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If the history of life were a movie, the fossil record would be a few
dusty, scratched and obscure frames, telling widely separated parts of the
story. The palaeontologist’s task is to deduce the missing parts of this
very jerky and incomplete movie to find the whole story of the comings and
goings of bacteria, fungi, plants and animals over the past 3500 million
years.

Most of the evolutionary events in this film have been packed into the
final 600 million years. Imagine that the movie runs for one hour – 3600
seconds. Each second of the movie represents about one million years of
real time. The great explosion of marine life 570 million years ago in the
Cambrian period, took place with only about 10 minutes of the movie left
to run. Big stars, such as the dinosaurs, came and went in a couple of minutes,
disappearing with just one minute of the film left. But in this final minute
flowering plants developed, primates evolved and most other groups of living
mammals appeared. Humans made an entrance in the last half second.

But in the background of every frame of the film there are primitive-celled
organisms, which stay virtually unchanged during all the coming and goings
of the animals and plants. In terms of the three ways of measuring evolutionary
success – the range of environments a group of organisms occupies, their
effect on the ecosystem and the period of time over which they survive –
these simple organisms have no peers. They have been found in some of the
oldest rocks on Earth, they persisted for the first two-thirds of their
history without company, and they are still living in a few parts of the
world today such as Western Australia.

A wide diversity of microorganisms ranging from photosynthetic prokaryotes,
cells such as bacteria that lack both a nucleus and chromosomes, through
microscopic algae to a variety of other microbes make ecosystems known as
microbial communities. The dominant members are simply photosynthetic prokaryotes
called cyanobacteria (formerly known as blue-green algae).

The clearest evidence for such ecosystems in ancient rocks comes not
just from the fossilised remains of cells and filaments, but more commonly
from the complex structures that they built – columns and domes of limestone
sediment called stromatolites. Ancient stromatolites show up in the rock
record as layers with distinctive convex upward segments separated by cusps.
When the domes and columns were so closely spaced that they coalesced, the
layers of limestone undulate.

In recent years, contemporary stromatolites have become a focus of attention
among researchers wanting to reconstruct the earliest ecosystems of the
Earth. The microbial communities and the wide range of environments in which
stromatolites grow today may shed some light on why, after the microorganisms’
solitary domination of something like 2000 million years, first plants,
then animals took over. And, as a bonus, stromatolites in ancient rocks
can also help to estimate the length of a day hundreds of millions of years
ago and so indicate changes in the speed of the Earth’s rotation.

Contemporary stromatolites were first recognised in the early 1950s
at Hamelin Pool, at the southern end of Shark Bay in Western Australia.
Rising like serried ranks of concrete cauliflowers from the shallow sea
these domed mounds grow about 30 centimetres high. Radiocarbon dating carried
out recently at the Australian National University by Allan Chivas and his
colleagues indicates that individual stromatolites may be as much as 1000
years old. They grow as slowly as 0.4 millimetres per year. These original
discoveries have been used as the modern analogue with which all fossil
examples have been compared. But with the discovery of more stromatolites
in a wide range of environments in Western Australia researchers have found
that the Shark Bay stromatolites are not typical.

One of the main differences between stromatolites in different environments
arises from the way they grow. All stromatolites form from the calcium carbonate
that the microorganisms precipitate from the sea water, but in detail there
are two methods of building. In one, each cell secretes a sticky film of
mucus which traps fine sediment, then it binds the sediment grains together
with calcium carbonate. Because the cyanobacteria are both photosynthetic
and able to move toward the light, they can keep pace with the build up
of sediment. They always stay on the outer surface of the stromatolite.
But the cyanobacteria can also build stromatolites by precipitating enough
calcium carbonate from sea water to form the framework using little or no
sediment. Most of the Shark Bay stromatolites are formed by the first of
these methods, with sediment grains.

In addition, Stan Awramik of the University of California at Santa Barbara
and Robert Riding of University of College, Cardiff have shown that not
all of these stromatolites are built wholly by cyanobacteria. The dominant
species depends on where the stromatolites are on the shore. Those in the
intertidal zone are constructed by cyanobacteria in two different communities;
one dominated by Entophysalis, the other by Schizothrix. But subtidal forms
are quite different. Not only are they composed of much coarser grains of
sediment, but their microbial community is more diverse; it is dominated
by a mixture of cyanobacteria and a range of more complicated organisms,
including diatoms. Like cyanobacteria, diatoms secrete mucus which traps
sediment. But the structure and microbial community of these stromatolites
do not compare closely with ancient types; Awramik and Riding think that
they represent a more recent development in stromalite formation.

