「植物看得見你」公開課筆記／1.8 Plant Evolution

Photosynthesis is the ability of a plant to take CO2, mix it with water, and through an enzymatic reaction the plant can yield oxygen, which we need to breathe, and sugar, sucrose, which is the basis of everything we eat.

The earth was formed about four and a half billion years ago. Once the Earth was formed, how was life formed? There are a couple different theories.

Early conditions of Earth contained the chemicals and the energy necessary for life to form, or for at least for organic molecules to form spontaneously – this was actually the hypothesis of Stanley Miller, who was then a young scientist in the early 50s at the California Institute of Technology.

What was the experiment they did? They tool ammonia gas, heat, hydrogen, water and electricity, then mixed them together and checked what they found in the water. They found organic molecules, including nucleic acids. But there's a big assumption here, that these were the conditions early on Earth's development.

If we have early cells being formed, what type of cells were they?

(1) Heterotrophy, the type of life that survives by eating, by finding and absorbing food. Heterotrophs are consumers.

(2) Autotrophy, These are organisms that can make their own food, make their own energy. Autotrophic organisms are producers.

What do you think were the first cells that evolved on earth? Consumers or producers? Most of you answered autotrophs. That it seems logical to you that the first organisms were those that made their own food.

But actually, the first organisms on the planet were the heterotrophic bacteria. The ones that absorbed their food. They're consumers. We know that from the fossil record, fossils of bacteria.

The first billion years of life on earth was only heterotrophic bacteria that were floating in the ocean. About 2 and a half billion years ago, the first photosynthetic, the first autotrophic bacteria appeared at least in the fossil record.

From a philosophical point of view we can understand this that heterotrophy is simpler than autotrophy. It's much easier to eat than it is to produce your own food. So the heterotrophs, which evolved quite early in the history of the Earth, had the world to themselves for the first billion years.

About 2.5 billion years later, according to the fossil record, that autotrophic bacteria began to appear. So these autotrophic and heterotrophic bacteria coexisted in the oceans, with the autotrophs making food and the heterotrophic bacteria eating the food.

Unicellular eukaryotic cells evolved about 1 billion years later. Eukaryotic cells are the type of cells that our bodies are made up of, and the bodies of all animals and all plants.

Multicellular organisms started to appear about 1 billion years ago. All of these organisms evolved in the oceans, as the oceans provided an environment that was most nourishing and protective. The land was originally uninhabitable, because it was dry, had extreme temperatures and it was exposed to harmful UV radiation.

Through photosynthesis, these autotrophic bacteria, the photosynthetic multi-cellular organisms were making and releasing oxygen into the atmosphere eventually led to the formation of the ozone.

Once there was ozone to protect the land from harmful UV radiation, the dried parts of the Earth could be colonized by plants and animals and this happen about 500,000 million years ago.

One of the ways we see this happening is to the huge reduction and the amount of CO2 that was present in the atmosphere at this time As plants left the oceans and colonized land, more and more photosynthesis led to a reduction in CO2 levels and an increase in atmospheric oxygen.

But as plants left the oceans and got onto dry land, how did this effect plant evolution? What were the pressures in evolution that plants felt on the land, that they didn't have in the oceans? What are the different problems a plant would have on the land that it doesn't have in the ocean?

One of the most obvious problems is a problem of water. A plant, a photosynthetic organism in the ocean, doesn't have any problem of absorbing water and having it in all of its cells. An organism in the ocean can float around, whereas an organism on dry land has to be rooted. It has to find a way of holding itself against all of the pressures of wind, of sun, of cold.

And while there are various forms of higher plants, ranging from the first, earlier organisms, all the way up to the modern ones, in this classroom we're only going to be dealing with higher plants which we call angiosperm.