remains in coal
The evidence is that coal was formed very recently. It was not a gradual but a catastrophic process. It may not be the only sudden geological phenomenon, many rocks bear the signs of nearly instantaneous formation.
Coal deposits have been analyzed and show that they consist of plant and animal remains from around the globe. Not only that but they contain human remains too! Human body parts, pottery, artefacts and even a cartwheel have been found.
As you see above there are polystrata tree trunks which span many coal seam layers. They show how suddenly these layers were formed. Some intense heat source has literally cooked thousands of feet of organic material and turned it into coal within a short time.
There will probably never be another situation like the one 7000 years ago when Earth faced such a catastrophe. Coal will never be formed again in that case.