PUZZLE
Many of the creatures found in the fossil record from the time immediately
preceding the Cambrian are so unlike modern forms that deciphering what they
are and how they lived continues to challenge paleontologists.
In
1909, Charles Walcott, a paleontologist and secretary of the Smithsonian
Institution, discovered one of the greatest and most famous fossil
troves high in the Canadian Rockies on Burgess Pass in British Columbia. The
slabs of Burgess Shale that Walcott excavated contained the earliest known
examples at the time of many major animal groups in the fossil record, in rocks
that were about 505 million years old.
Walcott’s 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.
In
“On the Origin of Species,” Darwin posited that “during these vast, yet quite
unknown, periods of time, the world swarmed with living creatures.” But he
admitted candidly, “To the question why we do not find records of these vast
primordial periods, I can give no satisfactory answer.”
It
took a very long time, and the searching of some of the most remote places on
the planet — in the Australian Outback, the Namibian desert, the shores of
Newfoundland and far northern Russia — but we now have fossil records from
the time immediately preceding the Cambrian. The rocks reveal a world whose
oceans were teeming with a variety of life forms, including primitive animals,
which is certainly good news for Darwin.
Now,
this once-worrisome gap in the fossil record is a period of intense interest to
geologists as well as paleontologists. The former have even given it its own
division in the geological timescale. The Ediacaran Period, from 635 to 542
million years ago, is the first new geological period to be named in more than a
century. Moreover, geologists have developed some intriguing
theories about how dramatic changes in the Earth’s climate and chemistry during
the Ediacaran may have allowed for the evolution of animals.
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.
But
finding these fossils has posed many new mysteries. Many of the creatures are
so unlike modern forms that deciphering what they are and how they lived
continues to challenge paleontologists. Prof. Andrew Knoll of Harvard
University has likened the Ediacaran forms to a paleontological
“Rorschach” test because different scientists often interpret the same fossil
very differently.
Dickinsonia,
for example, has been interpreted as being a relative of jellyfish, a marine
worm, a lichen, or even as a member of a completely extinct kingdom. The
challenge to classifying most Ediacarans is that they lack some features that
are characteristic of modern animals, a mouth or an anus in the case of Dickinsonia,
or the shells and hard parts typical of many Cambrian groups. But, in fact,
such simple bodies are exactly what should be expected of primitive forerunners
of later animals.
On
the other hand, scientists have had to explain how such creatures functioned.
Some of the very flat-bodied Ediacarans, for instance, lived on sediments and
appear to have fed by
directly absorbing nutrients by osmosis.
The
kinds of animals that paleontologists have been especially eager to identify in
the Ediacaran are those with bilateral body symmetry, the feature
characteristic of the majority of modern animal groups, including ourselves.
Bilateral
animals flourished in the Cambrian so tracing their origins is crucial to
understanding the pace of animal evolution. Several bilateral Ediacaran animals
have been discovered, including Kimberella, a possible mollusc. Hundreds
of Kimberella specimens are known that date to about 555 million years
ago, 50 million years before the animals of the Burgess Shale.
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.
from the New York Times, July 27, 2010
