第20期：巨兽世界

Except for one big difference. Like the Meganeura, they're monsters.

It's a world full of giants, where millipedes are 6 feet long, and scorpions are the size of wolves. This is because the oxygen level is much higher than it is today,

which allows their respiratory systems to be more efficient, fueling their bodies to grow larger.

So far, animals have been laying their eggs in the water. But a lizard-like creature, called the hylonomus, prefers land.

Its eggs contain all the water and nutrients that the developing fetus needs.

The babies grow in their own self-contained pond. The egg is a major evolutionary breakthrough.

Allowing animals to leave the water behind, and conquer land. This baby hylonomus leads the advance, as a new creature, the reptile.

Inevitably, with life, comes death. Dead plant matter builds up and decays into dense soggy layers.

Over hundreds of millions of years, rocks cover these layers. And heat from the Earth's core and pressure from the overlying rocks transform these layers into seams of coal.

Each lump of coal burned today is made of plants that died 300 million years ago.