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It was the longest empire. For 300 times as long as the dinosaurs roamed,
single-celled organisms held undisputed sway over the Earth. The rule of the
tiny, slimy microbes began almost four billion years ago, and they fell from
grace a mere 500 million years ago.

Slime held their empire together—sticky polysaccharides and proteins
that glued them to their microscopic surroundings of clay flakes and sand
grains. These slimy mats were a perfect way for microbes to exploit their
environment, taking nutrients from the water above, the sediment below and even
their neighbours’ wastes.

There are few fossils of these ancient microbes, and the slime itself has
long since disappeared. So we need to look at modern examples to get a feel for
what slimeworld looked and felt like. Thus armed, we can look for subtle echoes
of the microbes’ activities in the ancient layers that were once their home.

Where to look? Well, half a billion years ago, a hungry dynasty of
metazoans—multicellular animals—arose, and chewed their way through
the slimebugs’ empire. Now, whenever microbes try to regain their hold on modern
environments, somebody has them for lunch. So slimeworld’s last surviving heirs
eke out an existence in places too wretched for multicellular animals. Metazoans
can’t stand the heat of the hot springs of Yellowstone National Park, for
example, or the high concentrations of salt in Australia’s Shark Bay.

At Yellowstone, a multitude of bacteria bask in the heat. The rings of colour
around each hot spring come from the different bacterial pigments. Some capture
the Sun’s energy and others protect the bacteria from harmful UV radiation.

In Shark Bay, slimeworld has a different incarnation. In the salty water,
microbes build what look like weirdly misshapen sandcastles from lime mud
suspensions. Sticky microbial mats, half-cyanobacteria and half-algae, trap
layer after layer of the lime mud. As the layers pile up, natural limy cement
reinforces the slime particles, and the whole structure hardens, though it still
has a living, bug-laden veneer. This process yields distinctive mound-shaped
limestones called stromatolites.

You can find microbial mats in more familiar environments—but they are
short-lived. In the past few years, James Hagadorn and David Bottjer,
researchers from California, have watched microbes colonise the shallow pools
left after high tides on the Texas coast. In the few days before the heavy mob
of crabs and snails moved in to chew through and churn the sediment, Hagadorn
and Bottjer saw the microbes weave beach sediment into cohesive mats. They
describe different sorts of mats as being like skin on top of a pudding, like
the pudding itself, or like tough bread. While powerful currents can tear these
slime-bound mats apart, gentler currents merely wrinkle them, just as dragging
your hand across a tablecloth can rumple it. Hagadorn and Bottjer compared this
texture to elephant skin.

Where can we look for the ancient ancestors of these modern slimebugs? Well,
stromatolites show up in rocks that are 3.5 billion years old. The composition
of these ancient microbial mats differs from that of the modern ones: for a
start, algae didn’t evolve until a billion or so years later. Yet modern and
ancient stromatolites look pretty much the same.

In the time before metazoans, stromatolites ranged much more widely, safely
ungrazed. The fine lime mud they needed, though, was not very common in the
Precambrian, so stromatolites still make up only a small portion of this very
ancient rock record. Most sediments, then and now, consist of the more familiar
silica-based sands and muds.

Within the past few years, evidence has begun to emerge that the tendrils of
slime reached beyond the shallow, lime-rich environments of the Precambrian to
colonise far larger swathes of muddy and sandy seafloors. Simple layers in the
rock, though, do not necessarily indicate the presence of a microbial scum.
Purely physical processes can create sand and mud layers on the seafloor. The
sweeping action of turbulent eddies will do it, for instance, or gradual
settling when mud-laden river floodwaters pour into the sea. So a different type
of evidence is needed.

Stickiness turns out to be the key. Researchers have found slime-influenced
sticky consistencies in ancient sediments, very like those seen in modern
environments by Hagadorn and Bottjer. Pure sand is not in the least bit
sticky—the grains act as individual, loose particles when currents drive
them across the seafloor. So when ancient sandstones shows signs of having acted
like sticky mud, and mudstones show evidence of super-stickiness, the prime
suspect is the slime of long-vanished microbial mats.

Tattered mats

For more than a decade, Jurgen Schieber of the University of Texas has been
discovering telltale elephant skin textures on ancient seafloors. He has
recognised wrinkled, folded and torn layers as the remnants of tattered flaps of
microbial mats disturbed by the passage of, say, a particularly violent storm or
a submarine avalanche. Last year, Bruce Simonsen and Karen Carney of Oberlin
College, Ohio, reported extreme examples from 2.5-billion-year old rocks in
Western Australia. Here, Simonsen and Carney think, the sediment layers were
literally rolled up into miniature carpet-rolls as earthquakes shook the
seafloor.

Friedrich Pflüger of Yale last year came up with more examples of the
mats’ sticky activities. He has studied a fossilised, 400-million-year-old
seafloor from Libya, an environment poor in oxygen in which metazoans were kept
in check, echoing the conditions of older, Precambrian seafloors. Pflüger
found sand volcanoes, which are normally small, upward outbursts of sand from
water-saturated sandy sediments. Here, though, the outbursts had been stifled,
as if by some invisible cloak, leaving only fossilised upward bulges;
long-vanished sticky mats were the obvious candidates for the restraining
cloak.

