At present, various national populations are now at different stages of the demographic transition. This transition may have far-ranging consequences for future humans. With the envisaged artificial support for human life, the significance of these non-metabolic processes may increase. Though the Earth is a thermodynamically open system receiving energy from the universe, the amount of energy flow is limited and the way its flow is structured on the globe restricts human development. Therefore, the future relationship between human population and the Earth may be constrained by a number of conditions; it may no longer be a simple conquest of the world by technology-wielding humans. Human cultures are still adapted to a world of high mortality, high fertility and little mass migration, where the structure and function of the human body was automatically adjusted by natural selection requiring medical intervention only rarely in cases of acute diseases or injuries. Moreover, human population may also continue to increase its “genetic load”, leading to a further decline in population fitness. This article will provide possible future scenarios for humankind from both evolutionary and cultural perspectives which may reduce long-term human fitness.