How old are microbial mats?

The Ancient Origins of Life on Earth

Deep in the geological record, clues to Earth’s earliest life forms lie preserved in ancient rock structures known as stromatolites. These dome-shaped formations, found in places like Western Australia, were built layer by layer over immense spans of time in shallow waters. Scientists from Caltech and NASA’s Jet Propulsion Laboratory (JPL) have determined that some of these stromatolites are about 3.4 billion years old—and they hold a remarkable secret: they were built by microbial mats.

Microbial mats are dense communities of microorganisms, including photosynthetic bacteria, that grow on moist surfaces. They trap sediment and, over time, cement it into thin layers. As new layers form, the structure grows upward, creating the distinctive banded patterns seen in stromatolites. These ancient mats represent some of the earliest evidence of life on our planet.

What makes this discovery so profound is not just the staggering age, but what it tells us about the tenacity of life. At a time when Earth was still a volatile, inhospitable world—bathed in volcanic activity and bombarded by asteroids—microbial life was quietly thriving in tidal pools and shallow seas. Their fossilized remains offer a rare window into the dawn of biology.

“These structures are more than rocks—they’re archives,” says one researcher involved in the study. “Every layer is a chapter in the story of how life took hold.”

Today, living stromatolites still exist in a few isolated locations, such as Shark Bay in Australia. But the ancient ones, like those studied by Caltech and JPL scientists, are silent witnesses to a planet just beginning to stir with life. In their delicate layers, we find not just history, but a testament to life’s quiet persistence in the face of time.