UPSC 2018: Consider the following statements: 1. The Earth's magnetic field has reversed every few hundred thousand years. 2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide. 3. When living organisms originated, they modified the early atmosphere of the Earth. Which of the statements given above is/are correct?

Palaeomagnetic studies show the geomagnetic field flips polarity irregularly, on an average every ~200–300 k years, so statement 1 is correct. Geological evidence indicates the primordial atmosphere was CO₂-rich and virtually devoid of free O₂ (<0.01 %), not 54 % oxygen, so statement 2 is false. Oxygenic photosynthesis by early cyanobacteria later enriched the atmosphere, making statement 3 correct. Thus only 1 and 3 hold. ([jagranjosh.com](https://www.jagranjosh.com/articles/upsc-civil-services-ias-prelims-csat-exam-2018-question-paper-and-answer-key-1527851395-1?utm_source=openai))