The type of world in which our ancestors lived for hundreds of thousands of years women probably had four to six children in a lifetime. Half of those children would die before they could reproduce. So the only thing we can be sure about in human population studies is that for the last 100,000 years, people on average, had two children to succeed them, or there would have been a population explosion a thousand years ago.
In just the last nanosecond of human history, which began ten thousand years ago with the advent of agriculture, we began to change the way we looked at the earth; something that we separated ourselves from. It is the basis for our civilization today. We have spread notions of proper sanitation as we've vaccinated for diseases, as we've provided famine relief and basic levels of health care, we see an unintended consequence of our best intentions. By adding fossil fuels to our agriculture, we have allowed population to simply skyrocket.
In the 1960's, population was growing at an unprecedented rate, the highest in human history; as famine developed in South Asia spreading fear to the rest of the planet. The world started grasping the urgency of the situation. Advocacy groups, such as Zero Population Growth (ZPG), emerged in the US in the 1960s. For the first time, population growth was linked as a major factor responsible for the global environmental crisis, at the first Earth Day in 1970.
40 years later, the environmental message has not changed. Apart from a few persistent groups, population growth is barely being mentioned. It's as if the issue has been diluted among all the others; Even though population growth and human consumption are the major factors in our on-going environmental crisis. We are adding about 50 Million new middle class each year; modeling their consumption habits from the unsustainable lifestyle of the developed world. A triumph for progress and poverty, but a ticking bomb for our civilization.