The original ink drawing by Rachel Rogge at the left is an artists
representation of a living Tribrachidium commissioned
by the museum (©2002 HSU NHM).
- The cast of Tribrachidium heraldicum
in our collection is from the famous Ediacara fossil beds of
Australia courtesy of James Gehling. It may represent a more
advanced form of medusa, with more similarity to modern cnidarians
than with other Vendian medusa.
- Pteridinium is a frond-like
organism with three-fold symmetry, which may indicate a relationship
to the Tribrachidium discussed above. On the other hand
its frond-like body plan could argue for a closer relationship
to Charnia, and a colonial nature. The Museum's cast of
a White Sea specimen is courtesy of M. A. Fedonkin (Paleontological
Institute, Moscow) and the University of California Museum of
Paleontology.
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Bilateria: The bilateria (animals with bilateral symmetry
such as the majority of living forms) are represented by two
types of Precambrian fossils: trace fossils and impressions.
Trace fossils of burrows and tracks are perhaps the best evidence
for the existence of animals ancestral to modern forms in the
Precambrian. Unlike the various fossils represented in the Vendian
impressions, these animals would have possessed the key characteristic
of most animals - mobility. Unfortunately there are no known
fossils of these organisms themselves, so we can't know who made
these tracks and burrows.
Among the best known and common impressions of animals thought
to represent bilateria are the fossils of Dickinsonia. These
discoidal, segmented animals are thought by some experts to represent
early annelid worms, while others feel they are unique Ediacaran
species with no descendants. The Dickinsonia varied greatly
in size, from small coin-sized to doormat sized. Some impressions
are surrounded by smoothed areas indicating the organisms expanded
and contracted while maintaining their shapes and segmentation
numbers. Some specimens appear to have orifices which have been
interpreted as a mouth and an anus at the ends of the central
ridge, though others dispute this interpretation. Perhaps future
specimens will enable us to determine the true relationships
of these and other Ediacaran animals, but currently we just don't
know. The original ink drawing by Rachel Rogge at the left is
an artists representation of a living Dickinsonia commissioned
by the museum (©2002 HSU NHM). The museum has two casts
of Australian specimens provided courtesy of James Gehling:
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The engravings are from Dana, James D. (1870) Manual of
Geology, Dana, Edward S. and William E. Ford (1922) A Textbook
of Mineralogy, and Strasburger, Eduard et. al. (1920) A
Text-book of Botany.
