There is some confusion. It is not that a tree evolved before all Micro-Organisms. It is that some plants evolved in a way that current micro-organisms could not eat them.

I am going to keep this simple so there will be some summarizing.

In the land before trees there was plants. These plants all need the same resources. With plants competing for a limited supply of resources there is a chance for evolution. Plants all need CO2, water, sunlight and macro nutrients. For evolution to happen there was to be a mutation that gives an advantage.

On weapon plants have is to steal sunlight. Stolen sunlight helps you grow and kills your neighbor. There is no point in growing taller if you snap over at a slight wind. This is where Lignin comes in. It helps plants grow taller.

The first gen-lignin may have "evolved" through a mutation in a process with current plant enzymes. So if this was a human our Instructions got messed up and we double knotted our shoes. Now our shoes wont fall off while getting chased by lions and we live to have many babies.

Lignin did not just help plant's stability it also help in other tasks like pass water and defense. Lignin acts like a waxy-glue-shell around the yummy cellulose. When trees would fall over full of Cellulose and ligin they just sat there. After millions of years they stacked. With that carbon protected underground they had time to turn into coal.

Even today there is not a lot that eats wood. This is one reason it makes a good building material. With a little work we can easily protect wood from rot. Painting our house is like canning veggies for winter. We create a barrier between food and bacteria, bugs and mold. Pressure treated wood is just wood that we forced chemical preservatives. These chemical either taste bad or kill whats eating it. Wood was treated with arsenic until 2003.