Problem with stromatalites is how much organic material actually remains. Mostly textural, trace fossil evidence.

Even so, these look pretty convincing. Pretty clear bedding planes. I don’t see any tectonic layering.

Eventually, there would be thousands of human colony worlds, communicating and trading, competing and fighting. Earth might even be forgotten, or abandoned as a polluted environmental hellhole. Or left as a natural park, to remind future generations of where we came from, what our ideal world looks like.

Actually, some mathematical models of human expansion into the Galaxy have been formulated, and even at sub-luminal speeds, the Milky Way could host hundreds, perhaps thousands, of worlds in only a few million years. After all, it only took us about a hundred thousand years to random-walk our way across the planet. Think of the multiplicity of cultures, and perhaps even alternative biological evolutions. All the diversity we could wish for.

More than likely, we wouldn’t be looking for earth-like worlds, but stable suns surrounded by asteroids and satellites were we could duplicate some of our own Solar System orbital habitats. We’ll probably have much better-developed techniques for living in zero-g vacuum than for adapting to alien worlds and biologies at the bottom of deep gravity wells.

Then again, maybe we will find excuses not to go. I can just imagine some future politician telling some future legislature,

“Why bother going to the stars, we have plenty of problems right here in the Solar System that need to be addressed. And there’s plenty of undeveloped real estate right here near Sol for us to move into.”

The analogy isn’t colonialism. The analogy is humanity moving out from Africa.

The Isua Greenstone Belt (IGB) is dated at 3.7 Ga, so there are several hundred million years in that “immediately.”

So, what the hell is a Greenstone Belt, anyway? It is a regionally extensive series of rocks that have been subjected to moderate pressure and temperature metamorphism, known as “greenschist.” Here’s a graph:

https://upload.wikimedia.org/wikipedia/commons/f/fe/Metamorphic_Facies.jpg

I’ve done a fair amount of work in the greenschist rocks of the Southwest United States and while they are a full billion years younger than those in Greenland, the metamorphism is similar. Here’s the thing about interpreting metamorphic rocks, even those of the greenshist facies, you have to assume they are not what they used to be. And greenstones are weird.

Within a greenstone belt, the deformation of the rock can vary greatly, even if they were subjected to the same pressure and temperature conditions. This makes interpreting original textures in the deformed rock challenging. Furthermore, the tight folding of rocks in greenstone belts results in some rather interesting structures. We had a saying in the field: “the rocks are so F’d up they look normal.” Look, I could go on about psuedo-structures in transposed isoclinally folded lithologies, both micro and marco, for hours – days, really – but I’ll spare you. Basically, we need to be very careful of classifying original sedimentary structures in greenschist facies metamorphic rocks.

That being said, they could be stromatalites.

The bad news is that we’re talking microbial, single-celled organisms. Complex multicellular organisms didn’t appear in the fossil record until the so-called Cambrian Explosion, about half a billion years ago. They very quickly dominated the earth, but it seems that the step from single to multicellular life didn’t occur until three billion years after life first appeared, maybe longer. It appears to be a real bottleneck in evolution, much more so that the appearance of microbial life.

This suggests life is very common in the galaxy, but that complex life
is much rarer, and much more vulnerable to astronomical catastrophes and planetary disasters that hit all planetary systems periodically.

It took half a billion years of critter evolution and relative stability on earth before one species arose capable of building spacecraft and radio telescopes. Unless intelligent races tend to survive a long, long time, this tells me that its very unlikely there is more than one in the galaxy at any one time.

I don’t know about you, but I find that terribly depressing. If you could talk me out of this conviction, I would be eternally grateful.