Unless I'm misunderstanding, the remaining third would be between the advent of agriculture and the industrial revolution.
There are clearly agricultural societies, where >90% of calories come from cultured plants, and there are clearly hunter-gatherer societies that predate them, where ~0% do. We used to believe in a relatively sharp cutoff between these, where once people learned to grow food, they quickly moved to mostly grow their food. This is no longer thought to be the truth, and there was likely a "transitional period" of many thousands of years as people very slowly hunted and gathered less and planted more.
(Why believe in a sharp transition? Because there is a lot of archeological evidence for it, so it clearly happened in lots of places. It's just that this doesn't represent people inventing agriculture, but it spreading to a new area and displacing older lifestyles, either by migrations of people or of ideas.)
Though that may not change much -- depending on your estimates, Neanderthals probably spanned 3-30 billion person-years, as little as 0.1% of modern humans. All human precursors (5-10My worth) might be margin of error on the modern humans' totals.
On the other end of things, if we plateau at around 10 billion humans, it will only take about 75 years to accumulate the next third (well, quarter) of human existence.