And there are no gates or anything to keep them back. So humans tend not to have much to carry around for their own safety. So their way to tell something is missing or added is by seeing that the form of the pile has changed. When the pile of apples has become bigger, some apples have been added. The bigger the image they have in their mind the more they have.
People move from a nomadic hunter-gatherer lifestyle to a settled lifestyle.
Their numbers grow and they specialize on professions. In time wealth and other things start to accumulate and numbers become larger. Methods of visualizing quantities are of course very subjective, the need for better means of telling quantities and at the same time keeping track of them, increases. As always the means are invented or improved upon need. As the need to represent larger numbers grows, early man employs readily available materials for the purpose.