This chapter examines James Lovelock's assertion that the Earth's atmosphere is a biological construct that is distinctly different from any expected abiotic chemical equilibrium. This claim can be broadened to the wider claim that Earth's environment bears the definite and considerable imprint of biological processes and is distinctly different from the environment that would be present if Earth did not possess life. Based on the evidence covered in this chapter, from atmosphere and oceans, Lovelock was clearly correct to claim that the coexistence of oxygen and methane in Earth's atmosphere is evidence of life, and that life can alter the planet. Other examples that could also be used as evidence to prove this point include the effects of vegetation in creating and stabilizing soils, the effects of plant transpiration on the cycling of water, and the fossil evidence for the effects of diatoms on silicon concentrations.