In her office in Building 68, Dianne Newman keeps a polished, spherical rock striped with bands of iron. Found on every continent, such rocks are the most common source of iron ore. And to Newman, specimens like this 2.4-billion-year-old example yield insights that could help unravel a very knotty part of Earth’s history. When did ancient microbes start producing the oxygen we breathe, and what kind of microbes were they?
Newman didn’t plan to spend her career exploring such questions. She came to MIT in 1993 to get a master’s degree in engineering, thinking she’d work in the field for a few years before going to law school to become a patent attorney. But a class in environmental microbiology left her fascinated by the diversity of bacteria’s metabolisms–the chemical reactions they perform to live. “I learned that bacteria could eat toxic compounds and transform them into benign ones,” she recalls. A project that induced bacteria to convert arsenic into a semiconducting material got her interested in how bacteria might have shaped Earth’s chemical makeup, and she moved to the geosciences department, where she earned her PhD. After seven years at Caltech, she joined the MIT faculty in 2007 as a professor of biology and geobiology.
“Microbes are the best chemists on the planet,” marvels Newman. Good enough, in fact, to have reshaped its environment. When our solar system formed 4.5 billion years ago, Earth’s atmosphere was almost devoid of oxygen. The first single-celled life forms, which arose about 3.8 billion years ago, probably lived in the seas and had metabolisms that neither required oxygen nor produced it as a by-product. Some of them subsisted on iron; their metabolic processes changed the iron’s chemical state and created the deposits in Newman’s rock. Others probably fed on sulfur.
And then something happened that would make possible animal and plant life as we know it. Some bacteria began using sunlight to split water into hydrogen, which they used to make fuel, and oxygen, which they released as waste. Thanks to oxygenic photosynthesis, the atmosphere and the shallowest ocean water had significant levels of oxygen by about 2.4 billion years ago; by about 540 million years ago, oxygen levels were comparable to those seen today.
The question of which organism first began producing oxygen, and when, is one of the great mysteries in Earth’s history. “It’s a really hard problem but really seductive,” says Newman.
To answer it, Newman and others at MIT and around the world focus on rocks like the one in her office. Just like dinosaur bones, the remains of bacteria living in the ancient seas were incorporated into rock over millions (in the bacteria’s case, billions) of years. Researchers know that certain compounds are made only through processes carried out in living organisms, so when they see such compounds in a rock, it means that the rock reflects traces of ancient life. Geobiologists interpret these bacterial fossils by comparing the chemical compounds in them with those created by modern bacteria that still rely on ancient metabolic processes. Through such analysis, they hope to pin down which microbes made the chemical compounds left in the rocks. “You have to look at the function of these chemicals in lots of living organisms,” says Newman. “This kind of logic links us to the past.”