Gaian Type planet. This is often the next stage in development from an EoArean to a full Gaian world. The oceans are typically large and are prolific with simple microbial life. This life has released great amounts of carbon dioxide and methane into the atmosphere, swaddling the planet in a covering of brownish-red clouds. Geologic activity remains quite high, and impacts are still common in the young system. Land areas are barren, and highly eroded. Tectonics may be quite active as well.
Terrestrial worlds of average mass and ongoing geological activity form thick primal atmospheres while between the ages of 100 and 800 million years. During this period the earliest oceans may form, and indeed some of the earliest forms of life might evolve, albeit very primitive microbial-monocellular in nature. The worlds themselves are wrapped within a dense atmosphere of nitrogen, methane, carbon dioxide, and even varying amounts of primal hydrogen and helium. In many cases, this atmosphere is reduced through the slowdown of the geological cycle, although some worlds may lose much of their atmospheric mass because of tremendous impacts with moon-forming planetoids. If the world is massive enough and the atmosphere is not lost, the planets may well evolve into Cytherean Types as they mature. Example: Arcadia before Terraforming.