As a geobiologist, Tanja Bosak studies the co-evolution of geology and biology, or how microbes helped form both the rocks under our feet and the atmosphere that sustains life on Earth. “Geobiology is both an old and very modern science,” explains Bosak, associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “The discipline began in the 1700s when people dug up fossils and realized that previous life forms were different from our own.” Today, Bosak looks for microbial fossils, and combines fieldwork with the tools of molecular biology, genomic sequencing, imaging, and geochemistry to learn about the world the microbes lived in. Her work is filling in gaps in understanding how Earth’s environment changed from one antithetical to complex life forms to one supporting an explosion of such organisms 540 million years ago.

For the first three billion years of life on Earth, the landscape was devoid of trees, grasses, flowers, mosses, ferns, algae, and animals of any size. The atmosphere contained sulfur, methane, and carbon dioxide, but no oxygen. Water saturated with silica, calcium carbonate, and other minerals burped up noxious-smelling gases from the volcanic activity just under the surface. Yet shallow waters teemed with bacteria that grew in dense, multispecies colonies. These colonies produced slimy substances called biofilms that formed thick mats. “Some organisms didn’t want to be coated in minerals, so they secreted sticky slime to prevent that mineral nucleation,” Bosak says. “Then other organisms that degraded the slime came as the layer of active organisms moved up. Today we see these layers of biological interactions frozen in stone.”

To study these interactions, Bosak travels to places like Yellowstone National Park, where similar microbial mats in vivid greens, deep reds, and mustard yellows form in scalding, bubbling sulfurous pools and slowly evolve into rock. Her lab has also replicated the biochemistry of the Precambrian ocean of around 575 million years ago, when multicellular animals first emerged. To do so, the lab grows bacteria in custom-built wave tanks with water adjusted to the pH, temperature, and mineral content believed to have existed at that time.

The bacteria form mats with microscopic features similar to those seen in fossilized formations, and also in some of Yellowstone’s microbial mats. Using this system, Bosak explained how the physics of photosynthesis and competition for nutrients led to conical patterns in microbial mats that occurred as cyanobacteria began emitting oxygen into the atmosphere, creating conditions amenable to more complex life forms.

In a 2014 paper in Nature Geoscience, Bosak’s lab explained a long-standing puzzle about the millimeter-wide ripples characteristic of microbial mats. “We didn’t know how these stone ripples formed or why they disappeared,” Bosak says, “but we knew they appeared when thick microbial mats probably coated the ocean floor. We see a lot of them in Precambrian times before the appearance of more complex life forms.”

When Bosak’s lab grew bacterial mats in a tank, they observed ripples forming when fragments of the mat broke off and rolled along sand. She speculates it was tiny burrowing or grazing animals (not waves) that increased the fragmenting of the tough mats of ancient times. “You need some, but not too many, early animals to break off these fragments. They can’t form once you have larger animals that burrow deeper or that chomp on the mats. Finding these features tells you that certain activities were present and others were absent.”

The animals themselves were too softbodied to be fossilized. But like many features of Earth’s past three billion years, their lifestyle can be gleaned from microbial traces in the rocks.

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