In the nineteenth century the origin of life seemed an intractable
problem for palaeontologists, with large complex animal fossils appearing in
the Cambrian explosion, but scientists having access to neither examples of
earlier fossils nor the means with which to examine them. However from the
early twentieth century onwards improvements in microscopy and petrography led
to the discovery of a range of putative fossils of microscopic organisms, with
the oldest dating back to almost 3.5 billion years ago. The search for the
earliest life has concentrated on looking for simple cyanobacteria-like
fossils, which our understanding of modern microbiology leads us to believe may
be the earliest forms to appear likely to have left recognizable fossils.
However comparison with the fossil record of larger organisms suggests that
this may not be best approach; nothing we know about modern biology would have
prepared us for the existence of Sauropod Dinosaurs, yet we have found numerous
specimens in the fossil record and have little difficulty in recognising them
as true fossils.
The Gunflint Chert is found at Schreiber Beach by Lake Superior in
Ontario, Canada. It comprises an ancient shoreline laid down over an eroded
Archean Lavabed during a marine transgression (period of rising sealevels).
During this transgression boulders on this shore were draped with layers of carbonaceous
chert and dolomitic carbonate, and then overlaid with banded ironstone. The
chert layers of these deposits are noted for the presence of numerous tubular
filaments, described as the a fossil Oscillatoriacean Cyanobacteria, Gunflintia.
Typical preservation of 1.88-Ga filamentous
microfossil Gunflintia within the
chert of the Gunflint Formation, Schreiber Beach, Ontario.Optical images of a
30-μm thin section through a stromatolitic microbial mat, showing carbonaceous
filaments of Gunflintia (white arrow
points to exampleswith septate appearance) plus rounded vesicles of cyst-like Huroniospora (black arrow). Brasier et al. (2015).
Although the Gunflintia
fossils were originally interpreted as septate tubes, similar to modern
Cyanobacteria, it is now thought likely that the divisions observed could be a
taphonomic artefact, produced by decay processes within nonseptatetubular
sheaths. Brasier et al. examined both
newly made thin sections of the chert (i.e. slices of rock cut very thin to
enable examination under a light microscope) and three-dimensional FIB-SEM
models (computer generated models based upon serial Scanning Electron
Microscope images) of the same, and found that while apparent septation was
visible in the light microscope preparations, it could not be seen in the
FIB-SEM models, and conclude that these septa are in fact an artefact.
The filamentous Gunflintia
fossils are often surrounded by a second fossil-type, rounded cells with excystment-like
openings, which have been named Huroniospora.
These fossils may represent heterotrophic organisms feeding on the Gunflintia filaments, or possibly some
form of spore or cyst, which may relate to the life-cycle of Gunflintia, or represent an entirely
separate organism.
3D reconstructions from FIB-SEM sequential slicing of pyritized Gunflintia filaments (various colours)
showing clusters of tubular sheaths bearing small epiphytic cells (orange/red). Brasier
et al. (2015).
A third type of fossil is also present in the Gunflint Chert, to
which Brasier et al. pay particular
attention. Eosphaeratyleri comprises
small spherical fossils 28–32 μm in diameter, which appear to have a thick
walled inner sphere within a thin walled outer sphere. Each such body is
surrounded by up to 15 smaller tubercle-like spheroids, though there seems to
be no pattern in the way these are arranged. Examination of one of these
fossils in a FIB-SEM reconstruction recovered a 30 μm outer sphere and a 20 μm
inner sphere surrounded by ~10 1-7 μm spheroids in a regular pattern, each
separated by about 5μm from its neighbours.
