Potential Impact of Space Environments on Developmental and Maturational Programs Which Evolved to Meet the Boundary Conditions of Earth: Will Maturing Humans Be Able to Establish a Functional Biologic System Set Point under Non-Earth Conditions?

Abstract

Mammalian development and maturation, particularly processes for humans have evolved in the context of the boundary conditions of Earth (i.e. 1 g gravity, geomagnetic field, background radiation) to yield functional individuals, although the process is not perfect and "errors" do occur. With the advent of spaceflight to low Earth orbit (the International Space Station), humans are now exposed to microgravity and increased exposure to radiation. However, thus far, only adult humans have served as astronauts, but this will likely change with plans to explore deep space and colonize planets. Thus, conception, fetal development, post-birth maturation, puberty and skeletal maturity will occur in the context of boundary conditions that did not shape human evolution and influence physiological and biomechanical systems designed to function within the Earth's boundary conditions. Thus, processes utilizing the 1 g environment (i.e. walking upright) and the geomagnetic field (i.e. the electrical/biomagnetic basis of neural interactions) will have to adapt to new boundary conditions, providing opportunity for additional errors or alterations in processing during development which could impact functional outcomes at multiple levels. This review/perspective will discuss some of these issues and attempt to provide direction for addressing the potential issues to be encountered.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

“…With the exposure to microgravity on the ISS, there is a fairly rapid induction of bone loss and muscle atrophy, as discussed in [ 11 , 12 , 17 ]. The bone loss is more from the lower extremities than the upper extremities, likely indicating that the bones of the lower extremities are more exposed to GRF on Earth, and thus, are more likely to be influenced by its loss.…”

“…While the answer is not known definitively at the present time, there may be clues in what is known about the regulation of bone on Earth. These include the following: Astronauts lose more bone from the lower extremities than the upper extremities, as discussed in [ 11 , 12 , 17 ]; From the work of Frost [ 34 , 35 , 36 ], bone adapts to mechanical stimulation in response to GRF; The current exercise protocols do not mimic GRF, as GRF loading is likely more of an impact loading than a resistance loading, as discussed in [ 11 , 12 , 17 , 34 , 35 ]. …”

“…Astronauts lose more bone from the lower extremities than the upper extremities, as discussed in [ 11 , 12 , 17 ];…”

“…As human evolution could not have anticipated exposure to microgravity during space flight or living in LEO, this heterogeneity in response to microgravity arising during evolution must have remained silent as long as humans stayed on Earth, or the heterogeneity could contribute in some manner to disease development and progression during aging, a time when the integrated physiologic systems may be undergoing senescence and a loss of integrity. Actually, some reports have hypothesized that space flight is a model of aging [ 10 , 11 , 12 ]. While this analogy between space flight and aging remains to be confirmed and validated, it is clear from the response of humans to space flight that at least one of the boundary conditions of Earth, namely the 1g gravity, was a factor in the design of the upright mobility of humans.…”

Reproduction and the Early Development of Vertebrates in Space: Problems, Results, Opportunities

“…Gravity is only one of the factors that influence the formation of complex organisms. Factors such as the Earth’s geomagnetic field, different light cycles during the day and year, and the planet’s background radiation must also be taken into account [ 2 ]. The effect of these factors on mammalian reproduction is largely unknown.…”