What was the first drum instrument? Was it wood or stone? Maybe there is an answer out there, but all I found were mere discussions.
I don't think we can know exactly. There are no records of something that far back. Then again, which part of the world are you talking about? There were probably a few different versions of the drum invented at about the same time, and they were probably made from different materials. And also,what can you call a 'drum'? Do two pieces of stone, not shaped in any special way, count as an instrument? The first drum that had any special acoustic shape was probably made of wood, since it's easier to carve. But the very basic drum- two pieces of something- might have been stone, too.
The first 'official drum' was from medieval Europe and was called a Tabor. However in Asia they were using percussion already, aboriginals in Australia were using rhythm sticks for thousands of years and people in Africa had been using percussion and instruments which would evolve into Djembe's. Who knows maybe even neanderthal man hit some sticks against a rock thousands upon thousands of years ago.
The simple answer is that it was most likely some sort of animal skin stretched over hollow wood. There is no definite answer.