Humanity may hold key for next Earth evolution

Human degradation of the environment has the potential to stall an ongoing process of planetary evolution, and even rewind the evolutionary clock to leave the planet habitable only by the bacteria that dominated billions of years of Earth’s history, Harvard geochemist Charles Langmuir said Thursday (Nov. 13).

Langmuir, Higgins Professor of Geochemistry and an expert on the undersea volcanic vents that are contenders for life’s birthplace, said that planets may proceed through a natural series of evolutionary steps that transform them from lifeless balls into the home of bacteria and other microscopic life and then into a place that supports more complex life.

Along the way, life interacts with the nonliving parts of the planet in planet-changing ways. The oxygen in today’s atmosphere, for example, was initially given off by early bacteria that changed the air’s chemical composition over billions of years.

Langmuir described this planetary evolution as a series of steps and said there’s no guarantee that a planet will proceed from one to the next. Each step represents a moment of both crisis and opportunity. So far, the Earth has surmounted each step, while other planets, such as Mars, which may have once had microscopic life, failed to cross the evolutionary hurdle where life is sustained and becomes abundant.

“It seems likely that all planets go through stages, but some get stuck and stop,” Langmuir said. “Life, I think, is a natural planetary process, likely to occur on any planet with the proper climate systems.”

The Earth today may be at the brink of another step, Langmuir said. Complex life has evolved into intelligent life that dominates the planet — ecosystems, food webs, and energy flow — as no species ever has before.

Whether the planet takes the next step or not may depend on us. If we recognize humanity is an integral part of the planet and begin working for a healthy Earth, then planetary evolution could move forward to some unknown future.

On the other hand, Langmuir said, if we continue to view the Earth as something that is separate, that we merely use, then the resulting practices could damage the environment enough to stall planetary evolution, even causing it to fall back to a level where it supports just microscopic life.

“The story of the Earth is our story. We are intimately connected [to the Earth] in every fiber of our being, in every breath we take. We’re inseparable from the Earth,” Langmuir said.

Langmuir spoke to an audience of several hundred at the Geological Lecture Hall in the first talk of the Harvard Museum of Natural History’s “Earth Matters” series. Langmuir’s talk, “Earth and Human: A Planetary Perspective,” sought to look at humanity and human impacts on the Earth from the long viewpoint of the planet.

Langmuir developed his views as he explored the Earth’s ocean basins, where volcanoes along the mid-ocean ridges constantly renew the continental plates. It was while in research vessels’ close quarters with scientists from other disciplines who were exploring the living communities clustered around these vents that Langmuir came to appreciate how deeply intertwined living and geological systems are.

“Here we see an entirely different living environment from other life on Earth, based on the planet itself. It has completely changed our view of life on Earth. We see that volcanoes and life are related to each other; Earth and life are related,” Langmuir said.

Through the course of the talk, Langmuir brought audience members on a tour of Earth’s history.

The early Earth would have looked a lot like the moon, pockmarked and bombarded by meteors, with very active volcanoes, no protective ozone layer, high carbon dioxide, and very likely high temperatures. The early atmosphere, he said, had no oxygen at all, so early bacteria developed ways to live without it. When those bacteria figured out how to harness the sun’s energy through photosynthesis — which Langmuir termed Earth’s first energy revolution — oxygen was a byproduct. Released in tiny amounts by untold numbers of bacteria over billions of years, oxygen levels gradually built up in the atmosphere. That not only changed the atmosphere, Langmuir said, it also changed the rocks, as oxygen is very reactive and interacts readily with iron and other elements in the rocks.

To those early anaerobic bacteria, oxygen wasn’t a boon; it was a poison. Because oxygen is so reactive, it was toxic to those microscopic creatures. That is why, Langmuir said, chemicals such as antioxidants developed, to protect against oxygen’s damaging effects. As oxygen levels rose, however, some creatures learned to use it in a way that provided an enormous boost to their metabolism compared with that of oxygen-free processes. Langmuir described the incorporation of oxygen into metabolism as the planet’s second energy revolution.

More complex single-celled creatures, the eukaryotes, joined bacteria in the Earth’s microscopic menagerie, eventually teaming up to form multi-celled creatures. Eventually, enough oxygen built up that ozone was formed high in the atmosphere, shielding the land from harmful radiation, and allowing life to emerge from the seas.

From there, large animals emerged, then mammals, and finally, humans.

All the while, Langmuir said, physical systems supported and were influenced by living systems, with chemicals cycling into and out of the Earth to maintain the chemical balance of the seas and the air. The end result, Langmuir said, is that we humans are creatures that are very much a part of the Earth and will remain dependent on it for our future.

“We are … an agent for planetary evolution or an agent for planetary destruction,” Langmuir said. “Do we relate to the current environmental problems as if we are users of the Earth, or do we recognize that we are the byproduct of 4.5 billion years of planetary evolution? What we do may determine whether the planet is able to move into its next phase of development.”