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The missing link

Last updated: 09 August, 2010
New York Times News Service

Evolution

The Ediacaran fossil record 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? asks Sean B Carroll

IMPRINTS FROM THE PAST: An  undated handout photo of  Ediacaran fossils discovered in southern Namibia. (James G Gehling via The New York Times)

IMPRINTS FROM THE PAST: An  undated handout photo of  Ediacaran fossils discovered in southern Namibia. (James G Gehling via The New York Times)
In 1909, Charles Walcott, a paleontologist and secretary of the Smithsonian Institution, discovered one of the greatest and most famous fossil troves in the Canadian Rockies on Burgess Pass in British Columbia. The slabs of Burgess Shale contained the earliest known examples at the time of major animal groups in the fossil record, in rocks 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 542 million to 490 million years ago. Though 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.

Now, this gap is a period of intense interest to geologists and paleontologists. The former have even given it its own division in the geological timescale. The Ediacaran Period, from 635 million to 542 million years ago, is the first new geological period to be named in over a century. Geologists have developed some intriguing theories about how dramatic changes in the Earth’s climate and chemistry during the Ediacaran may have allowed for evolution.

The first major advance towards finding the earliest animal life was in 1946 when Reginald Sprigg, a geologist for the South Australia government, was checking out old mines in the Ediacaran Hills of the Flinders Range north of Adelaide. He noticed some striking disc-shaped impressions up to four inches in diameter on the exposed surfaces of rocks nearby. Sprigg interpreted the patterns as fossil remains of soft-bodied creatures like jellyfish or their relatives. But when he showed them to leading authorities, they dismissed them as artifacts made by weathering of the rocks. However, later that year, when Sprigg found the frond-like forms he called Dickinsonia, he was certain that such impressions could have been made only by living creatures.

Sprigg’s discoveries overlooked

Sprigg was excited by the unusual appearance of the fossils and their age, which he believed to be the beginning of the Cambrian, and made them the oldest animal forms yet seen. But, Sprigg’s discoveries were ignored at an international geology meeting and his paper was rejected by the journal, Nature.

Attention to these 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 established as 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 over 30 other locations on five continents. The global distribution of these disc, frond, tube, branch or spindle-shaped forms shows that life was complex and diverse in the Ediacaran.

A challenge to paleontologists

But many of the creatures are so unlike modern forms that deciphering what they are and how they lived continues to challenge paleontologists. Professor Andrew Knoll of Harvard University has likened the Ediacaran forms to a paleontological “Rorschach” test because different scientists 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 Ediacarans lived on sediments and appear to have fed by directly absorbing nutrients by osmosis. The kinds of animals that paleontologists have been eager to identify in the Ediacaran are those with bilateral body symmetry, the feature characteristic of the majority of modern animal groups, including us. Bilateral animals flourished in the Cambrian and tracing their origins is crucial to understanding the pace of animal evolution. Several bilateral Ediacaran animals have been discovered, including Kimberella, a possible mollusk. 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 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?

Etched history of oxygen levels

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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