Precambrian Life, Sept. 15, 2005
Marine diversity I
Precambrian time
Greatest hits of the Precambrian
First molecular fossils
First body fossils
Cyanobacteria, banded iron formations and oxygenation
First eukaryotes
First metazoans
Lessons of the Precambrian
Recall…
Important vocabulary for the history of life:
heterotrophic - organisms that obtain metabolic energy by breaking down molecules
absorbed from the environment. We are heterotrophs, as are clams, dinosaurs, sponges, and
most animals and many forms of single-celled life.
autotrophic - organisms that absorb external energy and use it to build up food internally photosynthesis, for example, allows organisms to be autotrophic. Plants are autotrophs, as
are many forms of single-celled life. Chemoautotrophs use energy from chemical reactions.
anerobic - needing to live in the absence of oxygen. Many bacteria are anerobic.
aerobic - needing oxygen in order to live. Most metazoan (multicellular)organisms are
aerobic.
prokaryotic - is a cell type that does not have a distinct nucleus. Asexual reproduction only.
Bacteria are prokaryotes.
eukaryotic - is a cell type that does have a distinct nucleus as well as other distinct bodies
(mitochondria, chloroplasts, etc.) within the cell. Asexual and sexual reproduction. All
eucaryotes are aerobic. All multicelluar forms of life are composed of eukaryotic cells.
Animals, plant, fungi, and many single-celled forms are eukaryotic.
metazoan – multicellular forms of life (plants, animals or fungi).
Six kingdoms:
Kingdom
Archaea
Bacteria
Protista
Fungi
Animalia
Plantae
First appeared
3 – 4 billion
3 – 4 billion
1.5 billion
1 billion
700 million
500 million
Structure
unicellular
unicellular
unicellular
uni- or multicellular
multicellular
multicellular
Photosynthesis?
no
some
some
no
no
yes
A pizza with at least one ingredient from four of the six kingdoms would contain…?
The Precambrian (Archean and Proterozoic) fossil record: life on the early Earth
Greatest hits of the Precambrian:
0.6 billion years - oldest metazoans
(multicellular animals)
0.6 billion years – Snowball Earth
1.6 billion years - oldest acritarchs
(eucaryotic cells)
2.5 billion years - abundant stromatolites;
oxygenation of atmosphere
(Archean/Proterozoic boundary)
3.5 billion years - first evidence of life
(stromatolites, cyanobacteria?) controversial
3.8 billion years - oldest sedimentary
rocks
4.5 billion years - age of the earth
Benton and Harper, 1997
Talking about Precambrian life requires an even greater perspective on geologic time than just
the past 540 million years. While most of the fossil record is concentrated in the last 540 million
years of Earth history, the story of life on earth begins much, much earlier.
Phanerozoic –includes Paleozoic, Mesozoic and Cenozoic)
Proterozoic
Informally known as “Precambrian”
Archean
We don't have much direct evidence left on earth of what the earth's rocks and environments
were during the first half billion years or so of its history. This is because the earth's surface is
active, with uplift, metamorphosis and erosion tending to alter or destroy older rocks.
The first 650 million years of earth history (between the 4.5 Ga date for the origin of the Earth
(based on dates from moon rocks and meteorites) seem to have been characterized by intense
bombardment of meteors - sufficient to vaporize any surface water and maybe event melt parts
of the crust – not a “friendly” environment for life. The bombing ceased at about 3.9 Ga
The physical evidence from the rocks (as well as models of planetary evolution) also suggests
that the earth's early atmosphere did not have oxygen. This suggests that the first life was
anerobic. – Archaea and many bacteria today are anerobic.
The oldest rocks on earth are about 4 billion years old, from northern Canada and the oldest
sedimentary rocks are about 3.8 billion years old, from Isua, in Greenland. These Isua rocks
were laid down in water – the first direct evidence of surface water.
Molecular fossils?
The rocks also contain tiny spherules of carbon that some have interpreted as evidence of life
because of their carbon isotope signature. Stable isotopes of Carbon (12C and 13C) are typically
selectively used in biochemical pathways: 12C, the “lighter” isotope tends to be present in grater
amounts in living material. These Isua carbon spherules have a somewhat “lighter” ratio of 12C
to 13C. Other workers disagree that these isotopic values are indicative of life.
Body fossils?
The first fossil evidence for life comes from rocks in Australia called the Apex Chert, from the
Warrawoona Series, dated at about 3.5 Ga. The evidence is strings of cells that form filaments in
a chert (silica). These filaments are interpreted by Schopf et al. (2002. Nature 416: 73-76) as
microfossils of cyanobacteria.