Ancient and modern

There are now closer living analogues for the Precambrian forms. Kay
Grey of the Geological Survey of Western Australia has been studying some
of the fossil types in great detail. She has shown that some are so distinctive
that they can be used to match up rocks formed at the same time in different
sedimentary basins. In addition, the Precambrian stromatolites are potentially
important source-rocks and reservoirs for oil deposits as well as sites
for mineralisation. Moreover, the conditions under which the ancient stromatolites
grew can be deduced by comparing them with living Western Australian stromatolites
that match them closely.

Grey has shown that different fossil forms are restricted to certain
types of sediment. For instance, in the 2000 million-year-old Duck Creek
Dolomite, in the southwestern Pilbara region of Western Australia, there
are two forms of stromatolites: one shaped like a column and one with a
broader, domed form. In Lake Thetis and Lake Clifton in Western Australia,
she found that the columnar variety grows in shallow lagoons and the lower
intertidal zone, whereas the broad domed forms inhabit intertidal or supratidal
regions.

The Lake Clifton stromatolites show a number of major differences from
those of Shark Bay. One fundamental variation is in the salinity of the
water in which they grow. The Shark Bay stromatolites grow in hypersaline
sea water (about twice the salinity of normal sea water). Few other organisms
can live in these conditions. So is this high salinity the factor that allows
the stromatolites to persist here? There is some evidence for this; with
the rise of animal life between 500 and 600 million years ago, there was
a corresponding decline in abundance of stromatolites that has been attributed
to the evolution of invertebrates that live by grazing on algae and other
microorganisms. But the argument does not hold, now that stromatolites have
been found in an incredibly wide range of environments as well as the hypersaline
seas of Shark Bay. While some of these, such as glacial lakes and volcanic
springs, are extremely hostile environments, and restrict other life forms,
stromatolites are also found in normal marine conditions, off the Bahamas
for example, and in saline, brackish and freshwater lakes in Western Australia.

Unlike the Shark Bay stromatolites, those in Lake Clifton are largely
the product of the precipitation of calcium carbonate by the cyanobacterium
Scytonema. Linda Moore, of the University of Western Australia, has found
that differences in discharge of ground water from the floor of Lake Clifton
affects not only the distribution of the stromatolites, but also their structure.
This seepage of ground water appears critical to the development of the
stromatolites because it allows Scytonema to flourish; the ground water
provides the prime source of carbonate and bicarbonate ions that the bacteria
need to build stromatolites. Furthermore, these stromatolites grow in brackish
water and carry a diverse invertebrate fauna containing many active grazers.
Stromatolites provide both a source of food and refuge for these animals,
which include isopods, amphipods, gastropods and bivalves. The salinity
of the water in which stromatolites grow seems to have little to do with
their formation or whether they can persist in spite of continual pressure
from the grazers.

To test this observation, Moore and her colleagues set up a laboratory
tank in which stromatolites were kept with a community of grazing invertebrates.
After 14 weeks, they found that not only had the stromatolites not deteriorated,
they had actually grown. So there must be other reasons why it is now unusual
to find stromatolites.

As well as growing in a diverse range of environments today, Western
Australia’s stromatolites also have an extensive fossil record. The earliest
recorded evidence for life on Earth is provided by stromatolites in the
Phibara region, in an area known ironically as North Pole where summer temperatures
rarely drop below 40 °C. They are between 3450 and 3550 million years
old. While most scientists accept that these structures were probably formed
by microbial activity, there has been some controversy over whether filaments
found in fine-grained silica rich sediments at the same site really are
fossilised remains of the organisms responsible for building the stromatolites.
Awramik and his colleagues have argued strongly that these filaments are
related to the ancient stromatolites. They also point out that the filaments
are remarkably similar to those of modern cyanobacteria; there is evidence
from the fossil record that stromatolites have undergone some diversification
since the early Precambrian, but the cyanobacteria have changed little.
Indeed, some late Precambrian genera, even though they have different names,
cannot be distinguished from some living forms on the basis of their shape
alone.

Studies of living stromatolites carried out in recent years at Yellowstone
National Park and at Shark Bay, principally by Awramik and James Vanyo of
the University of California at Santa Barbara, reveal that stromatolites
may be useful in other ways. By comparing living and fossil stromatolites,
researchers can assess the latitudes at which the rocks formed, the number
of days per year and the movement of the Earth and Moon over million of
years. Their method is to compare growth lines, a method also used on fossil
corals, bivalves and brachiopods. Growth and development of these invertebrates
is known to be affected by a range of factors such as diurnal rhythms, tidal
activity and the seasons. The resulting series of growth lines allow a sophisticated
‘tree ring’ dating to be made in these animals. Whereas trees only record
seasonal changes, invertebrates record daily, monthly and annual events.
In the 1960s John Wells of Cornell University at Ithaca, New York, found
that on fossil corals, living about 370 million years ago, about 400 daily
growth rings formed each year. So, 370 million years ago each day must have
been only 21.9 hours long, and each year had 400 days.

