Life is thought to have evolved anaerobic metabolism first because at the time:

1. Anaerobic pathways do not use oxygen at any step. 2. A pathway evolves to fit the conditions available when it appears. 3. Living organisms came into existence about 3.8 billion years ago, when the atmosphere lacked oxygen. 4. Organisms probably evolved anaerobic metabolism to survive those conditions. 5. Glycolysis, which does not use oxygen, is used by nearly all organisms on earth today. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 6 'Metabolism', sections 6.1-6.5_