Life's Grand Explosions

By Lori Oliwenstein|Monday, January 01, 1996
Evolution favors grand entrances and dramatic exits. Thanks to the dinosaurs, we’re most familiar with the latter, in the form of sudden extinctions, but there have been equally spectacular, if less ballyhooed, bursts of creation as well. Figuring out just what drives the process has long been a favorite focus of scientific theorizing, and in this past year alone, researchers weighed in with three new theories about possible agents of change, involving underwater volcanoes, gluttony, and the evolution of feces.

Although life began on Earth some 3.5 billion years ago, for the first 3 billion of those years it pretty much progressed at a numbingly slow evolutionary pace. Then 545 million years ago, in the Cambrian Period, it burst wide open. It was then that almost all the major groups of animals alive today, including our vertebrate ancestors, got their start. There’s this period of time when animals are clearly around, but they haven’t really taken off, says Australian biogeochemist Graham Logan. And then, at the start of the Cambrian, animals just explode. It’s a pretty sudden thing.

Why the change? According to the fossil record, says Logan, it was only some 40 million years before the Cambrian explosion that the first multicellular animals appeared in the ocean, which at that time was still dominated by single-celled plankton and bacteria. Some of these new animals, he notes, could process food the way animals do today, by taking it in at one end and excreting it out the other. In other words, they had guts and they were defecating. And feces, says Logan, are what ultimately allowed the Cambrian to explode with life.

Before then, the thinking goes, animal evolution was held back by low levels of oxygen in the ocean. Photosynthesizing plankton, living near the surface, were pumping oxygen into the water, but the animals couldn’t get any--it was being used up by bacteria. The bacteria were feasting on dead plankton that would slowly sink through the water, and nearly all the ocean’s available oxygen would go to fuel their digestion.

Enter surface-dwelling, plankton-eating, gut-carrying animals and their fecal pellets, which dropped quickly to the ocean floor. Surface bacteria had less food, their population shrank, and they consumed less oxygen; oxygen became available to other creatures and fueled their evolution into more energetic forms.

Logan backs up his theory with data that he and his fellow researchers--from both the Australian Geological Survey Organization and Indiana University--have gathered from rocks formed from the tissues of these organisms. In particular, they looked at the ratio of two isotopes, carbon 12 and carbon 13. When an animal eats a plant or another animal, it tends to preferentially incorporate carbon 13. So the further along the food chain you are when you die, the more carbon 13 you’ll have in your body. Thus we humans have more C13 than the cows we eat.

Before the Cambrian, Logan and his colleagues found, the C13 ratio was high, which is what you’d expect from a bacteria-dominated, oxygen-poor ocean--food stayed around the surface a long time, creating a very long food chain, with wave after wave of bacteria having their fill. Each group of bacteria was coming in, eating the previous material, and becoming isotopically enriched compared with the things that they ate, explains Logan. But when the gut arrived on the scene with its subsequent fast-sinking fecal pellets, food wasn’t around as long to be gobbled by bacteria. The result was a shorter food chain and a drop in C13 ratios reflected in the sediment. And the result of that was more available oxygen, a more stable environment, and an evolutionary playground.

The fecal theory of life hasn’t captivated everyone. Geerat Vermeij, an evolutionary biologist at the University of California at Davis, thinks that undersea volcanoes caused not only the Cambrian explosion but also another flurry of evolution 170 million to 100 million years ago, when flowering plants and social insects became widespread. At both times, he says, volcanoes on the seafloor may have been erupting on a massive scale. These eruptions would have profoundly changed Earth’s climate. The carbon dioxide from underwater eruptions would have made its way out of the oceans and into the atmosphere--in this respect, the volcanoes would be similar to those on land, adding a greenhouse gas to the air and warming the planet. But while terrestrial volcanoes would also kick up sulfur compounds and other particles that could have kept out sunlight and resulted in a net cooling effect, underwater volcanoes would have simply dispersed this stuff into the ocean. Without any compensating cooling, the volcanoes created a huge warming effect.

Unlike today’s man-made global warming, these natural superwarmings were a boon to life. Higher temperatures enable you to do a lot of biological things energetically, says Vermeij, and the reason is that, like most chemical reactions, biochemical ones are strongly temperature dependent. And at higher temperatures water viscosity decreases, which means it’s easier to swim, to respire, and to feed. You can jump faster and move faster. The result is, he says, just like an economy that is energized with greater productivity: more diversity in the ways organisms make a living.

Paleontologist Ronald Martin of the University of Delaware likes Vermeij and Logan’s general approach. I think most people would agree that somehow you have to break down the old ecosystem so that organisms have a chance to try something that’s different, he says. But after looking at the fossil and isotope records, Martin has chosen a more paradoxical- sounding explanation: many bursts of evolution in the ocean, he says, come from bursts of extinction created by bursts of plentiful food.

Overabundance comes about when erosion increases on land, delivering more nutrients to the ocean, or when ocean circulation speeds up, pumping up unused nutrients from the deep sea. Though you might think that an increase in food would be good for everyone, studies suggest that these windfalls can knock over delicate ecosystems. The result, Martin explains, is that the organisms that can reproduce fastest will do so, and they’ll outcompete and outreproduce the ones that reproduce more slowly. They take over and they swamp the system, so to speak. It basically blows up.

The resulting extinctions provide the breathing space evolution needs to try out new tricks. Once the system has collapsed and it starts to reorganize itself from the survivors, says Martin, my interpretation is that the new organisms are more productive. The amount of carbon that is used seems to increase relative to the nutrients that are available. I can’t completely explain that, other than to say that somehow the disturbance is incorporated into the ecosystem.
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