Exceptional preservation and novel morphology of
1.88-Ga complex carbonaceous microfossil Eosphaera tyleri
from the Gunflint Chert, Schreiber Beach, Ontario. (A) Four levels of optical
focus through a thin section in non-stromatolitic microfabric, showing a
well-preserved Eosphaera complete with
inner sphere (red arrow) and outer sphere (green arrow) plus several rounded
tubercles (e.g., yellow arrow) within the intervallar space. (B−E) The 3D
reconstructions (from FIB-SEM sequential slicing) of a different Eosphaera specimen. Note the thicker and
more robust inner sphere (red, 20 μm across) with linear rupture (beneath white
arrow), thinner and more membranous outer sphere (green, 30 μm across), and
about 10 hollow, spherical to elliptical cell-like tubercles (various colours
including yellow, 1–5.8 μm)plus two external tubercles (blue, is less than 7 μm; pale
green at left, 1.8 μm). (B and C) Viewed from centre of specimen visualizing
approximately half the organism; (D and E) Viewed from outside the specimen
showing both inner and outer spheres (D) or just the inner sphere (E), plus
tubercle locations. Scale bar is 10 μm.). Brasier et al. (2015).
This regularity, which was not observed in previous light-microscopy
based studies, strongly supports the idea that Eosphaera tyleri is of biological origin, although different from
any living micro-organism. Brasier et al.
suggest three possible explanations for this. Firstly, Eosphaera tyleric ould be a member of an extant group such as the
Cyanobacteria, which inhabited an ecological niche available to such organisms
but now no longer available, due to the evolution of more sophisticated
competitors such as Eukaryotes (organisms with cells that have true nuclei).
Secondly Eosphaera tyleri may
represent an extinct group of organisms otherwise unknown in the fossil record
with a lifestyle unlike any known organism. Thirdly, it may be a symbiotic
organism, with a large host cell and either the inner sphere or the surrounding
cells or both representing symbiotic organisms. This is at first sight a
slightly exotic explanation, but is a stage predicted in the evolution of
Eukaryotic cells, and could therefore potentially be found in the early fossil
record.
The Apex Chert comprises chert inclusions in the 3.46 billion year
old Apex Basalt of the Warrawoona Group, Western Australia. This is interpreted
to be one of the oldest known silica-rich hydrothermal systems, and is
associated with other interesting deposits, such as pumices with putative
biological signals. The Apex Chert has produced putative microfossils assigned
to eleven species, considered to be similar to Cyanobacteria, though not
formally assigned to any group. These specimens have widely been accepted as
the oldest known cellular fossils, though older biomarkers (chemical traces
thought to have been produced by the actions of living organisms) are known.
Apex chertputative fossil holotypes and paratypes,
reimaged from the type thin sections, plus newly discovered comparable
microstructures from sample CHIN-03. Archaeoscillatoriopsis disciformis
holotype (A) plus comparable examples from CHIN-03 (B and C). Primaevifilum delicatulum holotype (D)
plus comparable examplesfrom CHIN-03 (E and F). Archaeoscillatoriopsis grandis paratype (G) plus comparable examples
from CHIN-03 (H and I). Brasier et al.
(2015).
The original description of the Apex Chert locality interpreted the
cherts as having formed on a wave influenced beach or stream system with
hydrothermal input. However Brasier et
al. re-examined and remapped this locality, and came to the conclusion that
the deposits represent a subsurface hydrothermal vein system, possibly as much
as 100 m bellow the ground. This is a highly unlikely environment for a
Cyanobacteria-like organism as Cyanobacteria, and therefore presumably other
organisms with a similar lifestyle, are dependent on light for photosynthesis,
and therefore cannot survive in an underground environment.
Brasier et al. next
inspected the fossils themselves for mineralogical composition. The original
material from which the fossils had been described was mounted in thick
preparations and stored at the Natural History Museum in London. These proved
to be unsuitable for mineralogical analysis, but a new sample of the material, CHIN-03,
from which new specimens were obtained comparable to the original material. The
original descriptions of the Apex Chert material suggested that it comprised carbonaceous
cell walls with interior spaces infilled with silica. However after examining
the new material Brazier et al.
conclude that the ‘fossils’ are in fact vermiform aggregates of plate-like
aluminosilicate grains, and therefore pseudofossils of non-biologiacal origin.
Nanoscale structure and chemistry of a vermiform
pseudofossil comparable to Archaeoscillatoriopsis grandis from
populations in the Apex Chert dyke (microfossil site, sample CHIN 03). (A and B)
Optical photomicrographs of Archaeoscillatoriopsis grandis
before and after extraction of an ultrathin wafer for analysis by Tunnelling
Electron Microscope (TEM). Position of wafer indicated by red dashed line in (B).
