"The doubling of life expectancy is largely down to medical advances. Without medical care, we’d probably be at a much lower population level. I can’t find projections, but given that even some fairly routine injuries and sicknesses would be life-threatening without medical intervention, one would expect a more pyramidal pyramid, and 2 billion people doesn’t seem unreasonable to me. This is also amplified by [there] being [fewer] people to innovate ideas like the green revolution, which [led] to higher populations. In nature, population overshoot is usually remedied by a higher die-off rate, not a lower birth rate. But humans think that medical care and increased lifetimes are a sign of progress. Medical advances can even allow some who wouldn’t have been able to have children to have children, thus exacerbating overshoot."