360U: Prehistoric fiction book from 5th grade

My book was read to our 5th grade class on 1989-1990 It is set in prehistoric times in either North America or Europe. The protagonist is a girl. She is somehow separated from her hunter-gatherer tribe and survives on her own, basically developing the historical advances that made it possible for humans to settle in one place, such as planting crops. I think she befriends a boy partway into the story but she’s mostly on her own. At the end her tribe is passing through and they meet. The tribe is starving and she invites them to join her, with the last line in the book somewhere along the lines of “Come in. We have fire, and food in plenty.”
There is no romance, no science fiction, no fantasy element. It seemed to be geared towards children and possibly published for use in a classroom setting to teach prehistory, but it was fictional. A novel or novelette. I have asked my elementary school library, my elementary school principal, my city library, Google, reddit, Facebook, a list of over 300 prehistoric fiction books on Goodreads, Name That Book, and Amazon. I also tried the Scholastic archive, but the one I found doesn’t go back far enough.
I’m really desperate by this point and want to find this book to read to my own kids.

246I: A “cave boy” survives on his own

This is the first book I remember reading on my own, which would have been about 1954-55. The book was much older than that. From my hazy recollection of its appearance, I’d guess late 1920s through early 1940s. I have no memory of the title or the author. It was written at what was then probably considered a third- or fourth-grade level. There were some facing-page illustrations, but I don’t remember them clearly. It’s a story about a “cave boy” who becomes separated from his family, survives on his own, grows to young manhood, and is reunited with his family at the end. Besides that, I can remember only one specific incident: lightning strikes a tree, causing a fire, which is the occasion for the boy to discover the use of fire.

Would be delighted to find it again.