Ahoy, AakashM-2014. It is I, AakashM-2023 with what I am sure is the answer to your question.
In 1973, John Christopher (of Tripods fame) wrote a short novel titled Dom and Va, also known as In the Beginning. In 1982 he published a short-story adaptation of the same tale, and called it In the Beginning. Confusing huh. From the blurb of a single-volume edition of the two of them, we have:
In the beginning, many, many years ago, there were two tribes. Dom [you were close!] was
a boy from the hunting tribe, and Va was a girl from the farming
tribe. The two tribes fought, but that couldn't stop Dom and Va from
falling in love.
I eventually found this work by phrase searches, dropping various words from the phrase you remembered, until I landed at this google books reference:
https://www.google.co.uk/books/edition/_/Jan5CAAAQBAJ?hl=en&gbpv=1&pg=PA194&dq=%22nothing+can+grow%22+%22strong+arm%22
"But what I said was true. Nothing can grow unless a strong arm
shields it."
Having read this single-volume edition, I can now say that this phrase appears only in the novel, not the short story adaptation (the plot of which is different in some key points). It is on page 194 out of 214 in this printing.
The book is set earlier than the Agricultural Revolution, I think, although I also think anthropological thinking has moved on since it was written, making it hard to know exactly when it's set. Don's tribe eat their meat raw, having not yet tamed fire, while Va's tribe keep domesticated cattle - I'm not sure that it's now believed that these two peoples could coexist.
Being subjective for a moment, I will say that the novel is... not great? And the blurb isn't exactly accurate about the events described. The short story is even worse, written in an oddly telegraphic, almost Simple English way.
But the identification question is solved, at least.