LogFAQs > #965556325

Trees weren't always biodegradable:

Today, bacteria and fungi eat away at fallen trees, but that wasn't always the case. Bacteria had to evolve to eat wood, so hundreds of millions of years go, trees would fall at death, leaving large piles of dead wood. Forest fires of unimaginable proportions would burn the massive mounds of dead wood. And that's where most of the coal today on Earth came from, according to National Geographic.