In what way is membrane fluid?

Cell membranes are comprised of phospholipids which are loosely attracted to each other by van der wahl forces and hydrogen bonding. That means they are able to individually respond to the environment and water by their hydrophillic and hydrophobic properties. This individual response is why cell membranes are 'fluid'.

we do indeed have an oxygen-rich atmosphere!

the organism known as cyanobacteria uses sunlight,water, and CO2 to produce carbohydrates & oxygen!