I think there#s a bit of a misconception about "hunter- gatherers". People seem to imagine them as wandering randomly over the landscape, gathering fruit, grains and nuts as they found them, and running blindly across herds of cattle or sheep and spearing a few, then setting up camp for the night. It's better to see them as moving between quite definite, well thought out sites in a cyclical pattern according to year and season, within a definite territory. Neighbouring groups would do the same, there being only occasional disputes over the fringe areas. Serious incursions into another group's core territory would only occur under pressure of shortage.

Remembering that they gather the best fruits etc, and leave the grotty ones behind, now see what happens:

They take them back to the same camp site every year.
They eat them or keep them (grains say) for the winter.
They discard the stones with the rubbish, which they also enrich every year because that's the latrine area too. They discard old grain that has started germinating after winter.. in the spring.
So the best fruits grow better around the place where they always camp, and grains grow there too.
Not only that, but herbivores are attracted by these better fruits and growing grains. That means hunters don't have to range so far.
They build barriers to funnel the cattle (say) into a convenient killing zone.

It's not difficult to see the genesis of almost settled communities in this- let's throw away the fruit stones over this part, and spread the spoilt grain over here, let's kill the big beasts but keep the calves in the killing enclosure till they've grown a bit, then we don't have to hunt for a while.

Such a society would have had a surplus of time, though where they derived the impulse to use the time in this way I don't know.And don't run away with the idea that they did it all at once. They say the hill appears man- made. I'd like to section it, to see if this developed top layer was preceded by a series that starts simpe at the bottom, them gets ever more elaborate as time goes on.

Here's a scenario- it's a coming-of-age rite. Every year, all the young men (or women, or both) of an age group from a great distance round get together, and carve one stone. They set it according to which age set they are (scorpion, fox, lion etc). It's also a big party, where you make lots of friends. When the ring is completed they start another one. It only takes twenty five years or so to complete one ring, in two hundred years you've got lots of rings, so they bury the oldest one and start the next ring on top, and so over a thousand years the mound grows without any single huge community effort.

Say it takes 50 people to carve and set a 10- tonne stone, and life expectancy is 50 years. A population of 10000 would easily provide enough bodies to do this, and that population could be spread over hundreds of square miles.

Sounds quite a fun place to be.