How do we know our upper limits unless we push through the limits that are already in place? As humans, there are physical limits on what we can do. However, since the earliest days of our species, Homo Sapiens have always tried to push through these boundaries. During our hunter and gatherer days, we pushed through these boundaries in the search for food or while trying to avoid our enemies. In the book Born to Run, Christopher McDougall even hypothesizes that we used endurance running to track down prey, literally running animals to death.