The history of ancient life on Earth is written in its oldest rocks. Paleontologists, detectives who track the ancient denizens of that early world, continue to gather fossil evidence of the earliest multicellular life, the Ediacara.
These mysterious creatures preceded the famous Cambrian explosion that began 541 million years ago.
The Cambrian, abundant with the fossils from the ancient ocean sediments, including the Burgess Shale in Canada, justifiably captures the popular spotlight.
Almost everyone has seen a trilobite, an animal that appeared in the Cambrian and roamed the ancient seas for 270 million years, right up to the Permian-Triassic extinction, the so-called “Great Dying” 252 million years ago.
Of course, even kids know the names of dinosaurs, which came to rule the Earth after the Great Dying until only 66 million years ago.
Ediacaran life has attracted much less celebrity
Beginning in the 1950s, a few fossils began to tell of complex animals that were not even supposed to exist prior to the Cambrian.
This pre-Cambrian time, the Ediacaran period, between 575 and 542 million years ago, followed the Marinoan glaciation or “Snowball Earth,” a period when the entire surface of our planet was frozen solid.
Not that long ago, the Cambrian was considered the boundary before which no multicellular life existed. That’s all changed.
Simple one-celled life appeared on Earth about a billion years after the planet formed 4.5 billion years ago, a time punctuated by massive meteor bombardments, including a whopping collision with a Mars-sized planet that sent molten rock into space to form our Moon.
Within less than a billion years life appeared. Life on Earth may have gotten its start near deep-sea geothermal vents, still the home of extremeophile bacteria and the animals that feed on them.
Whatever process led to the emergence of life, soon single-celled plants began converting the Earth’s carbon dioxide into oxygen and carbohydrates, the stuff you sprinkle on your cereal.
The first oxygen was consumed by iron dissolved in the oceans which became iron oxide or rust, laid down as rich ore deposits later used to make your car.
The conversion of a carbon-dioxide in the Earth’s atmosphere into oxygen may seem like a good thing to mammals like us. But carbon dioxide traps heat much better than does oxygen. Simple plants made too much of a good thing! Oxygen cooled the Earth.
Our planet froze into a Snowball Earth. Several times.
Why did ancient single-celled life dominate Earth for so long?
Appearing “only” a billion years after the Earth formed, single celled life took more than three billion years to begin forming more complex multicullular life forms during the Ediacaran Period about 600 million years ago. Meditate on this time line.
Notice how quickly simple life appears 4 billion years ago, at the end of meteor bombardment of Earth. Yet, it takes a full 3 billion years and longer for animals to finally appear. Then, in less than a billion years, animals conquer the Earth, with vertebrate animals only making land in about the last third of a billion, and mammals like humans showing up very, very late to the party.
Some scientists think that low oxygen levels in the oceans 1.8 to .8 billion years ago kept life simple. Others believe that Snowball Earths caused, ironically, by plentiful oxygen in the atmosphere, delayed the appearance of complex life. However, some paleontologists continue to push back the time of the first animals. perhaps even before the last “Snowball Earth.” In any case, the Cambrian is no longer the first word in multicellular life.
Ediacaran Ancient Life Finally Gets Some Respect
These ancient critters now have their own explosion, the Avalon Explosion, a rapid increase in the diversity of Ediacara body forms that preceded the Cambrian Explosion by 33 million years. If 33 million years doesn’t sound like much, it’s a quarter the length of the dinosaurs’ reign and half as long as the Cenozoic, the Age of Mammals.
A Sample Collection of Ediacaran Life Forms
Unless noted all photo credits: Wikipedia
Dickinsonia grew as big as bath mats and apparently lay flat on the sea bottom, possibly grazing on microbial mats. The segments of the soft-bodied Dickinsonia have been described as liquid-filled chambers, something like an air mattress. First discovered by Reg Sprigg in 1946 in the Flinders Ranges of South Australia, apparently while eating his lunch, Ediacaran fossils were described in a paper submitted to, and rejected by, the journal Nature.
Charnia was first discovered in 1957 in rocks in Charnwood Forest in central England by Tina Negus, a 15-year-old schoolgirl, but her geography schoolteacher ruled out the possibility of Precambrian fossils.
Later, a schoolboy Roger Mason, who later became a geologist, brought the fossils to the attention of scientists. Charnia lived attached to the sea bottom, too deep for sunlight to allow photosynthesis, suggesting that they absorbed nutrients directly from the seawater.
Also first discovered in Charnwood Forest, Charniodiscus was first discovered as only the circular base or holdfast that anchored the stalk and frond to the sandy sea floor. Similar to Charnia, the frond structure is different, although the source of nutrition for this an Ediacaran fossil was presumably similar. Two Charniodiscus forms have been found, one with a short stalk and fat frond, the other with a long stalk that held the frond 20 inches above the sea bottom.
Eoandromeda has eight radial spiral arms, spiraling either clockwise or counterclockwise. A few dozen fossils are known, ranging from 1 to 4 cm in diameter. This specimen from Australia has longer, more tightly coiled arms compared to Chinese Eoandrommeda. Photo credit: http://www.flickr.com/groups/complex_life/
Named for Ron Sprigg, this segmented Edicaran organism reached about 3-5 cm in length and may have been a predator. Two rows of tough interlocking plates cover the bottom, while one row covers its top. Its front segments are fused to form a head, which may have borne eyes and antennae.
These are but a sample of a growing collection of Ediacaran organisms. Below is an artist’s conception of an Ediacaran garden
The Ediacara appear to have become extinct before the celebrated hustle and bustle of the Cambrian. Some suggest they are a “failed experiment” that left no descendents. As objects of scientific study the Ediacara are an active field with a growing list of Ediacaran genera.
Here’s how the Earth’s continents changed from Ediacaran times.
Scientific Picture Book of the Ediacarans
Highly rated, with foreword by Arthur C. Clarke, The Rise of Animals by five respected authors is considered the best account of Ediacaran life. Organized by fossil location, the book includes an Atlas of Ediacaran paleontology.
Take a Peek
Even if you are just curious, the “Look Inside” button at Amazon offers a generous sampling of the Ediacaran “zoo,” with fossil photos and interpretations of how these strange organisms lived.