Why is it necessary to explain early Earth's instability through both external and internal causes?

Correct answer: Both impacts by external bodies and volcanic activity driven by Earth's internal heat affected it.

Early Earth was affected by external processes such as frequent impacts from meteorites and planetesimals. At the same time, residual heat from formation and heat produced by radioactive decay promoted melting, volcanism and the release of gases from inside the Earth. Hence, its instability requires both types of causes. Option B is incorrect because the ozone layer and life are associated with much later stages. Exam tip: mention impacts as external causes and internal heat, volcanism and differentiation as internal causes.