(Picture from here.)
There are a number of revolutionary events in the biological history of earth. I've spoken about many of them.
But one extremely large one was the invasion of the land by plants.
We had life in the seas nearly four billion years ago. The most likely origin of life is in the sea unless we were planted here by the Prometheans. However, life on land didn't happen until significantly later. And by life on land, I mean plants. Animals might have ventured on land before then but they didn't stay. Likely the looked around, realized this was a bad idea, and ran/skittled/slithered back to the water before they cooked.
And it was inhospitable. Imagine a bleak bare rock environment. No soil-- that's a product of life. Sure there's sand but soil requires an organic constituent. And without plants that didn't happen. No fungi-- fungi requires decaying organic matter.
There might have been photosynthetic single celled organisms here and there. For example, an algal cell blown up past the water might have landed in a lake and survived. It's not an invasion of the land but it's getting away from the sea.
Even then, it would have been a rough life. The organisms in freshwater lakes also depend on incoming organic material and specific minerals. Much of which happen as a consequence of-- you guessed it-- plants on land.
This was a hard problem.
Consider that nice green algal cell in the ocean.
No problem with water-- it's surrounded by it. It can get all the water it needs. Minerals and organic material? Not a problem: it's in the ocean. Even a billion years ago the ocean was a functioning ecology and a veritable soup of edible organic and inorganic material. All carried by the water. Reproduction? Dump eggs and sperm in the water. The probabilities are that a few will find each other. The water is doing all the work. Radiation? The water stops most of it. Heat and cold? It's going to range from freezing to boiling-- physics prevents anything else. But the temperature is always highly buffered by the mass of water itself. It takes an enormous amount of heat energy to push the water temperature around. (Question: why doesn't global warming happen more quickly? Answer: It's in the water.)
You see a pattern here. Water serves as a wonderful compendium of capabilities: medium of reproduction, temperature and chemical mediator, container of nutrients. Take away the water and what's a poor little algal cell to do?
Plants comprise the kingdom Plantae. All land plants and the green algae are in this kingdom. They are multicellular eukaryotes just like mammals. Unlike mammals-- or any other animal-- plants have have cell walls. The land plants have cell walls of cellulose. Cellulose is just like starch except for how the individual sugars bind to one another. That little difference in binding is the difference between the contents of a potato and the heart of an oak.
Plants photosynthesize-- a feature they share with the red algae and brown algae, which are not part of Plantae but are photosynthesizing eukaryotes.
There are two main divisions in Plantae: the land plants and the green algae. The current presumption is that a species of green algae made its home in fresh water lakes. This would have forced them to evolve mechanisms for coping with the lesser nutrient load, increased radiation load (depending on shallowness) and increased tolerance for temperature and chemical shifts. I.e., it prepared them for land.
This isn't all that far fetched. If you've ever been to a fresh water lake where the level fluctuates regularly, you'll see a band of dried algae at the high water mark. It's black, dried and crusty and looks like nothing more than burnt toast. Toss it in the water and wait a day and you'll get bright green algae.
But it's far cry from that to aforementioned oak. There are a lot of steps between.
The first, obviously, is to be able to operate in a dry, or relatively dry environment.
The land plants are called embryophytes.
Molecular evidence suggests the following clade relationships between living embryophytes:
It's chancy to examine a clade of modern organisms and then attempt to project back in time. For example, humans and chimps share a cladistic relationship: in the great ape family humans and chimps share the most recent common ancestor.
However, that happened several million years ago. Chimps kept evolving since then. All that means, then, is that this common ancestor had shared traits between humans and chimps that evolved over time into humans and chimps. Looking at that last common ancestor, we'd likely expect a blend of traits.
In this cladogram, we would expect the last common ancestor of modern plants branched off to liverworts before it branched off to something else. But that's not the same as saying plants descended from liverworts. Like our chimp example, liverworts have been evolving right along with the rest of plants. The best we can do is examine liverworts and compare them with other modern plants and try to tease out what traits might have been preserved from that ancestor and what traits liverworts evolved on their own.
That said, what's a liverwort?
The Marchantiophyta are bryophyte land plants. Bryophytes consist of mosses, hornworts and liverworts.
They're small, usually less than an inch. If you find a rock in a relatively moist environment you'll probably find lichen-- that flat, gray, leaf-like material-- and liverworts. They have no stems. No leaves. The ones that I've seen look like bits of green clay spread over a rock with a spatula.
They are non-vascular. Vascular plants have tubes inside of them that are used to transport fluid and nutrients-- the plant analog to blood vessels. Liverworts lack this. So it can't get very big.
Liverworts need water to reproduce. They build reproductive structures for their gametes and the gametes have flagella to propel themselves through thin films of water. It's not surprising that they are not prevalent live in excessively dry environments and are not terribly tolerant of direct solar radiation. That rock you found them on was likely shaded. Although, there are desert species.
The liverwort plant body is called a thallus-- interestingly enough, the same term is used in describing algae and fungi. It's anchored to the ground by a rhizoid. Rhizoids can be as little as one cell across and are hair like. Since the liverwort lacks vascularization, diffusion of nutrients, oxygen and waste products has to be across cells. Hence, it's thin. It's outer skin is often covered with a waxy material called cutin to protect it from drying out. Liverworts lack lignin, the main support polymer of vascular plants, so it can't get very tall.
(See here.)
In short, the liverwort does pretty much the bare minimum to survive on land. It looks as if a green algae thought of the cheapest and easiest ways to survive on land and implemented them. It's a long way from a liverwort to our towering oak. Still, it's a step in the right direction.
It's not hard to imagine liverworts (or something like them) in the shade of a boulder a few feet from a lake, in a lonely and barren world, bare rock in all directions.
The liverwort doesn't care. It's got a foothold.
More reading:
Early evolution of plant evolution
Invasion of the land by plants: when and where?
Climate change caused by land plant invasion
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