Life came to the land as an extension of the cooperative community in the ocean. This community may have lived in the water between grains of decomposed rock, as the first soil; or it may have lived as the hardy symbiosis between fungus, algae and bacteria that is called a lichen. These inconspicuous forms of life are now found living on bare rocks in almost all environments. They greatly enhance the weathering of rock. Together with the bacteria that live within the rocks of the crust it may be more correct to say that mountains are digested rather than eroded. Lichens chemically degrade the rocks they live on, which supplies them with chemical nutrients. Over long periods of time, this degrading mediated a large scale process whereby calcium silicate rocks take CO2 from the air as they are degraded, turn this into carbonate, which is swept into the ocean to be deposited by algae as limestone. Seen in balance with the outgassing of CO2 by volcanoes, this could be the major negative feedback cycle that has controlled the temperature of the surface of our planet for the last billion years. The cycle runs more or less like this: the more CO2, then the more lichens , but more lichens use and therefore decrease the amount of CO2.
Whether the first residents of the land lived as communities in the soil or as communal lichens living on bare rock, the cooperative microbial community from the ocean was waiting on the land to incorporate larger, more complex forms of life when they emerged from the ocean.