Five different times during the Earth's history, the planet has entered a prolonged period of reduced temperatures.

When this happens, massive ice sheets form and sea levels drop.

While some of these events occurred billions of years ago, not all of them were in the distant past.

In fact, the last such event had a profound impact on the development of humans as a species.

Learn more about Ice Ages, how they affect the planet, and how they affect humanity,

on this episode of Ice Ages with what an ice age is.

An ice age is nothing more than a prolonged period of time of reduced temperatures

such that large continental size ice sheets and glaciers will form.

These periods do not last decades, centuries, or even millennia.

Ice ages are periods that are millions of years long.

Much of the confusion comes about from the colloquial use of the term the ice age, which is usually

in reference to the most recent glaciation, which I'm going to get to in a bit.

According to geologists, there have been at least five ice ages in the history of the Earth.

The earliest would be the Pongolan glaciation, which occurred about 2.9 to 2.7 billion

years ago. This is often not included on the list of Ice Ages because it occurred so long ago, and

the evidence for it is the shakiest.

This is why I said there were at least five Ice Ages.

What is usually considered to be the first ice age was the

Huronian glaciation which was around 2.4 to 2.1 billion years ago. The next

was the Cryogee and Ice Age. This occurred about 850 to 635 million years ago.

This was still before the Cambrian explosion which saw the rapid rise of multicellular life.

One or both of these early ice ages may have been a period known as

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