9 November 2019 | New Scientist | 39LE >
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geological record in the 19th century, and
they quickly noticed something puzzling. The
oldest rocks they could find seemed devoid of
fossils. Biologically complex marine animals,
including woodlouse-like trilobites, suddenly
appeared in abundance in the rocks assigned
to the Cambrian period.
The pattern troubled Charles Darwin
because it clashed with his idea of evolution
by natural selection as a slow and gradual
process. To make sense of it all, he suggested
that simpler life forms must have evolved
before the Cambrian but left little or no fossil
evidence of their existence.
We now know Darwin’s hunch was correct.
Geologists have spotted signs of microbial life
in rocks more than 3 billion years old. They have
also identified an important transition roughly
2 billion years ago, when those microbes
became slightly larger and more biologically
complex. This was a necessary step on the way
to animals, broadly defined as organisms that
are multicellular, capable of locomotion and
responsive to their environment.
But the Cambrian explosion still seemed
to mark the sudden blossoming of animal
life. This remained the case even though,
in the mid-20th century, geologists began
finding fossils of large organisms, some a
metre or more across, in rocks that predate the
Cambrian explosion by 30 million years. These
organisms were dubbed the Ediacaran biota
because they date to the Ediacaran geological
period, which precedes the Cambrian. But we
couldn’t quite figure out what to make of them.
This wasn’t only because none of these
organisms seemed to possess obvious animal
features like a gut or a mouth. Some, including
those in a group called the rangeomorphs,
also had a very unanimal-like fractal anatomy,
in which tiny parts of the organism looked
like miniature versions of larger parts
(see “Pushing back the clock”, page 40).
The influential palaeontologist Adolf
Seilacher argued that the Ediacaran organisms
were so clearly unrelated to the animals of
the Cambrian that they were effectively as
strange to our modern eyes as life on another
planet would be. Seilacher was one of many
researchers who felt that Ediacaran species
and ecology looked so alien that it was
impossible to escape the conclusion that the
Cambrian was indeed a dramatic explosion
of familiar animal life.
In the past 10 years, however, geologists have
shifted their thinking. Sophisticated analytical
techniques have started to suggest that
some of the weird species of the Ediacaran
were animals after all, and that they behavedThe Cambrian explosion is feted as evolution’s
big bang, but now some enigmatic earlier creatures
are rewriting the history of life, says Colin Barras
“ Ediacarans were
as strange to our
eyes as life on
another planet
would be”
L
IFE appeared on our planet more than
3.5 billion years ago and consisted
exclusively of microbes for the next
3 billion years. Then, about 539 million years
ago, everything changed.
In the geological blink of an eye, the seas
were filled with large and complex animals,
including worms with legs and fearsome
spikes, creatures with a trunk-like nose and
five eyes, and giant shrimp-like predators
with mouths like pineapple rings.
This evolutionary starburst is known as
the Cambrian explosion. It is one of the most
significant moments in life’s history on Earth
because it is the point at which species that
are clearly related to today’s animals first
appeared. It is seen as evolution’s big bang.
But over the past few years, geologists
have begun to have second thoughts. Newly
discovered fossils and careful analysis of ones
found decades ago suggest that animals were
thriving in the period before the Cambrian.
As a result, some people are now arguing
that the explosion of animal life started
about 12 million years earlier. Others are
questioning whether it is possible to define
a distinct explosion at all.
You could be forgiven for thinking that
shifting the dawn of the animal revolution
from 539 to 551 million years ago isn’t that big
a deal. But evolution can do a lot in that length
of time: the entire span of human evolution
probably fits within 12 million years, the length
of time since our lineage separated from that
of chimpanzees. What’s more, shifting life’s big
bang back could have important implications
for the quest to figure out what sparked
evolution’s most spectacular spell of invention.
Scholars first worked out how to read theThe leaf-like
rangeomorphs (left),
the largest of which
grew to 2 metres in
height, are now thought
to have been some of
the earliest animals.
The same goes for
Tri b r a chid iu m (above)