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Plants invade the land

Plants first invaded the land about 470 million years ago, around the middle of the Ordovician period.

The first plants (liverworts) the first fungi

As far as I know, little is known about how they did it. But there are plenty of possibilities.

One is river estuaries. Life probably evolved in salt water, and underwater plants will have been no different. But in estuaries, the distinction between salt and fresh water gets blurred. Bacteria will have colonised the estuaries long before, and will have been busily converting the raw rock particles into fertile mud. This mud will have been a very attractive niche for the seaweeds of the time to expand into.

But to colonise this fertile mud, the seaweeds will have both to adapt to a partly freshwater environment and also to develop roots of some kind. In some parts of the world the tidal range will have been many metres, just as it is today. So to get the best out of this new niche, these already-halfway-plants would have to learn a new trick. They would have to cope with regular drying out.

By the time they had done that they were land plants; and had a whole new world to conquer.

The first plants

Palaeo-botanists have found spores of the very earliest true land plants, and they are almost identical to those of modern liverworts. So that is what they believe the plants themselves were like. This picture of modern liverwort was taken by Virginia Kline.

To begin with of course these liverworts will have concentrated in the tidal zone and above the high water mark. One of their first strategies seems to have been to put out those underground ‘runners’, still beloved of plant growers today. ‘Vegetative’ processes in general remain an effective strategy for keeping the competition out of your patch, and of expanding it slowly but steadily. And they will have become possible as soon as the plants expanded from their rocky underwater environment to the softer beach.

But the pressure was also on to get their spores distributed as widely as possible. To do this, you had to grow tall, so that the wind would carry them further. This required proper roots to get a good firm grip on the soil. And it also generated one of plant life’s great breakthroughs, the vascular system (more). The vascular system enabled plants to grow tall and strong, and within a hundred million years or so we had full-grown trees.

Meanwhile there remained a viable niche for ground hugging plants; and to prove it the liverworts are still here today, apparently little changed.

The first fungi

I’ve not been able to find out much about where fungi came from. I suspect that there’s a good reason for that. Fungi have no hard parts, and will only be preserved under exceptional conditions.

But these exceptional conditions do occur (more), and fungi appear in the fossil record at almost exactly the same time as plants. It’s not at all clear which came first, or how the fungi evolved. It appears that fungi are actually more similar to animals than to plants, though I’ve read a report that disputes any notion that they evolved from animals.

All the four major groups of fungi, known to us today, have now been found in Devonian strata. This picture of fossil fungal hyphae comes from the UCMP site (University of California, Berkeley).

Today, a major feature of fungi is the way they form symbiotic relationships with plants. It seems that they started doing this at least as early as the Devonian.