How can you make a fire without wood?
Well, you can burn almost anything organic if you can get it dry. Grass burns easily, but it’s gone in a flash. Reeds last a bit longer, and if they're still a bit damp then they’re good for making smoke to cure hides.
Dung will burn well, and it's easy to collect dung - as long as a lion or hyena doesn’t eat you while you’re searching for it.
Bones can be made to burn, but you have to get the fire so hot it’s really not worth the trouble.
If you’re very lucky your territory might have deposits of coal or peat or lignite (which is half way between coal and peat).
But most of us won’t be that lucky.
It’s a good job we can cure hides to keep ourselves warm, isn’t it? Or, Mr (or Ms) Homo sapiens (the name means wise man, after all), do you have a better solution to the problem?