Precambrian microfossils (e through i are from the Apex Chert)
Figure 1 Optical photomicrographs showing carbonaceous (kerogenous) filamentous
microbial fossils in petrographic thin sections of Precambrian cherts. Scale in a represents
images in a and c–i; scale in b represents image in b. All parts show photomontages, which
is necessitated by the three-dimensional preservation of the cylindrical sinuous
permineralized microbes. Squares in each part indicate the areas for which chemical data are
presented in Figs 2 and 3. a, An unnamed cylindrical prokaryotic filament, probably the
degraded cellular trichome or tubular sheath of an oscillatoriacean cyanobacterium, from the
770-Myr Skillogalee Dolomite of South Australia12. b, Gunflintia grandis, a cellular
probably oscillatoriacean trichome, from the 2,100-Myr Gunflint Formation of Ontario,
Canada13. c, d, Unnamed highly carbonized filamentous prokaryotes from the 3,375-Myr
Kromberg Formation of South Africa14: the poorly preserved cylindrical trichome of a
noncyanobacterial or oscillatoriacean prokaryote (c); the disrupted, originally cellular
trichomic remnants possibly of an Oscillatoria- or Lyngbya-like cyanobacterium (d). e–i,
Cellular microbial filaments from the 3,465-Myr Apex chert of northwestern Western
Australia: Primaevifilum amoenum4,5, from the collections of The Natural History Museum
(TNHM), London, specimen V.63164[6] (e); P. amoenum4 (f); the holotype of P.
delicatulum4,5,15, TNHM V.63165[2] (g); P. conicoterminatum5, TNHM V63164[9] (h); the
holotype of Eoleptonema apex5, TNHM V.63729[1] (i). Schopf et al., 2002. Nature.
However, Brasier et al. (2002. Nature 416: 76-81) contend that the Apex Chert structures are
artifacts of preservation and are not fossils of early life. The controversy continues.
The oldest undisputed fossils of prokaryotes and are about 3.4Ga old from the Fig Tree Series of
South Africa.
Trace fossils
Stromatolites: sedimentary structures consisting of low mounds or domes of finely laminated
sediment.
Stromatolites form today in some isolated marine and fresh water environments. They are made
by mats of cyanobacteria - sometimes called blue-green algae. Very simple, but nevertheless,
photosynthetic (thus autotrophic) bacteria.
Stromatolites are the most common macroscopic evidence of Precambrian life.
-The late Archean and early Proterozoic saw a big increase in the abundance of stromatolites,
suggesting widespread colonies of blue green algae. The world was probably a pretty slimy
place.
Cross-sections of Archean stromatolites.
Image from
http://www.wf.carleton.ca/Museum/stromat
olites/ARCHEAN1.htm
Modern stromatolites, Shark Bay., W.
Australia. Photo by Paul Hoffman.
The evolution of life and the Earth’s atmosphere
The stromatolites did more than make things slimy - they gave off oxygen as a byproduct
of their photosynthesis. In considering the effect that life has on the physical environment of the
earth, this is the most profound effect of all. The fact that the earth has free oxygen in its
atmosphere is thanks to photosynthetic activities - dating back to the slimy Archean
cyanobacteria.
Photosynthesis simplified:
light
6CO2 + 6H20
C6H12O6 = 6O2
For more about stromatolites, see
http://www.wf.carleton.ca/Museum/stromatolites/CONTENTS.htm
At first, all the oxygen given off by the cyanobacteria went into oxidizing minerals - especially
iron minerals:
Banded Iron Formations are sedimentary rocks made up of alternations of iron ore and chert,
sometimes very, very finely laminated. Banded Iron Formations are dominantly found in rocks
older than 1,800 million years. They make up thousands of meters of rock and are not forming
today.
http://www.ucmp.berkeley.edu/
http://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/Banded_Iron.html
The idea is that as the cyanobacterial mats gave off oxygen, it oxidized dissolved iron in the
water, which then precipitated out to form the sedimentary deposits.
Oxygen then began to build up in the atmosphere after most of the available iron was oxidized.
Free oxygen was then built up in the oceans and atmosphere.
Another byproduct of oxygen in the atmosphere is the formation of the ozone (O3) layer,
shielding the surface of the earth from excess UV and cosmic radiation.
Eukaryotic life
Recall that eukaryotic cells are exclusively aerobic - they need oxygen in order to live.
So these more complex forms of life had to wait until enough oxygen had build up in the
atmosphere.
