- Photosynthesis created and maintains the current composition of the athmosphere. By removing too much CO2, it likely also contributed to several ice ages.
- Early life sequestered almost all the iron in the primordial Earth's sea into sediments, which turned into iron ores.
- We take the presence of such a basic thing as soil for almost granted, yet soil is created and maintained by life itself. At the same time, soil is only absent in environments where only extremophiles can survive. From this follows that it must have taken a lot of time for life to conquer the land and make it hospitable by creating a soil covering.
From these things, I (perhaps hastily) conclude that planets with life could actually look a lot like Earth. Or they can at least be recognized by the presence of chemically and physically unlikely features such as reactive elements like oxygen in their athmospheres.
The point is that plants are capable of terraforming the planet on their own. It isn't even a slow process if humans help the plants do their job.
"Whatever the advantages are, they do not apply in all habitats because some groups have also lost carcinization; one modern example being the frog crab."