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TAKE a few buckets of coral sand and sieve. Place a deep layer of the fine
grains in an aquarium. Add a generous pinch of the most basic sorts of animal
life鈥攁 tube-dwelling anemone or some flatworms will do. Leave for a few
minutes, watching carefully. The recipe is simple, but the results are very
satisfying: tracks that resemble the earliest trails preserved in the fossil
record.

The anemones and flatworms are doing nothing special, simply getting around
using one of the oldest forms of locomotion. They lay down a ribbon of mucus and
paddle themselves along it. But until now, no one had made the connection
between the slimy paths they create and tracks left by enigmatic trail-makers
hundreds of millions of years ago. 鈥淭he implication is that some of the earliest
traces might have been made by more simply organised animals than previously
thought,鈥� says Allen Collins, a biologist at the University of California,
Berkeley.

The first undisputed fossil trails date from about 565 million years ago,
during the Neoproterozoic period, at least three billion years after life began
on Earth. Some researchers claim there is evidence of earlier trails鈥攎ost
famously the billion-year-old 鈥渨orm鈥� trail in rocks from India鈥攂ut all are
controversial. The oldest trails are featureless and only a few millimetres
wide. Within a few million years, trails became abundant and more complex, often
with grooves and ridges, but still very narrow. With no 鈥渂odies鈥� linked to the
trails, the identity of their creators remains a puzzle. But most
palaeontologists agree that whatever made the trails had to be a tad more
sophisticated than a flatworm.

A few years ago, Jim Valentine of the Museum of Paleontology at Berkeley
described the lowliest sort of trail-maker. He argued that for an animal to
leave a trail robust enough to become a fossil, it had to displace enough
sediment to leave troughs and ridge on the seabed.

Valentine鈥檚 pioneering trailmaker was some sort of early 鈥渂ilaterian鈥�, a
bilaterally symmetrical animal that was triploblastic鈥攎ade of three layers
of tissue鈥攁nd, most importantly, had a fluid-filled body cavity to provide
something firm for the muscles in the body wall to push against. This type of
body structure allows a worm-shaped animal to send a wave of muscle contractions
rippling along its length, pushing the animal over or through the ground,
forcing aside sediment as it goes.

The fossilised creatures from 565 million years ago would not have moved in
this way. Most of the bodies belong to bizarre organisms known as the Ediacarans
that may not even have been animals. Ancestors of the sea anemones were
undoubtedly there, but like today鈥檚 anemones, they would have had only two
layers of tissue and no fluid-filled body cavity. And some sorts of ancestral
flatworm were probably around, though they too lacked a fluid filling. The
earliest fossils of what are clearly triploblastic animals appear during the
Cambrian, which began 543 million years ago.

But Valentine鈥檚 model implies that, despite the absence of fossils, complex
triploblastic animals were alive and well and crawling all over the
Neoproterozoic seafloor. His model pushes back the date of the evolution of a
body pattern that most animals, including humans, have today.

The notion of the 鈥渕inimal trail-maker鈥� was widely accepted, but Valentine,
along with Collins and another colleague, Jere Lipps, decided to test the idea
with living animals, including some very simple ones. Tests with some local
species of flatworm initially seemed to support the idea: they didn鈥檛 leave any
marked trails.

However, Collins and Lipps tried again, this time crossing the Pacific to the
island of Moorea in French Polynesia, where Berkeley has a field station. In the
island鈥檚 lagoon, they collected some anemones and flatworms, put them in tanks
and watched their progress over the sand. Much to their surprise, both sorts of
animals left impressive trails. 鈥淢ucus and bits of sediment combine to form a
three-dimensional trail,鈥� says Collins.

As the animals travel along, tiny hair-like cilia on the underside of their
bodies beat against the mucus. With each beat, the mucus ribbon is pressed
firmly against the seafloor, picking up sand grains, which are incorporated into
the slimestream. The result is a firm, gritty structure that stands proud of the
seabed and is solid enough to pick up with your fingers. Some trails are simple
slimy sausages between one and two millimetres across, others are edged by a
pair of ridges flanked by tiny furrows like miniature earthworks.

The simplest of the experimental animals, with just two tissue layers, was
Pachycerianthus, a purple anemone with stripy tentacles. It is about 10
centimetres long and normally lives in a burrow lined with slime secreted from
glands all over its body. Tube-dwelling anemones rarely leave their burrows, but
when Collins and Lipps forced these animals to travel over the sand, they got
about perfectly well by paddling along on a strip of slime.

An anemone travelling along the sand secretes slime only from a narrow belt
of glands where its body touches the sediment. 鈥淚t leaves a really nice trail,鈥�
says Collins. Sometimes, Pachycerianthus left a single featureless
ribbon, between two and three millimetres wide. Other times, it left a more
structured trail with low embankments to either side. Both closely resemble
well-known types of trace fossil from Neoproterozoic rocks.

The flatworms, which are triploblastic but lack body cavities, also left
narrow trails almost identical to well-known trace fossils. Collins and Lipps
observed several species of Moorean flatworms鈥攕potted, striped and knobbly
species up to five centimetres long. All of them normally live on hard surfaces,
but they had no trouble travelling over fine, loose sand. Like the anemones,
they secreted a ribbon of slime, this time from a pair of glands near the head.
The paired strings of slime created trails with two distinct ridges, bordered by
tiny trenches where sand was removed and taken into the slime trail.

These animals probably didn鈥檛 exist 565 million years ago. But others with
the same basic types of bodies did鈥攁nd many of them are likely to have
moved about in this way. Slime, suggests Collins, could be the key to many of
the earliest fossil trails. Different arrangements of mucus glands or different
methods of moving along the mucus ribbon could explain the variation in the
shapes and sizes of trails. 鈥淚f the mucus came out in pulses鈥攇lob, glob,
glob鈥攖hat might produce an uneven trail,鈥� suggests Collins. 鈥淥r if an
animal spat out mucus in a more haphazard way, you would get very different
辫补迟迟别谤苍蝉.鈥�

The discovery that animals with very simple body plans can make just as big a
mark on the seabed as those with more complex ones doesn鈥檛 really solve the
great mystery of what made the most ancient trails. It doesn鈥檛 rule out
sophisticated animals, but it does rule in the simpler sorts. 鈥淭his shows that
there鈥檚 a larger palette of trace-makers than we thought,鈥� says Andy Knoll, an
evolutionary biologist at Harvard University.

鈥淢ost of these trails could have been made by the mucus-and-cilia form of
locomotion, but we suspect that some more complex things were there at the same
time,鈥� says Collins. The challenge now is to find a way of telling them
apart.