The Rock Doc — An explosion in the history of life


My brother likes to build buildings in his free time. He has a couple of timber-frame structures on his property that he put up over the years, and now he’s working on a more traditional “stick” building made of 2x6’s and 2x4’s. One thing all of his efforts have in common is that they begin as drawings and become blueprints. And even though he’s built with quite different approaches over the years, all his buildings have some things in common: windows, doors, and stairs to name just a few.

Many scientists would say that life on Earth also has a complex but single blueprint. Many animals are “bilaterally symmetric,” meaning they have left hand and right hand parts that are mirror images of each other. There are lots of other regularities, such as the fact that all mammals have backbones.

Some of the most interesting parts of life’s blueprint are the relationships that have predators feasting on their prey. The predator-prey relationship took time to become established Here’s the story:

Life has been around for about 4 billion years. At first life was mostly single-celled organisms living in the sea. Then a form of colonial algae turned up that, although simple by modern standards, may have had a lot to do with slowly producing oxygen for what had naturally been an oxygen-poor atmosphere.

As time went on some unique soft-bodied life forms appeared. It took a while for geologists and paleontologists to see them in the fossil record simply because they were small and didn’t have “hard parts” like shells that could clearly be preserved. Instead these animals looked something like worms or small fronds. A few of them made marks or impressions on the sediment of the seafloor and those impressions became preserved as a kind of subtle fossil. These animals, called the Ediacara fauna, were discovered in several places around the world once scientists knew what to look for. The Ediacara animals are long, like pencils, or flat, like pancakes. Most paleontologists think they had those body plans so that their skin could exchange gases with the water around them, meaning they didn’t need structures like gills or lungs.

The Ediacara animals were simple, but for a time they were the most complex creatures on Earth. All of that changed dramatically during Cambrian times. This was the period part of which is known as the “Cambrian Explosion,” a time that life became rapidly much more complex.

One place that preserved an interesting slice of life in the Cambrian is what’s termed the Burgess Shale from high in the mountains of British Columbia. You might say the rocks there are from the “mid-explosionary” times. What’s fascinating about them is they preserve some animals we can easily recognize as like their modern counterparts, like brachiopods, but also many strange animals. Just for example, one of the oddballs is Opabina, an animal that had five eyes and a nose-like structure a bit like an elephant’s trunk. Another of the strange animals is named Hallucigenia because it seems to be more of a hallucination than a standard animal. For reasons we won’t ever fully know, the “oddballs” went extinct and the animals that occupied part of the blueprint of life more like our own survived and flourished.

One of the interesting questions about the Cambrian explosion is what might have triggered it. Recently Harvard’s Erik Sperling and his colleagues published evidence they argue shows an increase in the air’s oxygen content at the time the rapid diversification started. Their paper appeared in the prestigious Proceedings of the National Academy of Sciences. Higher levels of oxygen, the theory goes, could support animals that had to move around to pursue prey, and then eat and digest them. Once there were predators on the scene, prey animals had to develop defenses like hard shells for protection. The “arms race” between prey and predator species had begun and has been firmly established in life’s blueprint from then until now.

And you thought there wasn’t a connection between the history of animal life and international relations!

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.