The chapter does not give one reason, so the honest answer sets out the reasons that follow from the evidence it does give, and says which are firmer than others. Start from the shape of the movement:
| Wave | Who | When | Evidence for it |
|---|---|---|---|
| First | Homo erectus — 'the first hominin to exit Africa' | Began moving out around 2 million years ago; dispersal between 2 and 0.5 million years ago | Its handaxes and cleavers have been found across Asia and Europe — 'indicating their origin in Africa and their dispersion beyond the continent' |
| Second | Early Homo sapiens — ourselves | Around 125,000 years ago (having evolved in Africa about 300,000 years ago) | Our species is 'now spread all over the earth'; reached Australia and the Americas between 50000 and 12000 years ago |
Reason 1 — hunter–gatherers move because their food moves. This is the strongest reason and it needs no extra assumption. A hunting and gathering economy has no stores and no fields. When a herd shifts its grazing or a fruiting season ends, people follow. Over hundreds of thousands of years, a band that shifts camp a few kilometres each generation covers continents without ever intending to 'migrate'. The chapter's own dates make this the natural reading: 1.5 million years is a very long time to cross from East Africa to East Asia — this is drift, not a journey.
Reason 2 — climate opened and closed the doors. Fig. 4.4 labels the deserts of the Sahara and Arabia as 'both wetter during interglacials'. That is the chapter telling you the route was not always desert. When the climate turned wet, grassland and water crossed the barrier, and animals and people could cross with them. The chapter also links the whole story of cultural evolution to 'changes in climate and the environment' through the Quaternary Period.
Reason 3 — population pressure. A growing band in a fixed territory eventually splits, and the daughter group settles the next valley. The chapter shows this force clearly at work later, when warming after 12000 years ago produced 'the first-ever population explosion in human history'.
Reason 4 — they had the technology to survive elsewhere. This one is important because it explains who could leave. Homo erectus did not leave empty-handed: it left as the maker of handaxes and cleavers. Tools that could butcher a carcass and process plants anywhere are what made new environments survivable. The chapter names this species as the tool-maker and as the first to leave, in the same sentence — the two facts belong together.