The fossil record shows that all Earth s species have been transient. Yet humankind espouses sustainable development (which is based on continued human occupancy of the planet) despite the probability that Earth may last another 15 billion years. Is this expectation hubris, denial of scientific evidence, or failure to engage in any fundamental change in social norms? If unsustainable practices continue, leaving a habitable planet for posterity will not be possible. The most crucial determinant of human destiny on the planet is the continual health and integrity of the biospheric life support system, which has produced conditions (e.g., atmosphere gas balance) favorable to Homo sapiens for 160,000 years. If major alterations occur in the function of the biospheric life support system, present human social systems will be in disequilibrium, and even human survival will be in doubt.