Creatures are the out-come of food. Food is produced by rain. The rain is produced by Sacrifice. The Sacrifice is produced by action.
अन्नाद् भवन्ति भूतानि पर्जन्यादन्नसम्भवः। यज्ञाद् भवति पर्जन्यो यज्ञः कर्मसमुद्भवः॥