Tracing origins of odd creatures
By Sean B. Carroll (China Daily)
Updated: 2010-08-16 11:35
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The Cambrian Explosion - the apparently abrupt appearance of complex animals in the fossil record in the Cambrian Period troubled Charles Darwin.
In Darwin's day, no fossils were known below those of the Cambrian Period, which dates from about 542 to 490 million years ago. This was an extremely unsettling fact for his theory of evolution; 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 did not know why no records of them existed.
It took a very long time, and the searching of some of the planet's most remote places - 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.
This gap in the fossil record, now called the Ediacaran Period, is of intense interest. Dating from 635 to 542 million years ago, it is the first new geological period to be named in more than a century. Geologists have developed some intriguing theories about how dramatic changes in the Earth's climate and chemistry 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 South Australian geologist, noticed some striking discshaped impressions up to 10 centimeters in diameter on the exposed surfaces of rocks in the Ediacaran Hills north of Adelaide. He believed their unusual appearance and their age marked the beginning of the Cambrian, and made them the oldest animal forms yet seen. But his discoveries were ignored by leading geologists.
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 in Newfoundland, 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.
Many of the creatures are so unlike modern forms that deciphering what they are and how they lived continues to challenge paleontologists. The challenge to classifying most Ediacarans is that they lack some features that are characteristic of modern animals - a mouth or an anus, or the shells typical of many Cambrian groups. Scientists have also had to explain how such creatures functioned. Some of the very flatbodied Ediacarans, for instance, appear to have fed by directly absorbing nutrients by osmosis.
The kinds of Ediacaran animals that paleontologists have been especially eager to identify are those with bilateral body symmetry, the feature characteristic of many modern animal groups, including humans.
Tracing the origins of bilateral animals is crucial to understanding the pace of animal evolution. The Ediacaran fossil record also raises the question of why, after more than 2.5 billion years, larger, more complex, forms emerged at that time.
A key requirement for larger creatures is oxygen, and the history of oxygen levels is also etched in rocks. The earliest Ediacarans were deep-water creatures that emerged 575 to 565 million years ago. Recent chemical analyses of Ediacaran sediments reveal that the deep ocean originally lacked oxygen, then became much richer in oxygen and stayed that way. That sharp rise may have been the catalyst to the evolution of animals.
Darwin once wrote to a friend:"Our ancestor was an animal which breathed water, had a swimbladder, a great swimming tail, an imperfect skull & undoubtedly was an hermaphrodite!"
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 doormat.
The New York Times