Larger groups of people to create and cooperatively share or use the built environment, yes. This requires communication of a certain mutual sophistication. But it does not mean necessarily they were all living together, all the time. We have churches standing today that have taken hundreds of years to build from start to finish. This is even with dense populations, the technology, tools and know-how to construct it in a particular style. Many intervening events delayed the build.

What the changes were that led to larger cooperation, presumably over long spans of time across dozens of generations, are where historical record meets speculation. Some agroforestry researchers believe balanophagy (acorns as the main dietary source of calories and nutrients) declined as demand for trees for other uses became prevalent, which pushed the rise of the less-nutritive and more effortful agriculture. Numerous other options have been suggested. Perhaps structures were initially created from trees for thousands of years before megaliths became the norm.