The first glaciers formed around 2.9 billion years ago, during the Paleoproterozoic era. This is based on evidence of glacial deposits found in rocks of that age in various parts of the world.

However, it's important to note:

* Earlier glaciers may have existed: There's the possibility of even earlier glaciers, but the evidence is less clear or has been eroded away.

* Glaciation was cyclical: Glaciers have formed and retreated in cycles throughout Earth's history, with periods of intense glaciation (ice ages) and warmer periods.

So while 2.9 billion years ago is the oldest well-established evidence, glaciers likely existed even earlier in Earth's history.