The first fossil evidence of eukaryotic cells comes from molucular fossils – characteristic lipids
(a type of biologically-formed molecule) that occur only in eukaryotic cells. From 2.7Ga old
rocks in Australia.
The oldest eukaryotic body fossils are acritarchs - spherical microfossils that have thick and
complex organic walls. They appear to be the resting spores of free-floating, aquatic, and
eukaryotic algae. The oldest ones found are from 2.1 Ga old rocks in the Ural Mountains of
Russia.
Fossil acritarchs (eucaryotes) from Proterozoic rocks of Russia. (Schopf, 1992)
Snowball Earth
There’s evidence of low latitude (even equatorial) glaciers about 600 million years go. The
evidence in the form of glacial “dropstones” (boulders from terrestrial glaciers that extended out
to sea, then melted, releasing the rocks they picked up while on land).
http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg
There’s other evidence too, from the geochemistry of late Proterozoic limestones. For more
information, see: http://www.bbc.co.uk/science/horizon/2000/snowballearth.shtml
The Snowball Earth is a problem: How did the Earth even warm back up? One theory is the
CO2 emissions from volcanoes created a greenhouse state in the atmosphere, allowing the
Earth’s surface to warm up and for the ice to melt.
And how did life survive the extended (about 10 million years, the estimates say) cold spell? By
hanging out near submarine hot springs, like those found today near spreading centers in the
deep sea?
But in any event, the post-Snowball warming is thought to have been a possible trigger for the
appearance and diversification of metazoan life, at about the time of the warming:
The oldest metazoan (multicellular)organisms date from about 600 million years ago:
The Ediacaran fauna (examples below) - known first from south Australia; now known
from around the world
--Vendian Period, last time period in Proterozoic
--all soft-bodied
--all simple geometries, with radial or bilateral symmetry, some with an anterior—
posterior differentiation
--ancestors of jellyfish and the like or evolutionary dead ends?
Benton and Harper, 1997
Ediacaran (575-560 Ma) fossils from Mistaken Point, Newfoundland. (Narbonne, 2004. Science
305: 1141-1144.
Fig. 2. Rangeomorph architecture from the Trepassey Formation at Spaniard's Bay. (A)
Isolated rangeomorph frondlet. Specimen whitened with ammonium chloride. (B) Enlarged
view of the area indicated in (A), showing details of the fractal-like branching pattern. The
smallest branches are indicated by the arrow. (C) Plumose rangeomorph. Latex mold whitened
with ammonium chloride. (D) Enlarged view of the area indicated in (C), showing details of
the fractal-like branching pattern and the cylindrical cross section of the branches in the ripped
and partially overturned frondlet in the upper left. Scale bars, 0.25 cm [(A) and (B)], 0.5 cm
[(C) and (D)] (Narbonne, 2004)
Recently, Chen et al. (2004) have discovered some very small fossils from the Doushantuo
Formation of China, dating from 40 to 55 million years before the beginning of the Cambrian.
The fossils are phosphatized and show bilateral symmetry, anterior-posterior differentiation, and
seem to possess a gut. They are unlike previously known Ediacaran fossils.
Fig. 1. Images of three different, fairly
well preserved specimens of the
bilaterally organized fossil animal
Vernanimalcula guizhouena. Left panels
show digitally recorded, transmitted light
images of sections about 50 µm thick,
which had been ground from larger rock
samples, mounted on slides, and viewed
through a light microscope. Right panels
show color-coded representations of the
images on the left. These were prepared
by digital image overlay. Yellow, external
ectodermal layer; ochre, coelomic
mesodermal layer; red, surface pits;
mauve, pharynx; light tan, endodermal
wall of gut; gray-green, lumen of mouth;
dark gray, paired coelomic cavities;
lighter gray, lumen of gut; brown, "glandlike" structures, with central lumen (B);
light green, mineral inclusions (C). The
scale bar represents 40 µm in (A), 55 µm
in (B), and 46 µm in (C). (A) Holotype
specimen, X00305, slightly tilted, almost
complete ventral level coronal section,
passing through the ventrally located
mouth. Chen et al., 2004. Science 305:
218-222.
Lessons of Precambrian evolution
very early origin of life
marine
mostly microscopic; stromatolites the commonest macroscopic expression
oxygen in atmosphere from photosynthetic activity
Snowball Earth and environmental change as a trigger for diversification?
oxygen build-up permitted eucaryotes and metazoans
soft-bodied metazoans (multicellular animals) before metazoans with hard parts