Either the earth will drown and collapse under an unsustainable level of humanity, or we aren't making enough new humans as the world rapidly modernizes and eventually we will collapse back to a fraction of the current population. Our whole economy is based on the second derivative–growth–which is only sustainable in a world where efficiency gains + population growth > 0. If that equation drops below zero, the whole economic system of the globe needs to be re-worked and I don't have a lot of confidence that that will be a smooth process.
The other interesting implication is, even if that equation stays positive, the balance between the rewards of labor and the rewards of capital is highly susceptible to the value of population growth / efficiency gains. Some postulate that during previous population collapses (plagues et al) wages increased rapidly while the ROI for investment dropped. When the population expands, wages stagnate and ROI's increase. Makes sense I guess; as more people means more competition for jobs, and less people means more competition for workers.
TL:DR I don't know...and now I have a headache. -Brian
Courtesy of Joe D. comes this really fascinating look at how humans domesticate animals. For the last sixty years, a lab has been breeding foxes for domestication and tracking how other things (cute faces, fluffy tails) develop through the hundreds of generations and tens of thousands of fox'eses lineages. -Brian