Effects of gravity on development: the importance of behavior.

Organisms live in four dimensions. With the dimension of time, we recognize that each organism is a lifecycle--an expression of its development. The cycles of differentiation expressed in the timeframe of biotic evolution yields the diversity of life on Earth. Thus, phylogenesis, can be understood as alterations in developmental processes. Species of organisms are thus groupings of individuals that share developmental processes. Natural selection acts throughout the lifecycle to select for developmental features that are adoptively appropriate for the organism's immediate environment. The process of ontogeny occupies an appropriately central position in gravitational biology. The evolution of all life on Earth, and thus the evolution development took place in the presence of gravitational forces. It is thus natural and necessary for biologists to wonder and to hypothesize about the role of gravity in development. We shall discuss some of the ideas and concepts that we believe are fundamental to appreciating the place of developmental analyses in gravitational physiology. We focus on problems relevant to the effects of gravity on vestibular development, and then emphasize issues pertinent to mammalian development.