In a new paper published in the International Journal of Astrobiology, Vincenzo Rizzo from the National Research Center in Cosenza, Italy, asks a provocative question: Why are many scientists reluctant to accept the use of geological methods to identify biological processes on Mars, when those methods are commonly used on Earth?

He points to a case in Germany from 1908, when a scientist by the name of Ernst Kalkowsky proposed that layered mounds, columns, and sheet-like sedimentary rocks called stromatolites were of biological nature. His contemporaries did not believe him. But Kalkowsky was later proven correct, when it was recognized that stromatolites formed because biofilms—composed of cyanobacteria and other microorganisms—trapped sediments. Stromatolites are now known to be the oldest evidence for life on Earth, stretching back at least 3.5 billion years, and they still exist in some remote regions, such as Shark Bay, Australia.

In his paper Rizzo follows in Kalkowsky’s footsteps by analyzing images from the Spirit, Opportunity, and Curiosity rovers on Mars that indicate the presence of biotic macrostructures such as stromatolites. He suggests that if no non-biological explanation can be found, the images should be considered as possible candidates for Martian stromatolites. Rizzo shows many examples of structures that have an amazing resemblance to stromatolites on Earth.

In principle I’m very skeptical of any evidence based only on visual resemblance or morphology, because the human brain has the tendency to see or fill in familiar patterns, even where they don’t quite exist. Rizzo goes well beyond appearances with his analysis, however. He made me realize that if—and it’s a big if—stromatolites did in fact exist on early Mars, they would probably look like the specimens he found in the rover images.

Carl Sagan’s saying, “Extraordinary claims require extraordinary evidence,” comes to mind. For every structure produced by biological processes, there might be some geochemical or physical process that mimics it. Some of these processes may be unknown to us on an alien planet such as Mars. On the other hand, we know more about Mars than we did in the past. We know there were lakes and possibly oceans on the planet early in its history, including at Gale Crater where the Curiosity images are from. Organic compounds have also been found at that location, and we know that rocks can travel from Earth to Mars (and vice versa), and that microbes are able to survive the trip. What, then, is the more “extraordinary” claim? That Mars during its early habitable period had life forms similar to those on Earth, which produced similar biogenic structures? Or that Mars is and always has been a dead planet?

Still, the bar for claiming to have detected life on Mars, past or present, has to be set high. Even on Earth it is sometimes difficult to decide whether a particular structure is the result of biology. Even though our ability to explore Mars has improved, we’re still not able to do what we routinely do on Earth—go out with a hand lens or other tool and study some enigmatic feature.

Future isotopic analyses of the stromatolite-like structures identified by Rizzo may strengthen or weaken the case for a biological origin (life prefers lighter isotopes, which we can use as a test). But I suspect that his hypothesis will have a similar fate as Ernst Kalkowsky’s. Only much later—likely when humans are exploring Mars—will we get a final verdict, which I hope will prove him correct.