Historically, up until the middle ages, human populations were mostly limited by

If you think of life in the not-so-good old days, what images come to mind?  Disease, famine and war.  (That's the sort of answer you're looking for.)  The Bible book of Revelation described much of human history as a rider on a sickly, pale greenish horse given power to kill on earth by the sword, famine and plague.

Only when better methods of food production helped reduce famine, and scientific advances in medicine helped reduce disease did population progress take off.  War, well, we're still dealing with that -- though with less deaths today in the world than there were in those older eras.  (The last huge death tolls in our time came during the World Wars.)