Great Filter-Doomers will find this alarming. If life evolves easily (and quickly - in thermodynamics that's the same thing) then it should be everywhere. And yet, where is everyone? The galaxy appears to be dead. There must be a Great Filter between the appearance of life, and technological civilization leaving its solar system of origin.
It's worth pointing out that until about 700 MA ago, Earth was a boring microbial planet. There was basic multicellular life but until the Ediacaran, nothing that you could actually see with the naked eye, or that had complex organization. In fact for over a billion years it was stuck in a simple oscillation of build up oxygen, die back, build up oxygen, die back, encoded in the banded iron formations (in different redox mixes) visible in extremely old rocks like the ones above in Australia. An alien visitor would have sampled the boring microbial soup and moved on. Visiting a hundred million years later, it would have observed exactly the same situation. Life on Earth was very much like a "blinker" in Conway's Game of Life, with little sign it would ever break out of it. Based on our N of 1, thermodynamically, the appearance of life is likely (it was almost immediately after the Earth cooled!) - and similarly, the appearance of multicellular life is UNlikely.
For that reason, if there is a filter, there's a big question is whether the Filter is behind humanity, or in front of it. The discovery of microbial life on Mars, especially exctinct microbial life, would be good news, in the same way that discovering an extinct civilization would be bad news. A thought experiment may help illustrate.
Imagine you send out a fleet of near lightspeed von Neumann probes. As they cover the galaxy, the reports come back to Earth: thousand then millions of planets with oceans of bacteria and/or blue-green algae, some living, some extinct embedded in clay like the ones on Mars. But absolutely nothing multicellular, anywhere, besides our freakish Earth. Everywhere, simple one-dimensional ecosystems, some "blinking" forever like Earth almost did, but no dusty ruined cities, or eerily silent half-built Dyson spheres, or even alien cockroaches. Nothing beyond a Kardashev 0.001![1] Time to uncorck the champagne! The Great Filter is behind us! We're the first!
Now imagine the opposite case: ghost planet after ghost planet, civilizations that blossomed and then burnt out. Some of them had even sent out their own probes and learned their fate. We would be looking at our own future.
If we assume this Mars finding really is extinct microbes, we now have N=2 for the denominator of how frequently life evolves, and N=1 - where it never got past the microbe stage - a 50% rate of the Great Filter being behind us.[2] Assuming the principal of mediocrity, 50% of the aliens we're not seeing are microbes embedded in clay. 50% is hardly a guarantee of our eternal future among the stars but it moves the needle in the optimistic direction. You might think it would be boring to explore the Solar System and find only microbes on Mars, Venus, under the ice of Europa and Enceladus - but such discoveries should make you happy for humanity's future, especially if they're extinct.
FOOTNOTES
[1] Recently there has been an effort to establish a continuous Kardashev scale (rather than having only discrete classifiers for civilizations which surpassed certain benchmarks, "1" being the power output of a star); one paper assigns Earth in 2023 a 0.7276 (Zhang et al 2023.) For very low Kardashev numbers, we The concept of the
5.1x10^14 m^3 x 0.7 x 10^11 algal cells/m^3 x 10^-11 Watts/algal cell
divided by 3.84x10^26 Watts = 10^-12 Kardashev
-70 is the lower bound for SQ, to single-celled organisms, so we can say that -70 SQ converts to 10^-12 Kardashev. Humans have an SQ about +13. However human civilization cooperates to control more energy than a single human, so +13 does not correspond to 0.7276, but whatever the SQ of the human race as a whole, does. You can't get the Kardashev of a single human just by dividing 0.7276 by 8 billion because of the non-zero-sum effects of civilized cooperation.
[2] As written before, Venus had oceans until about a billion years ago. I would have liked to include it here as another microbial blinker planet that ran out of time before its own Ediacaran, with the evidence of both phosphine and microbe-sized UV absorbers in its upper atmospheres, as the remnant of its ecosystem. This would give us a denominator of 3, and even more confidence that the Great Filter is behind us. However, the famous phosphine paper failed multiple attempts at replication and two papers (Jiang et al 2024 and Egan et al 2025) have advanced good candidate abiotic explanations for what the absorbers could be.
