Re: why did plants came before animals

Dear Thea,

Excellent question. But the fact that water is made of hydrogen and oxygen has nothing to do with the answer. In fact, animals, plants and fungi did all evolve at nearly the same time, and we are all still evolving. We can't be certain exactly when the first types of anything like a plant or animal or fungi evolved because they were all single-celled organisms that didn't leave much fossil evidence behind. But long before land plants and dinosaurs, long before bony fish and clams and other multi-cellular complex fossil-making organisms evolved, the single-celled precursors of what would eventually both plants and animals were evolving.

A few hundred million years before either plants or animals started to evolve, the first oxygen was produced by photosynthesis, but it was done by bacteria, not plants. For the first few hundred million years of oxygen production, none of the oxygen was released as free oxygen into the atmosphere. The first few billion tons of oxygen reacted with metals like iron and uranium, and sulfur and other atoms. The iron oxides and other compounds that formed settled out of the oceans and formed thick beds of what is now iron-rich rock. At the point in time about 2 billion years ago, when the uranium got oxidized and settled out, gigantic nuclear reactors formed.

Anyway, these bacteria were producing oxygen for over a billion years before anything like what we now consider to be plants, or even algae or seaweeds, evolved. Plants and algae and seaweeds began to evolve when some precursor of modern day eukayotes (plants, andimals, fungi and protists are all eukaryotes) took in some type of photosynthetic bacteria in a process now called endosymbiosis.

Today, most biology textbooks teach us that food chains all begin with photosynthesis, that solar energy is the source of all biological energy. This is true for most of the common food chains we see on land and near the surface of the ocean where sunlight is abundant. However, life on earth began long before photosynthesis, and it evolved with other sources of chemical energy. What came before the plants and animals of these modern day food chains we see now, was bacteria and single-celled precursors of the eukaryotes.

A lot of people think the term "animal" only refers to mammals. They think that fish and clams and insects are not animals. But multicellular animals, living things with complex structures that eat other life forms to get their energy, evolved twice: once to create sponges and once to create the other types of animals (everything from jellyfish to worms to insects to fish and mammals). This all happened 670 to 425 million years ago.

The multicellular plants began to evolve almost twice a long ago, between 1,300 million years ago and 425 million years ago. The first land plants followed the lichens, fungi and algae onto the land masses of earth close to 425 million years ago. Animals began living on land at essentially the same time.

Long before there were modern land plants like grass, and trees, there were ferns and giant mushrooms that were the largest living things on land!

We still don't know a lot about exactly how and when all the different life forms we see today evolved. But attempting to figure it out is a very exciting part of biological research today. It is also an important part of geology, because of the way that life has changed the rocks such as creating the vast iron and uranium deposits.