Examples of life-supporting functions of a living system:
Venus  Earth without life
(Calculated as an interpolation between Venus and Mars, including the chemical differences in the planets)
Mars Earth with life
Carbon dioxide
96,5%
 98%
  95%
 0,03%
Nitrogen gas 
3,5%
 1,9% 
2,7%
 79%
Oxygen gas 
traces 
0,0%
0,13%
  21%
Methane gas 
0,0
0,0
 0,0
 1,7 ppm
Temperature at surface
459°
240° - 340°
- 53°
16°
Pressure at surface (bar)
 90
60 
 0,0064
1,0
Calculations by James Lovelock

Adapted from: Lovelock, J., 1988. The ages of Gaia. Norton & Company, New York, London.