Szostak, well-known in biology for his work on chromosomal recombination, notes that his interest in evolution has caused him to establish strong collaborations with chemists: "I have people in my lab who are doing synthetic chemistry, and because we have to make molecules to build these systems, we collaborate with a number of other chemists, too." This connection between disciplines is being further strengthened by the creation of a new center at Harvard to study the origins of life. "Both in chemistry and biology," he says, "the origin of life is a fundamental issue."

For more than a billion years, single-celled organisms were Earth's only inhabitants. But these cells were evolving and diversifying, and in so doing they began to change their environment. As California Institute of Technology geomicrobiologist and HHMI investigator Dianne K. Newman puts it, "Life and Earth have coevolved."

The earliest organisms lived in a forbidding world. Earth's atmosphere contained virtually no oxygen and would have killed many of the organisms that live on the planet today. Instead, scientists believe, the atmosphere contained substances such as nitrogen, carbon dioxide, and water vapor. As a result, our ancient predecessors had to rely on these atmospheric components, not the oxygen that now sustains us.

Newman studies modern-day organisms that essentially breathe metals—they transfer electrons from one metal ion to another to produce metabolic energy. The distant ancestors of these metal-breathing bacteria ruled Earth early in its history, but bacteria evolved that released oxygen into the atmosphere. For many millions of years this oxygen was sequestered in rocks, producing the ore deposits now known as banded iron formations. Eventually, oxygen began to build up in the atmosphere, triggering an environmental and biological "crisis" by changing the composition of rainwater, streams, and oceans.

Newman's work on metal-breathing bacteria has led her to consider the broader question of how bacteria have changed Earth's environment over evolutionary time. For instance, she and a group of colleagues recently proposed that a particular kind of bacterium played a key role in deposition of the banded iron formations. "I'm not an evolutionary biologist," says Newman, who studied German as an undergraduate at Stanford before receiving a Ph.D. in civil and environmental engineering from the Massachusetts Institute of Technology. "What I hope to contribute is an understanding of the mechanisms whereby these putatively ancient bacteria do what they do, and then make connections back to the rock record."

Like Szostak, she stresses the importance of interdisciplinary collaboration. "It's imperative for someone like me to have good colleagues who are experts at looking at ancient rocks," she says. "We're also beginning to design experiments with bacteria in the lab to help geologists interpret certain structures they see." To that end, she recently visited South Africa with a team of geologists and biologists to investigate the banded iron formations there.

Newman's work has many practical applications. For example, because some metal-breathing bacteria can convert toxic metals into less toxic compounds, these descendants of Earth's first occupants may someday be hard at work cleaning up pollution produced by humans.

About 2.4 billion years ago, the atmosphere began to harbor appreciable amounts of oxygen. A few hundred million years later, according to the fossil record, new kinds of cells appeared. They were larger and more complex, possibly because they had evolved ways of using oxygen to support metabolic processes. Shortly thereafter, organisms appeared that were large enough to be seen without a microscope (had one existed)—algae consisting of cells in spiral chains.

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