A system of lagoons has been discovered in Argentina hosting a rare range of microbial communities previously unknown to scientists. The microbial communities form giant mounds of rock as they grow — like corals building a reef millimeter by millimeter. And the University of Colorado points out that "the communities could also provide scientists with an unprecedented look at how life may have arisen on Mars, which resembled Earth billions of years ago."

"If life ever evolved on Mars to the level of fossils, it would have been like this," said geologist Brian Hynek, a professor in the department of geological sciences at the University of Colorado Boulder, who helped document the ecosystem. "Understanding these modern communities on Earth could inform us about what we should look for as we search for similar features in the Martian rocks."

more details from CNN: Stromatolites are layered rocks created by the growth of blue-green algae, or cyanobacteria, through photosynthesis. The structures are considered to be one of the oldest ecosystems on Earth, according to NASA, representing the earliest fossil evidence for life on our planet from at least 3.5 billion years ago. "These are certainly akin to some of the earliest macrofossils on our planet, and in really a rare type of environment on modern Earth," said Hynek...

While the stromatolites are in an environment containing oxygen, Hynek said he believes the layers farther down in the rock have little to no access to oxygen and are actively formed by microbes using anoxygenic photosynthesis. This would make the structures similar to the ones found on ancient Earth... "We've identified more than 600 ancient lakes on Mars; there may have even been an ocean. So, it was a lot more Earth-like early on," he said.