When did the first trees appear? The first trees appeared about 350 million years ago and they evolved from seedless plants.
There had to be plants before there could be trees and the first plants evolved about 400 million years ago. The first plants evolved from a type of freshwater green algae, but there were a lot of steps along the way. It was not easy for ocean and freshwater based lifeforms to make the jump to living on the land and it might have taken the algae a few billion years to make that jump. There is evidence that algae might have evolved several billion years ago. The first life on Earth was microbes, about 3.7 billion years ago. They lived in the still pretty hot seas. Then cyanobacteria evolved about 2.4 billion years ago and they were the first photosynthesizes, producing large quantities of oxygen in the atmosphere. Algae evolved about this time as well. These organisms changed the atmosphere, but they also provided food for other organisms by dying and sinking to the bottom of the oceans.
The first plants came from algae that washed up onto the shore and thrived there, but that took a long time because the land was very inhospitable. There are possibly two reasons for this. The first is that the atmosphere had too much oxygen. Oxygen is highly toxic in high concentrations and it might have killed every organism that came out of the sea until the levels dropped enough. The second reason was that there was no soil for the algae to grow on. Soil is a mix of minerals, but it has a huge quantity of organic matter in it. That is why plants can grow in soil and why you couldn’t grow plants in the soil of Mars. The first algae that washed up had no organic matter to feed on. It took probably millions or hundreds of millions of years until enough algae had washed up and died to make a soil that algae could grow in. Then, once there was a soil, evolution took its course.
The first plants settled about 470 million years ago. As they got bigger, they evolved roots and a vascular system because they needed a way to take up and transport water. When algae and plants photosynthesize, they take in carbon dioxide, but in the process, water can evaporate out of them. They lose water far more quickly than they absorb carbon dioxide, but they have to have carbon dioxide. The only solutions to this would have been to live in very wet areas, so the water could be replaced externally, or to evolve a system of roots so that they could take up the water and replace it. Obviously, the roots system worked very well and the majority of plants now use it. Once they had roots, they needed a vascular system to transport the water around the plant.
Once plants had a vascular system, they could start to get taller. Height helps a plant get its leaves above other plants and objects, giving it more ability to photosynthesize. Height also helps plants get away from plant eating animals on the ground. The problem with being tall is that the plant needs more support. Plant stems are strong, but they are not strong enough to hold up a plant that is meters tall. The plant will be top heavy and it is very likely to break. Nothing could really be taller than about a meter until plants evolved the ability to grow wood. This appeared about 400 million years ago. Wood provides the support to hold up a tall tree and it also gives added protection from predators on the ground.
The first real tree was called Wattieza and it grew to be about 8 m tall. It didn’t have leaves but had fronds like a fern. It also reproduced with spores and not seeds. It was probably like a giant fern with a strong trunk that was close to wood, but not completely wood. Actual wood that we would recognize today evolved about 380 million years ago. It took a while for real wood to evolve because it needs something called secondary growth. Primary growth is what happens at the top and bottom of a plant, and is how it grows taller or longer. Cells divide and grow at the tips of branches. Secondary growth is cell division and growth that happens in stems and roots to make them thicker and turn them into wood. This took a while. Once it appeared, trees grew even taller and the Archaeopteris grew to be 30 m tall. And this is what I learned today.
Sources
https://treescharlotte.org/tree-education/a-brief-history-of-trees
https://en.wikipedia.org/wiki/Tree
https://en.wikipedia.org/wiki/Wattieza
https://en.wikipedia.org/wiki/Evolutionary_history_of_plants
https://en.wikipedia.org/wiki/Secondary_growth
https://naturalhistory.si.edu/research/botany/research/algae
Photo by Johannes Plenio: https://www.pexels.com/photo/two-brown-trees-1632790/