MesoGaian worlds are in the second stage of the life, after EoGaian, being between 500 million and 2 billion years old. With stable weather systems, erosion and tectonics beginning to kick in a cycle that removes carbon dioxide from the atmosphere. These worlds are not habitable, but may be a source of raw material for food if nothing better is available. Life, if it exists, will most probably consist of single celled bacteria.
Typical Habitability: III (II)
The defining characteristic of an Early Stage Meso Gaian world is that it has an oxygen atmosphere (normally either Low Oxygen or High Carbon Dioxide, so not yet ideal for human habitation) and an Aerobic life level.
All life is still in the oceans. Temperatures will have dropped to high but not immediately lethal, and atmospheric pressure is often below twice standard.
If the world is too small (possibly less than 4,000km in radius), then atmospheric pressure and temperature will continue to drop, until the planet becomes unable to support extensive oceans. At this point it becomes an EoArean world.
Ecology will initially consist of simple organisms such as bacteria and algae, but as life evolves to take advantage of the increasing levels of oxygen, more complex multi-cellular life will arise and begin to dominate.
It is possible that very early MesoGaian worlds suffer a crash in the availability of life, as the rising oxygen levels poison existing life forms. It is possible for Necro Gaian to form, where all life has been wiped out. The oxygen reacts with the rocks to leave a pure nitrogen atmosphere. Such worlds are completely dead, but also quite rare. Life often finds a way.
Typical Habitability: II (III)
If a MesoGaian world proceeds to the Complex Ocean level, then it is considered to be a Late Stage MesoGaian world.
Aquatic vertebrates are often the dominant life form, with the possibility of lichens or other simple life starting to conquer the land.