Oxygen Changes the World
2,700–635 million years ago (Proterozoic Eon)
When does photosynthesis begin?
As we all know, photosynthesis plays a major in the transfer of the sun's energy into food directly for plants and indirectly for animal.
The earliest photosynthetic organisms were bacteria, that converted carbon dioxide and water into sugars in the presence of light.
We also know that photosynthesis gives off oxygen as a waste gas, and some of earliest life may have slowly been doing this process
Around 2.4 billion years ago, oxygen was increasing and carbon dioxide was decreasing in the atmosphere, probably because photosynthetic organisms were becoming very successful throughout Earth
In fact, oxygen was so abundant that it was reacting with iron in the soil, creating "rust", or iron ore rocks (see below)
This also had a profound effect on early Earth's organisms, and set the stage for modern life.
This time in Earth's history is referred to as the Proterozoic Eon (2,500–540 million years ago).
Above: Banded Iron Ore formation from 2.1 billion years ago when oxygen on the Earth reacted with the iron-rich soils, causing them to create iron oxide or rust.
Above: Living stromatolites from Australia. The Proterozoic Era may have looked similar.
When do plants appear on Earth?
In order to answer the question requires a definition of the word "plant"...
If we use a narrow definition (i.e. true plant) which includes just organisms that have roots, stems, and leaves growing on land (e.g. oaks, pines, ferns, tulips), then we will have to wait until 500 million years ago!
If we use a wider definition which includes any complex organism that performs photosynthesis, then plants have a long history on Earth.
Some of the earliest visible organisms that could do photosynthesis were algae
The earliest algae appear in rocks ~1,800 million years ago.
For most of the Proterozoic, algae were very successful, evolving and adapting to every part of the ocean (e.g. shorelines, floating, deep water, etc).
Many groups of algae evolve: green algae, red algae, brown algae, golden algae, and many more!
When do animals appear on Earth?
The earliest animal-like organisms appear during the end of the Proterozoic Eon, around 630 million years ago, during the Ediacaran Period (635–541 mya).
These animals were less complex than modern forms
Jellyfish, sponges, and plant-like forms that filtered nutrients from the water were all present.
Above: Reconstruction of life around 600 mya
So when do true plants evolve on land?
To answer this question, let's move to the next section: Life Moves Onto the Land
What was life like before this time?
To answer this question, go back to the former section: Early Earth