Thin black lines in (B) separate images taken at different focal depths. (C) Bright-field
TEM image showing an overview of the pseudofossil (left margin denoted by
dashed red line) below the surface of the thin section. It comprises plate-like
phyllosilicate grains (dark grey, spikey in cross section at top of image),
minor quartz (q), and significant carbon (pale grey/white; four examples
arrowed). (D) Bright-field TEM image of the blue-boxed area indicated in (C)
showing the thin plate-like morphology of the phyllosilicates (dark grey) and
the interleaving of carbon (light gray) between the phyllosilicate grains.
Arrow points to a small hole within the carbon. (E) Bright-field TEM image and
(F−J) false-colour energy-filtered TEM elemental maps of the yellow-boxed
region indicated in (C). Carbon (yellow) is clearly interleaved between platy
phyllosilicate grains (red) and shows no resemblance to cellular compartments.
Minor iron (green) is also present, closely associated with carbon along many
grain boundaries (I). This suggests that carbon and iron followed the same
grain boundary conduits to enter the vicinity of the vermiform pseudofossil.
The three-color overlay (J) most clearly demonstrates the distribution of
quartz (blue), phyllosilicate (pink), and carbon (yellow) in this pseudofossil.
(K and L) Selected area electron diffraction patterns from the regions of the
TEM wafer indicated in E. DP1 is consistent with the [001] zone axis pattern
from a 2:1 phyllosilicate. DP2 shows a pattern of ring arcs, representative of a
set of closely aligned grains of a 2:1 phyllosilicate with the beam incident
parallel to the {00l} plane. Brasier et
al. (2015).
The Strelley Pool Sandstone is a 3.43 billion-year-old quartz sand
from Western Australia deposited during the oldest known marine transgression.
The grains are held in a chert matrix made up of multiple lamina (layers)
implying chert deposition was episodic. As well as the sand grains, trapped
within these chertlaminae are clasts of older black sandstone, grains of
rounded pyrite and carbonaceous items which Brasier et al. regard as candidate microfossils.
These candidate fossils comprise clusters of elliptical or rounded
bag-shaped bodies, as well as isolated coccoidal (spherical) and tubular cells.
These bare similarities to cells found in the later Gunflint and Rhynie Cherts
(the latter from the Devonian of Scotland) and preserved cells from modern
beach-rock deposits from around hydrothermal springs in New Zealand.
Optical images of a sheath-like microfossil candidate
preserved within dripstone-like chert fabrics of the 3.43-Ga Strelley Pool
Sandstone of Western Australia. (A) A small cavity (C) formed beneath a quartz
sand grain (Q) crossed by a distinct dark tube (white arrow) that takes the
form of a hollow, carbonaceous sheath-like structure. (B and C) Detail of the
structure, showing two images taken at different focal depths illustrating a
hollow and silica-filled construction provided with a rounded cross section.
All images are plain polarized light micrographs. Brasier et al. (2015).
The presence of pyrite nodules in the deposits suggests that they
were laid down in an anoxic environment, while the nature of the silica films
suggests that they were formed during periods of aerial exposure (i.e.
silica-rich water dried out leaving a mineral film over the sand grains during
periods of exposure to the atmosphere, possibly during periods of tidal
exposure). The quartzite (sand) grains are translucent, which suggests that
they were laid down in a photic environment (i.e. exposed to light), which is
consistent with the intertidal interpretation of the environment.
Working from this environmental interpretation, Brasier et al. suggest that the living organisms
are likely to have carried out a form of anaerobic photosynthesis, similar to
that seen in modern Purple Sulphur Bacteria. However they also note the cell
walls seen in the Strelly Pool fossils are much thicker than seen in modern Proteobacteria
(the group that includes both Cyanobacteria and Sulphur Bacteria), suggesting
that either they are members of an extinct group, which coated their cells in a
thick layer of exopolymers, or they are members of an extant group (such as the
Proteobacteria) showing a novel morphology in response to an environment no
longer found (anoxic intertidal sands).
See also…
