Q. Have any calculations been done on on the possible atmospheric make-up of Earth with 1/3 its current gravity?
A. Determining the atmosphere of the Earth 300 million years ago is difficult but I did attempt to do this in Chapter 7 of my book. I examined various lines of evidence that indicated the atmosphere was probably about a tenth to a third of its current atmospheric pressure, due to the reduced gravity, 300 million years ago.
Single celled life had evolved by 3000 million years ago but it took about another 2000 million years before multi-cellular life evolved. This long time scale makes sense if the atmosphere was very tenuous at this time and could only start to support multi-cellular life from about 600 million years ago. Multi-cellular life had evolved in the sea by 500 million years ago but then took another 100 million years to move onto the land. Again this long timescale begins to make sense if the atmosphere was slowly becoming more dense over time as gravity increased.