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Explosion of life
Jul 17, 2009

Knowing that we owe our very existence as a species to an event that happened 540 million years ago is quite fascinating and also rather humbling. Without the Cambrian Explosion, life on planet Earth may have been stuck in the simple organism phase. Prior to the Cambrian Explosion, the seas were dominated by single-celled bacteria, plankton and multi-celled simple algae, although there was the first inkling of mysterious multi-celled “small shelly fauna” as well as primitive soft-bodied jellyfish, sponges  and worms 570 million years ago. But there are few Precambrian survivors existing today and there appears to be a sharp demarcation between these Ediacaran faunas (fossils of the Proterozoic period) and the Cambrian animals.

According to a new study by University of California, Riverside, and Arizona State University researchers, the explosion of multi-cellular complex lifeforms resulted from non-vascular plants — primitive ground-hugging plants lacking channels to convey fluids (think of a tree’s vascular system delivering sap where it’s needed) that spread outward onto the  planet’s surface — greening our world.

The trigger for the Cambrian Explosion actually occurred some 700 million years ago, when the ground huggers began to pump great quantities of oxygen into the atmosphere.

“During this period, Earth became extensively occupied with photosynthesizing organisms,” said Arizona State University geologist L. Paul Knauth, as reported in a recent ScienceDaily  article. “The greening was a key element in transforming the Precambrian world — which featured low oxygen levels and simple bacteria dominant lifeforms — into a kind of world we have today with abundant oxygen and higher forms of plant and animal life.”

Researchers Knauth and Martin Kennedy, of the University of California, Riverside, studied the isotopic composition of limestone that formed during that period.

“There are three atoms of oxygen for every atom of carbon in the limestone,” explained Knauth. “We looked at the oxygen isotopes as well, which allowed us to see that the particular carbon isotope signature previously interpreted in terms of catastrophes was always associated with intrusions of coastal ground  waters during the burial transformations of initial limestone muds into rocks. It’s the same as we see in limestone forming today.”

Over a three-year period, the researchers gathered  published measurements and carefully plotted the carbon isotope data against oxygen isotope data. From the data, the researchers saw a world not subject to periods of life-altering catastrophe, but one greened by primitive plants. These primitive plants were ground huggers because they lacked a physical structure such as a stem or trunk to hold them up.

“Our work presents a simple alternative view of the thousands of carbon isotope measurements that had been taken as evidence of geochemical catastrophes in the ocean,” said Knauth. “It requires that there was an explosive greening of Earth's land surface with pioneer vegetation several million years prior to the evolution of vascular plants, but it explains how a massive increase in Earth’s oxygen could happen, which has been long postulated as necessary for animals to evolve big time.”

The Cambrian Explosion occurred between 540 and 520 million years ago and what resulted was the lineage of almost all animals living today.

One of the world’s best examples of the animal diversity of the Cambrian Explosion is the fossils found in the Burgess Shale, which author Simon Conway Morris termed The Crucible of Creation in his book of the same name. The Burgess Shale is sedimentary rock located in the Rocky Mountains of British Columbia. The shale contains extraordinary fossils of early multi-cellular lifeforms with many having their soft-body parts fossilized.

The scarcity of ancient rock containing animal fossils just prior to the Cambrian Explosion troubled Charles Darwin, whose famous and earth-shattering book outlining evolution, the Origin of Species, was published 150 years ago — an anniversary which is being widely celebrated across the world this year. “Darwin was fully aware that his theory might be difficult to reconcile with the seemingly abrupt appearance of the Cambrian animals,” wrote Morris. Darwin’s theory involved a slow, random evolutionary process through natural selection. 

But since Darwin’s time, the gaps are being filled (note the Ediacaran fossils referred to earlier) and the Cambrian Explosion is now known to be a genuine occurance. Evolutionary spurts do occur, such as the Cambrian Explosion, although calling it an “explosion” is relative, as the time period involved still covers millions of years.

The animals of the Cambrian Explosion are somewhat “alien” in appearance, although a closer inspection reveals their body type is still found today. One of the more eerie animals was the Anomalocaris, a flying-through-the-water multi-legged arthropod (think crabs, lobsters and insects with their jointed and segmented exoskeletons) with two large eyes and two spike-lined limbs used to capture its prey. This animal was the Great White Shark of the ancient sea, a terror to all other animals of the Cambrian. Then there is Hullucigenia, a spindly-legged weirdo with a series of defensive spines on its back. There are also trilobites, segmented shelled animals whose most famous fossil in recent years came from along the shore of Hudson Bay near Churchill. The Churchill trilobite (now in the Manitoba Museum) is from 445 million years ago and measures 70 centimetres in length, which until an announcement from Spain this month was the largest fossil trilobite ever found.

For our species, the one fossil of greatest importance found in the Burgess Shale is the Pikaia glacilens, a chordate, which swam through the water by contracting the muscles of its body wall, sending out a series of waves along its body, just as fish do today. Chordates have a stiff internal rod (notochord) running along almost the entire length of its body — the beginning of a backbone. From the chordate fish came fish with a backbone, progressing to amphibians, reptiles and mammals. 

From such humble beginnings as the Pikaia  arose man.