Stromatolites can be used in a similar way. Because the bacteria that
construct stromatolites are photosynthetic and so only active during the
day, they leave a daily layer of sediment or calcium carbonate. Annual cycles
can be identified from analysis of the inclination of living stromatolitic
columns such as those in Shark Bay and in Yellowstone National Park; they
grow towards the Sun. As the position of the Sun changes with the seasons,
the direction in which the stromatolites grow alters. If you slice vertically
through a column, the highest point of each layer lies in a slightly different
position. Joining successive highest points gives a curving line in the
shape of a sine wave. The number of layers of sediment – deposited daily
– within one wavelength tells researchers the number of days in a year at
the time the stromatolites formed. In this way Awramik and Vanyo showed
that a stromatolite living some 850 million years ago recorded about 435
days in a year. This means that day would then have been about 20.1 hours
long.

With the data from coral, this discovery implies that there has been
a decrease in the Earth’s rate of rotation, which is thought to have happened
because of tidal ‘friction.’ According to Frank Stacey of the University
of Queensland, this is caused by the tidal bulge that is produced by the
rotation of the Moon around the Earth. The Moon moves around the Earth in
the same direction as the planet rotates, but more slowly. The gravitational
attraction between the two is greatest at their closest point, making the
tides. High and low tides are slightly delayed by tidal ‘friction,’ largely
due to turbulence and drag. Because movement of the Moon lags behind the
revolution of the Earth, high tides happen at the place that was underneath
the Moon a short time earlier. The pull of the moon acts on this tidal bulge,
to pull it back into alignment. This imposes a slight twist on the Earth’s
rotation, and makes it slow down.

Astronomical observations over the last two centuries have shown that
the day is lengthening by about two hundred thousandths of a second each
year. Evidence from the stromatolites points to a fractionally greater rate
of about one hundred and thirty thousandths of a second each year.

Communities of microorganisms and the stromatolites that they construct
have dwelt on this planet for three-quarters of its existence; even though
they are rare, and grow at inaccessible sites in the main, they are not
escaping the attentions of humans. While some environments are fast disappearing
because of direct exploitation, stromatolites are beginning to suffer indirectly.
A case in point are the brackish water stromatolites of Lake Clifton. These
form probably the largest reef of its kind in the world; coalesced stromatolites
extend in a 30 metre-wide strip for more than 10 kilometres. But phosphate
levels in the lake have increased tenfold in the last decade, because more
people live in the area, and they have used more phosphate fertilisers.
The extra phosphate in the ground waters has disrupted the equilibrium in
which the microbial community normally exists and stimulated excessive growth
of Cladophora algae on stromatolites below the water level. The effect of
this has been to smother the cyanobacteria and so stifle stromatolite growth.
The long-term consequence is likely to be the death of the entire stromatolite
community.

Attempts are being made to preserve some of the Western Australian stromaltolites
by creating reserves, but this could prove to be counter-productive. The
Shark Bay stromatolites and those at Lake Thetis are both becoming tourist
attractions with rapidly increasing numbers of visitors. Signposts to the
stromatolites at Shark Bay have recently been removed because of the damage
caused by people driving cars over them. The local shire council, who erected
the signs, are now planning to build a track to Hamelin Pool, one of the
most important areas of living stromatolites. Even though the state government
has recently declared Hamelin Pool a marine nature reserve, the stromatolites
will suffer unless there is adequate local management and very careful supervision
of visitors. In particular, a programme of public education on stromatolites
needs to be instigated, emphasising in particular their great vulnerability.
This site is in great danger of irrevocable damage. Although a recent attempt
to mine beach sand next to the stromatolites was squashed by the state government,
following strong representation from scientists throughout the world, attempts
to promote stromatolites as a tourist attraction may prove even more damaging
in the long term.

The vulnerability of stromatolites arises from the fact that they grow
very slowly. Those at Shark Bay grow at less than half a millimetre per
year; the tracks of horse-drawn wagons which crossed platforms of stromatolites
earlier this century can be seen today. Until community awareness of their
fragility is heightened, stromatolites are in danger of being smothered
to death by people pressure. As a guide to the health of our planet, there
can surely be no better measure than a resilient ecosystem that has survived
for 3500 million years. When that starts to disappear, we might as well
start loading the space shuttles and head for the stars.

Ken McNamara is senior curator of invertebrate palaeontology at the
Western Australian Museum, Perth, and coordinates the Stromatolite Scientific
Advisory Group.