NOVA

(gentle music) - [Narrator] Plants and fungi created one of Earth's first complex terrestrial ecosystems.

And their partnership continues to this day.

It is often easy to spot mushrooms, which are the reproductive part of some types of fungi.

But most fungi live underground where we can't see them.

- So this plant has grown with a fungal partner, and you can see that with the plant roots being intermingled with fungal filaments, these wrap themselves around the plant roots and form these intimate associations.

- [Narrator] These associations are so vital that nearly 90% of plants living today are dependent on them.

- It's really easy to overlook fungi because for the most part, they live underground, whilst plants grow much taller and are more obvious.

- [Narrator] But some fossil evidence from around 420 million years ago suggests that this balance once looked quite different.

Something utterly astonishing has happened to some fungi.

They have become giants.

Colossal fungal spikes tower over the landscape.

They're called prototaxites.

Standing over 20 feet high, they reproduced by releasing spores that are carried by the wind.

- The prototaxites landscape would've been an alien world.

- [Narrator] So alien that when the fossils were first discovered back in 1843, scientists were not even sure what they were.

- It was a very strange and odd thing when people found it was shaped like a chunk of wood.

- [Narrator] But when they took a much closer look they discovered something incredible.

- So what this is, is a very thin slice of prototaxites.

And we find that unlike a log, which will be full of woody cells, instead we find a mass of these fungal filaments, reminiscent of fungi today.

- It creates, in my mind, one of the most bizarre prehistoric landscapes of all.

'Cause there's nothing like it today.

- [Narrator] The towering prototaxites dominate the landscape.

Plants, by contrast are still tiny, measuring just a few inches or less.