When photosynthesis first evolved, the extra oxygen in the atmosphere killed off nearly everything on the surface (by covering it with ice). It took a very long

When photosynthesis first evolved, the extra oxygen in the atmosphere killed off nearly everything on the surface (by covering it with ice). It took a very long time indeed to subsequently evolve cellular respiration and bring things back into balance (see the "Great Oxygenation Event" for more). So what I'm saying is that we won't even have been the first species to have done so.