Final answer:
Homo erectus is the most well-documented first hominin to leave Africa, with migrations occurring between 1.3 and 1.6 million years ago.
Step-by-step explanation:
The first hominin to leave Africa were likely members of the species Homo erectus. This migration occurred during the Pleistocene Ice Age, which suggests that H. erectus could have left Africa between 1.3 and 1.6 million years ago. Although there is debate about earlier migrations that might include Homo ergaster or even an as-yet-unknown species of the genus Homo around 2 million years ago, H. erectus is the most well-documented early hominin found outside of Africa to date.
Evidence supporting the presence of H. erectus out of Africa includes archaeological findings on the island of Java, now part of Indonesia, dating them to this period. It's posited that they traveled there by land routes facilitated by lower sea levels during the Ice Age. In terms of modern humans (Homo sapiens), they began migrating out of Africa much later, around 100,000 years ago.