There's actually some doubt about the lignin hypothesis: https://www.dispatch.com/story/news/technology/2016/05/12/geology-carboniferous-period-was-coal/2365731

There's actually some doubt about the lignin hypothesis: https://www.dispatch.com/story/news/technology/2016/05/12/geology-carboniferous-period-was-coal/23657316007/ https://www.dispatch.com/story/news/technology/2016/05/12/ge... Which I found out about recently.