What events became the boundaries between periods of world history?

The emergence of writing became the reason for the beginning of the history of the Ancient World, that is, 3500 years BC;

The beginning of iron processing led to the beginning of the Iron Age, which happened in 1200 BC.

The fall of the Western Roman Empire caused the end of the history of the Ancient World, and so the Middle Ages began.

The great geographical discoveries caused the beginning of modern times, and the Middle Ages came to an end.

The First World War and other transformations caused the beginning of modern times.