Pflüger also saw structures that resembled the lower halves of gas
bubbles trying to escape against invisible roofs. He believes the gases couldn’t
bubble freely into the sea because of the slimy mats coating the seafloor. Thin
mud layers, moreover, had cracked, as if baked in the sun, but these layers had
never been exposed to the air or the sun. Such cracking can also occur, though,
where mud layers contain large amounts of microbes and slime, which decay
rapidly after burial and shrink the soft sediment.

So a strange picture is emerging, of a curiously immobile, sticky Precambrian
seafloor, quite unlike that of today. In modern seas, the sediment is
continually churned into a soupy texture by burrowing and crawling animals, and
easily shifted by the waves and the tides. In the slimeworld, the sticky mats
would have ruled uninterrupted, keeping the sediment strangely still except at
the edges of the seas, where the pounding waves could have dislodged them, or in
the deep sea where rare landslides and earthquake shocks cut through the
slime-bound surfaces.

If the Precambrian seafloor was saturated with slime, how about the land? The
Precambrian land surface has often been pictured as a barren, lifeless desert,
sterilised by lethal amounts of UV radiation. But there are hints that microbes
might have tried to conquer the land even in those far-off times. In 1994, Bob
Horodyski of Tulane University, New Orleans, and Paul Knauth of Arizona State
University found microbial fossils in rocks that are 800 million and 1200
million years old. The surrounding limestone bears the telltale isotopic signs
of rainwater, suggesting that the rocks were land-based. But the evidence is too
slight to say whether these microbes produced slime-bound mats.

The slime lost its exclusive grip on the Earth when the metazoans invaded. By
the Cambrian, 535 million years or so ago, slimy mats were in retreat, becoming
lunch for the earliest representatives of the modern multicellular
world—worms, arthropods and molluscs that launched a blitzkrieg on the
biosphere.

Just before this takeover, a bizarre, earlier set of organisms, called the
Ediacaran fauna, forged a unique, transitory co-existence with the slime. There
has been much controversy over what the Ediacaran fossils represent. Are they
the earliest examples of multicellular animals or a completely distinct and extinct form of life
(see ¿ìè¶ÌÊÓƵ, 16 May 1998, p 26)?

Whatever the answer, microbial mats played a big part in the lifestyles of
the Ediacarans, says Adolf Seilacher of Tübingen University in Germany.
These mats did not, for the most part, serve as a food source, says Seilacher,
since he thinks that most of the Ediacarans were either filter feeders or made
their own food. Last year he wrote a paper building on earlier work with
Pflüger in which he suggested instead that the Ediacarans clung to the mats
to resist the tug of waves and currents. One common leaf-shaped Ediacaran,
Charnia, has a curious small disc as its base. According to Seilacher, this
disc, anchored just below a tough, stable, slimy seafloor mat, would be just the
thing to keep Charnia in place.

Seilacher identified another role for the slimy mats—as oxygen masks.
In those far-off days, when oxygen was rare, photosynthetic mats could have been
a vital source of this precious gas, he argues. Creatures that needed to breathe
oxygen could have burrowed just beneath them, and at the same time fed on their
decaying lower levels.

The slimy mats may also have helped to preserve the Ediacaran menagerie for
posterity by sculpting their death masks. Soft-bodied animals rarely preserve
well if they are buried in soft sediment such as beach sand. Their carcasses
simply decay too quickly and the sand closes up the holes, leaving no trace.

Seilacher and other workers have suggested that the Ediacarans were built of
a tough substance that let them leave a permanent mark in the sediment. But last
year, Jim Gehling of the University of California invoked slimy death masks to
explain the unique preservation of the Ediacarans.

Faithful image

Gehling found Ediacaran fossils associated with telltale elephant-skin type
textures, and also detected faint films of rust on their surfaces—a
possible by-product of ubiquitous slimy mats. The rare fossils of soft-bodied
organisms are usually preserved as indentations in the layers of rock both above
and below the vanished carcass. Many Ediacaran fossils, though, are counterpart
casts, which means that they form raised bumps on the lower layer and
indentations in the upper layer.

Gehling explains this by picturing a scene in the Precambrian in which
microbial mats and Ediacarans live in peaceful coexistence. Rare, violent storms
sweep sand from a wave-pounded beach on an adjacent shoreline, or rip large
holes through the microbial layers to expose the loose sand underneath and sweep
it over surviving parts of the mat. A new mat begins to form on the new
seafloor, while the old mat and the Ediacarans below begin to decay. The sand
and seawater sandwiched between the old and new mats becomes a unique chemical
microenvironment. The new mat prevents clean, oxygenated water from entering,
while the old mat restricts the movement of water and ions downwards into the
lower layer of sediment. Gehling suggests that this forces iron minerals to
precipitate around the tops of the old microbial mats and the carcasses, binding
the sand grains in a thin layer and faithfully moulding rigid death masks over
the Ediacaran creatures. As the beasts rot, loose sand fills the empty space,
pushing up from underneath until it comes into contact with the hardened base of
the death mask.

This peaceful coexistence between the slime and the Ediacarans did not last.
As the metazoans secured their grip on the world, the slime had to recede. But
has it gone for good? Probably not. If the temperature controls at the planet’s
surface become seriously unhinged by a runaway greenhouse effect, or if nuclear
Armageddon takes place, or—assuming we manage to avoid these imminent
dangers—when the Sun enters its final, swollen red giant phase billions of
years from now, then our delicate metazoan life will probably lose its temporary
hold on the planet, and the resilient microbes will be back to cast a sticky net
across the globe. The Earth will then have returned to its natural
state—an undisturbed slimy Eden.