November 2009

It’s now clear that humans have dramatically changed Earth’s atmosphere. More than 30 years ago, scientists realized that our production of chlorofluorocarbons was destroying Earth’s protective ozone layer; as we burn fossil fuels for energy, we inadvertently release chemicals like sulfur dioxide, which react with other atmospheric compounds and end up acidifying rainwater; and of course, our production of greenhouse gases, like carbon dioxide, is shifting the makeup of Earth’s atmosphere in a direction that actually changes the climate on a global scale. Many of our recent Evo in the News stories have chronicled how human-caused changes in Earth’s atmospheric chemistry and environment are affecting the evolution of life on earth. In celebration of this months’ Year of Science theme, chemistry, we bring you another story about how atmospheric chemistry impacts evolution — this one from Earth’s deep history.

2009 is the Year of Science! This year-long party celebrates all of science. Find out what’s happening in your neighborhood at www.yearofscience2009.org.

Where's the evolution?

Grypania spiralis might not seem like much to you — an extinct, single-celled alga that resembles a tiny slinky, with coils just a few centimeters in diameter. But two billion years ago when Grypania lived, it was a giant — the largest organism on Earth at its time, and a million times bigger than its earliest ancestors. Today, of course, it is dwarfed by the largest organism that has ever lived, the giant sequoia (yep, it’s bigger than any dinosaur!), which can reach a body volume of 4500 cubic meters — enough for a single tree to fill nearly fifty semis. Over its 3.5 billion years, life on Earth has evolved more than a quadrillion-fold increase in size from its microscopic beginnings. How and why did body size evolve so dramatically?

Last year a team of researchers led by paleobiologist Jonathan L. Payne tackled that question. One possibility they considered was a sort of non-process — that the increase in body size was not driven by anything in particular. After all, organisms necessarily started out small, so they had nowhere to go but up. As life began to diversify, perhaps some lineages randomly evolved larger body sizes, resulting in a smooth increase in maximum body size over time (see figure at right). To test the idea, the team consulted experts and the scientific literature and put together a list of the largest organisms known throughout life’s history, from the earliest bacteria up to modern organisms. They graphed the data to see if they showed the slow and steady increase in body size we’d expect if body size had randomly evolved away from its small beginnings — but the pattern they observed in the data supported another explanation entirely. Something more complicated was going on.

According to the evidence the team collected, life has experienced two big growth spurts. For the first 1.5 billion years of its history, life stayed small — barely increasing in size from its beginnings. Then, around two billion years ago some lineages rapidly evolved to be roughly a million times larger than the largest organisms had been before. Things held steady for next billion or so years, even dropping off a little — but around 500 million years ago, there was another dramatic increase, which took us from organisms the size of a thimble to eight-meter-long octopus relatives called endocerids — another million-fold increase in size. After that, body sizes increased slowly, eventually bringing us up to today. Almost all of the increase in organisms’ body size over time is accounted for by just two big jumps 500 million and two billion years ago.

What might the explanation for these growth spurts be? The scientists suspected that key innovations — evolutionary changes that open up a new range of ecological or evolutionary possibilities — might play a role. For example, the scientists noticed that the organisms that evolved larger sizes during the first jump in body size were all eukaryotes — cells that have a more complex structure than bacteria. And the second increase in body size only occurred in multicellular organisms. It’s easy to imagine that both of these advances (complex cells and multicellularity) would have allowed individuals to evolve larger body sizes — perhaps by enabling new mechanisms for delivering nutrients to the interior parts of the organism.

However, the scientists also noticed that there was a big delay — hundreds of millions of years — between the evolution of the key innovation and the increase in body size. What was holding life back in the interim, constraining lineages from evolving larger body sizes once they had the basic structures necessary to do it? The answer that immediately occurred to the team was oxygen. The two, big body size jumps lined up almost perfectly with times that Earth’s oxygen levels increased dramatically! The scientists suspect that these changes in Earth’s oxygen unlocked the evolutionary potential of many lineages allowing them to diversify and evolve larger body sizes. Most life on Earth needs oxygen in order to produce usable chemical energy in the cell, which is necessary for growth and reproduction — so it makes sense that oxygen levels might limit the body sizes that organisms can evolve.

Interestingly, these shifts in Earth’s atmospheric chemistry didn’t just influence evolution — they are also the indirect result of evolution. Four billion years ago, Earth’s atmosphere had little or no free oxygen. That changed when bacteria evolved the ability to photosynthesize, which releases oxygen as a byproduct. For example, the first jump in Earth’s oxygen levels may have been triggered by the proliferation of photosynthetic bacteria in the oceans. And this jump in oxygen levels may have, in turn, triggered the evolution of larger body sizes. Life is locked in a feedback loop with the chemistry of the Earth’s atmosphere. As we’ve seen from this story, the makeup of the atmosphere is powerful enough to trigger leaps in evolution, and as we know from our experience with global climate change, life is certainly powerful enough to change the makeup of Earth’s atmosphere.