In 1909, Charles Walcott discovered one of the greatest and most famous
fossil troves high in the Canadian Rockies. The discovery was further
evidence of the so-called Cambrian Explosion---the apparently abrupt
appearance of complex animals in the fossil record within the Cambrian
Period, from about 542 to 490 million years ago.
Although not seen before on the scale documented in the Burgess Shale,
the
emergence of trilobites and other animals in the Cambrian was familiar
to
paleontologists, and had troubled Charles Darwin a great deal. The
difficulty posed by the Cambrian Explosion was that in Darwin's day (and
for many years after), no fossils were known in the enormous, older rock
formations below those of the Cambrian. This was an extremely unsettling
fact for his theory of evolution because complex animals should have
been preceded in the fossil record by simpler forms.
The first major advance towards finding the earliest animal life
occurred
in 1946 when Reginald Sprigg, a geologist for the South Australia
government, was checking out some old mines in the Ediacaran Hills of
the Flinders Range several hundred miles north of Adelaide. Sprigg
noticed some striking disc-shaped impressions up to four inches in
diameter on the exposed surfaces of rocks nearby.
Sprigg interpreted the patterns as the fossil remains of soft-bodied
creatures like jellyfish or their relatives. But when Sprigg first
showed the
imprints to leading authorities, they dismissed them as artifacts made
by the
weathering of the rocks. However, later that year, when Sprigg found the
frond-like forms he called Dickinsonia , he was certain that such
geometrical impressions could have been made only by living creatures.
Sprigg was excited by both the unusual appearance of the fossils and by
their age, which he believed to be the beginning of the Cambrian, and
made
them the oldest animal forms yet seen. But despite their potential
importance,
Sprigg's discoveries were ignored at an international geology meeting
and
his paper describing the fossils was rejected by the leading journal, .
Sprigg moved on to other, more rewarding pursuits in the oil, gas, and
mining
industries.
Scientific attention to these strange forms was not revived until a
decade
later when more soft-bodied forms were found in the Ediacaran Hills and
in
England, and their age was firmly established as actually predating the
Cambrian. Deposits of similar aged forms have been discovered at
Mistaken
Point on the Avalon Peninsula of Newfoundland, in southern Namibia, the
White Sea of Russia, and more than 30 other locations on five
continents. The
global distribution of these disc-, frond-, tube-, branch-, or
spindle-shaped
forms demonstrate that life was complex and diverse in the Ediacaran.
The Ediacaran fossil record thus stretches the origins of animals to
well before the Cambrian Explosion. But it also raises the question of
why, after more than 2.5 billion years during which microscopic life
dominated the planet, larger, more complex, forms emerged at that time?
A key requirement for larger creatures is oxygen, and the dramatic
history
of oxygen levels is also etched in Ediacaran rocks. Geologists now
understand that the earliest Ediacaran organisms were deep water
creatures that emerged 575 to 565 million years ago, shortly after a
major ice age ended
about 580 million years ago.
Recent chemical analyses of Ediacaran sediments reveal that the deep
ocean
lacked oxygen before and during that ice age, then became much richer in
oxygen and stayed that way after the glaciers melted . That sharp rise
in
oxygen may have been the catalyst to the evolution of animals, including
our
ancestors.
Several weeks after the publication of "On the Origin of Species" and
amid
a torrent of criticism, Darwin added a mischievous postscript to a
letter
to his friend, the geologist Charles Lyell: "Our ancestor was an animal
which breathed water, had a swim-bladder, a great swimming tail, an
imperfect skull & undoubtedly was an hermaphrodite! Here is a pleasant
genealogy for mankind." The Ediacaran fossils tell us that Darwin was
being too generous.
Our earliest animal ancestor probably had no head, tail, or sexual
organs,
and lay immobile on the sea floor like a door mat.
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