Plant Evolution and Diversity
Topics in Depth
The Theme of Plant Colonization of Land in Plant Evolution and Diversity
Plants haven't always extended across the land as they do now. All life started in the ocean, and like animals, plants had to move to land. Cyanobacteria, bacteria that can photosynthesize, were the first photosynthetic organisms to move to land. They got there about 1.2 billion years ago, before everyone else jumped on the bandwagon. Anyway, cyanobacteria had the place to themselves for half a billion years, and made some pretty nice home improvements during that time. They released lots and lots of oxygen into the atmosphere, which made land a more appealing place to live. Once the other organisms figured that out, they started thinking, "Hey, now I want to live on land!" and then plants made the move to land too.
Plants sold their underwater homes about 500 million years ago in favor of new, oceanfront views above water. But once they got to their drier homes with fewer shark attacks, they found benefits and challenges in their new environment.
Benefits of living on land:
- Sunlight is brighter, since it doesn't have to go through water first
- More carbon dioxide in the atmosphere than in the ocean
- Mineral nutrients are plentiful in the soil
Challenges of living on land:
- Less water, so plants needed to avoid drying out
- No support against gravity
The algal ancestors of plants had uncomplicated bodies. We use the term thallus to describe the body of an alga, which doesn't have roots, stems or leaves. This works for algae because the water they live in physically supports them. Plants had to develop new structures to deal with living on land. New structures that plants developed included:
- Vascular tissue
These structures will be discussed in greater detail later. For now, know that the great diversity of plant life on Earth began with the move to land.
Because early land plants had no way of transporting water through their bodies and didn't have any support holding them up either, they were fairly small. Though we don't know exactly what the first land plants looked like, we know they:
- were small
- had alternation of generations,
- lacked a vascular system (a way of transporting water and nutrients throughout the plant body)
…and therefore had to grow in wet environments. Many plants have gone extinct in the history of life on Earth. However, there are some plants that exist today that also are small, have alternation of generations, and do not have a vascular system. These plants are the bryophytes, and are the closest guess we have to what early land plants were like. Bryophytes are non-vascular, seedless plants that include the mosses, liverworts and hornworts.
When plants first colonized land, oxygen levels in the atmosphere were much lower than they are today, and carbon dioxide levels were higher. Over time, as plants evolved new structures that allowed them to grow taller, wider and photosynthesize more, carbon dioxide levels dropped dramatically.
Even though algae is usually referred to as pond scum (which wouldn't be incorrect), algal products are important for a lot of foods and other products humans use. Carrageenan is an algal product used in ice cream, soups, pudding and salad dressing.
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