Translating the Stories of Life Forms Etched in Stone

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