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Life Makes a Mark


Consider the forces that have shaped planet Earth over time.

One tends to picture the grand geophysical events: earthquakes and volcanoes, erosion by wind and water, the drift of continental plates, the warming and cooling of the global climate. But there is another crucial force, microscopic in size yet global in its impact: the microbe.

Single-celled microscopic organisms—microbes—are the oldest and most abundant form of life on Earth. The term "microbes" spans a bewildering range of life-forms, from plants to animals to the ambiguously classified fungi. And microbes occupy an astonishing range of habitats, from the familiar (your shower curtain) to the most forbidding (inside volcanoes on the seafloor). In 1683, Antoni van Leeuwenhoek, the first scientist to view living bacteria through a microscope, exclaimed: "There are more animals living in the scum on the teeth in a man'­s mouth than there are men in a whole kingdom."

Since their first emergence on Earth perhaps more than 3.8 billion years ago, microbes have dramatically altered the chemistry of the atmosphere and, with it, the planet'­s surface. Among the countless kinds of microbes that have evolved, none have quite equaled the accomplishments of the first cyanobacteria: photosynthetic organisms that, like plants, drew on the Sun'­s energy to create oxygen, and in doing so helped create an oxygen-rich atmosphere. Earth today is habitable to multicellular creatures like us largely because cyanobacteria made it so. "One cannot separate the study of Earth'­s early atmosphere from the study of the evolution of life on this planet," says Jay Kaufman, a geoscientist at the University of Maryland. "They are intimately linked."

Wherever they may live, microbes thrive on basic chemistry. Think of them as molecular scrap-metal workers: They take apart commonplace chemical compounds and reassemble the individual parts, or ions, into altogether different molecules. For example, phytoplankton, microscopic plants that flourish near the ocean surface, convert carbon dioxide (CO2) and water (H2O) into carbohydrates and oxygen (O2). Though small in size, these microbes are so abundant that they generate half the oxygen we breathe.

What does a microbe earn for its labors? An infusion of energy, gained through the handling of miniscule, negatively charged particles called electrons. Every atom is surrounded by electrons, which help bind atoms together into molecules. As molecules are broken down and reformed, their electrons are exchanged and redistributed. A microbe, in the course of reshuffling ions and molecules, siphons off an electron or two for itself, to be used later in still other chemical reactions in its pursuit of food and energy. The molecules it generates, meanwhile, can become fodder for all sorts of other microbes. Leeuwenhoek was right: dental plaque is in fact an assembly line involving several species of bacteria, each playing a different role in the conversion of sugars and carbohydrates into cavity-causing acids. Your teeth are the platform for an entire atomic economy that runs on a currency of electrons.

Through eons of evolution, microbes have adopted impressive strategies to exploit and metabolize the many kinds of molecules that exist on Earth. The bacterium Pyrococcus furiosus thrives in the hot water that boils from undersea volcanic vents. This heat-loving microbe doesn'­t breathe oxygen; in fact, oxygen is toxic to it. Instead it takes in sulfur and releases hydrogen sulfide, the same gas that makes rotten eggs stink. This hydrogen sulfide is part of a bizarre seafloor food chain that never sees sunlight and includes creatures like albino clams and tubeworms.


A scrap-metal business thrives, or doesn'­t, depending on the availability of certain prized metal parts. So too with microbes. Oxygen-breathing organisms—microbes as well as larger, multicellular creatures like us—abound today only because there'­s plenty of atmospheric oxygen to breathe. Before about 2.4 billion years ago, when there was no atmospheric oxygen, different organisms, all of them microbial, dominated Earth. The sulfur-breathing bacterium P. furiosus is a descendant of the oldest branch of life, the Archaea, which some scientists believe may date back to that early oxygen-less era. Today, many Archaean microbes are relegated to murky, oxygen-free corners of the planet, including seafloor volcanoes and the intestines of cows.

Whatever a microbe produces—oxygen, methane, hydrogen sulfide—the task does require some effort, and the microbe must derive the initial energy to perform it from somewhere. Many scientists think that the earliest microbes derived their energy indirectly from Earth'­s internal heat, much as P. furiosus does today. Photosynthesis, the ability to convert the Sun'­s energy into microbial labor, developed somewhat later, perhaps as early as 3.5 billion years ago.

Photosynthesis was a major evolutionary invention, as it freed organisms from the ocean depths and enabled them to thrive just below the sea surface. But the greatest innovation was yet to come. Photosynthetic microbes, though able to utilize solar energy, were still restricted to the shallows; ultraviolet radiation from the Sun was so strong that nothing could live exposed on land. Then, around 2.7 billion years ago, a class of organisms called cyanobacteria appeared. Unlike their other photosynthetic cousins, these photosynthetic microbes produced oxygen. (To learn about the fossil evidence for cyanobacteria, watch the accompanying video "Early Fossil Life.")

The appearance of cyanobacteria signaled the beginning of a global transformation. Free oxygen began to accumulate in the atmosphere, forming two gases new to Earth. One was molecular oxygen (O2), well known to those of us who breathe it. The other was ozone (O3), a gas that forms high in Earth'­s atmosphere and, acting as a sort of global sunscreen, shields Earth'­s surface from the most harmful UV radiation. In the long run, the gradual rise of oxygen had two sensational effects: It permitted life to evolve on dry land, and it permitted the evolution of organisms that could thrive on oxygen. You can breathe easily, thanks to those early cyanobacteria.

"Imagine an early Earth that had no global sunscreen, no oxygen, hence no ozone," says Kaufman. "The production of oxygen through photosynthesis created that sunscreen. So biology actually made the surface environments habitable for future life by producing the oxygen we breathe today."

The role of microbes didn'­t end 2.4 billion years ago. Microscopic organisms are equally prevalent today, although they tend not to draw the same media attention that flashier, multicellular creatures do. And they'­re still churning out free oxygen, replenishing atmospheric O2 as quickly as other animals and chemical processes use it up. Like an earthquake or volcano, the lowly microbe is a planetary force to be reckoned with and respected.

"The atmosphere we have today is strongly influenced by biological activity," says Kaufman'­s colleague James Farquhar, a geochemist at the University of Maryland. "It'­s influenced by the types of gases that bacteria and other organisms produce. Life is critical in determining atmospheric composition, just as atmospheric composition is critical in controlling the conditions that are required to allow life to exist as it does."