Implications of a Single-Biome Colony

How long has this been going on again?

We're used to living in a metal-poor world, but that's because humans have been here a long time, using all the easily accessible metal. I gather that in the distant past, before civilization, there was actually a fair amount of metal (primarily copper) just lying around on the Earth's surface. Like, not 'forest of giant shiny brown crystals' common, but at least seams of it poking through the bedrock here and there. After people figured out how to melt it down in hot fires and shape it (around 9000 years ago), almost all of it was quickly found and converted into artificial objects. On your planet, metal present in this way would have been undisturbed until the arrival of the first humans.

However, the prevalence of metal is really going to depend a lot on the history of the planet and the star system it resides in.

That's what I'm trying to work out: how long it would reasonably take to oxygenate a planet and get a self-sustaining biome (sufficient to start supporting larger animals) running, using basically just the plants and bacteria from one small subset of the world - that which is in Britain.

I'm finding that the models we have for the build-up of oxygen in Earth's atmosphere are so complex that calculating how many plants and other organisms you need over what period of time is pretty much impossible. You're probably going to have to hand-wave it.

There are other ways for a planet to get an oxygenated atmosphere. There are several moons around Jupiter and Saturn that have substantial levels of oxygen, apparently as the result of water ice interacting with the Gas Giants magnetic fields. But I was thinking that the simplist way was "leadage" from Earth- if substantial numbers of creatures and humans make the transfer, why not air? It's still a hand-wave, but if you jiggle the numbers a bit, you should be able to make it look plausible.

Cheers for that.