We are always so tied up with the Great Oxygen Event that we forget what happened before. We are biased by our need for o2. Well, the early earth was really put together by big blobs of space junk. When it got big enough, it melted and all the heavy stuff went to the bottom, and all the light stuff into the early atmosphere. We really have no clue what it was, but we can assume it was thick with methane and co2, somewhat like Venus.
The Galactic Seeders were very smart, and they put in some RNA, DNA, and maybe some bacteria that could live in this muck. Or not. Then they went away to retire in some beach-galaxy. But we forget that these bacteria were happily munching away sulphur and methane, even though the air could have been as thick as molasses. That's a heck of a lot of carbon, and more carbon was bombing from the sky. But these guys were doing their work for a billion years before their 'flashy' cousins came along.
From the methane, they liberated the hydrogen, and tied up the carbon in endless types of carbonates, whatever they do. Who cleaned up their shit? Ah, the lowly janitor - Mr. Plate Tectonics. Without this process, all those sulphur-carbon compounds would have just poofed up again, when the oxygen came. Think of a burning Earth.
But Plate Tectonics shovelled it all away. But how much of it? We have no clue. I've seen some estimates, but I think it is many zeroes above that. Where is it? It's packed in big layers under the continents, and holds us up out of the water.
I used to think that continents kept my feet dry merely because they were composed of silicates, which are less dense than the black basalt of the ocean. If you are ever on a big lake or the ocean, pick up a black rock and a piece of quartz. See? But now I know that this only half of that is true, and if we didn't have excess heat, then we'd all be under water.
It's sad to have one of my cherished 'beliefs' shattered by new science, but I just can't accept that the heat is there because it is. Why is it there? Doesn't anybody ask these questions?