The earliest humans were hunter-gatherers who lived little differently than other animals, with only rudimentary tool use to distinguish themselves from less intelligent land mammals. The predators which culled the herds that humans also tracked posed active threats to humans, and death by predation, especially of the young and sick, would have limited possibilities for human proliferation. The use of fire and increasingly sophisticated tools, particularly weaponry, lessened these threats and permitted limited human population growth.
Other humans also posed a threat to the overall growth of human populations. Groups of people inhabiting the same region competed indirectly for vital resources such as food and water. They also engaged in direct combat over territory and other matters. Warfare continues to threaten human populations. In the 20th century alone, wars were responsible for the untimely deaths of tens of millions of men, women and children.
The environment itself was, and still is in many cases, a check upon population growth. Human reaction to and manipulation of the environment either lessened or exacerbated the problem. Hunter-gatherers, utilizing only the nutrition naturally growing in the form of plant life or roaming in the form of animal life in a given area, suffered nutritional deficits that affected their ability to withstand disease, sustain fertility and feed their young. In contrast, the development of agriculture, which successfully exploited soil in order to make it bear larger crops than are natural, preceded a continuous acceleration in the growth of humans' world population.
Disease has always been a factor limiting human expansion. For most of human history, people had no way to fight even the simplest of infections. Illnesses carried off many people before they managed to reproduce and, in fact, took the lives of most children before they reached the age of five. This helplessness was frequently exacerbated by poor understanding of sanitation and personal hygiene. Only in the last two centuries has human health been proactively aided by advances in technology and medicine, such as the discovery of antibiotics. Since World War II, death rates in less developed countries have been greatly reduced.