1.1. Purpose
Early life on Earth evolved in the context of temperature, available elements and molecules (including water), as well as the biophysical boundary conditions of the planet. The later include gravity (1 g), exogenous and endogenous radiation from particles emanating from beyond Earth and due to radioactive elements on Earth, and magnetic fields due to the geomagnetic field of the planet and local concentrations of molecules such as iron. Thus, evolution of life within the boundary conditions of Earth for millions of years, even with fluctuations and variations due to solar system cycles and solar events, could not have anticipated life as we know it today with the advent of electromagnetic fields from modern devices, and going beyond such boundary conditions via space flight.
The altered conditions of Low Earth Orbit (LEO) space flight have already exposed astronauts to severely decreased gravity and elevated risk for exposure to radiation, and spending time in such conditions has led to a number of biological responses, particularly due to the loss of the 1 g gravity. However, the influence of increased risk for radiation exposure has not yet been documented in detail, likely in part due to the small number of individuals who have spent time at LEO.
It should be noted that one boundary condition of Earth that has not yet been exceeded by space flight is that of the geomagnetic field of Earth. At LEO, the geomagnetic field is still evident, and perhaps even at the distance of the moon, some of its influence could still be evident [discussed in 1–3].
Thus, humans have not ventured beyond the geomagnetic field of Earth and since some of the central systems of humans (i.e. cardiovascular and neural systems) generate and respond to magnetic fields, this boundary condition likely deserves more study as humans plan to travel to Mars and beyond the geomagnetic field of Earth for the first time. Thus, this article attempts to discuss the various aspects of magnetic fields on life, the potential involvement of magnetic fields on cellular and integrated biological processes, and how a lack of this boundary condition for a protracted time frame could lead to loss of function. Thus, we know that we live in magnetic fields (i.e. the geomagnetic field and local concentrations of magnetic elements), but it is mostly a silent boundary condition and we as yet do not ascribe and functions to it in our daily life. However, as we venture beyond its influence, we likely need to be prepared to understand the potential consequences and anticipate solutions to such consequences as we were not designed for space travel.
1.2. Background
Life on Earth evolved slowly from simple single cell entities that could eventually reproduce themselves using information storage molecules such as DNA. Initially as prokaryotes, and eventually as eukaryotes with subcellular organelles including mitochondria, such cells adapted to a variety of environmental conditions (deep ocean vents, fresh water, cold to hot water, etc). Interestingly, most life consists primarily of water.
In addition to securing the abilities to form a plasma membrane and the machinery to perform essential functions, primitive cells needed to develop these abilities within the context of the physical and biophysical boundary conditions of Earth. These include background radiation and radiation from space that was not deflected by the geomagnetic field of the planet, the 1 g gravity of the planet, and the actual geomagnetic field plus any local magnetic influences. Thus, successful early life must have evolved mechanisms to negate such influences, or evolved adaptations to embrace their influences.
As life likely evolved initially in oceans, lakes or other water environments, perhaps initially boundary conditions such as the 1 g gravity were sensed but further adaptations were required when complex life emerged onto land and mobility and navigation were advantageous. However, the influence of the geomagnetic field would have been felt in early life forms, either with regard to incorporating elements such as iron ions into essential processes, or potentially in other as yet unknown defined manners. The commitment to an information containing molecule such as DNA to be central to reproduction fidelity is also an interesting choice as it can be sensitive to radiation damage and induction of mutations. However, early in evolution this sensitivity could have been used advantageously to use mutations to adapt to a changing biological and physical environment. Mutations could arise from lack of fidelity in copying the DNA and/or radiation-induced effects. As life became more complex and multicellular with differentiated functions, it is also likely that methods to minimize such influences would develop to decrease the likelihood of developing adverse situations that would compromise organism integrity. These latter could include tumor suppressor genes, DNA repair mechanisms, and controlled cell death.
While the 1g gravity of Earth would have been felt by organisms and resulted in settling to the bottom of a lake or near a vent in the ocean, organisms could have resisted this effect via the movement of the water or by attaching themselves to something in shallow water and nutrients could have come to them via water movement. The real impact of the 1 g gravity environment would likely have been felt when multi-cellular organisms emerged to live on land and the advantage of mobility and navigation against ground reaction forces required evolution of new adaptations. Thus, the development of legs for quadrupedal movement and legs and arms for bipedal mobility required evolution of effective adaptations and integration with visual or other sensors. Also, of interest in this regard are species that lived on land and developed legs, but then returned to the marine environment (i.e. whales and other marine mammals). The cardiovascular system of complex lifeforms living on land also required adaptations to function in the 1 g environment. As all tissues except perhaps articular cartilage are vascularized, these adaptations would affect all organ systems.
As evolution could not have predicted space flight and exposure to microgravity, it is interesting that atrophy of mechanically loaded tissues such as bone and muscles is rapidly evident after leaving Earth and living at LEO [discussed in 3]. In addition, cardiovascular effects are also very evident after exposure to microgravity. Interestingly, even prolonged bedrest on Earth leads to loss of bone and muscle and cardiovascular changes, so such tissues have evolved a “use it or lose it” paradigm even when still under the influence of a 1 g gravity environment on Earth [discussed in 3,4].
Another major boundary condition, the geomagnetic field of Earth, certainly “protects” life forms from the negative influence of solar radiation [discussed in
5]. Many forms of exogenous radiation originating from the sun, other cosmic sources (i.e. pulsars, black holes, supernovas] can be deflected by this magnetic field [discussed in
5; and others], and thus protecting the DNA from mutational events or resulting in cell death. For example, it has been reported to enhance radiation resistance by promoting DNA repair processes in cells [
6]. From reading of the literature, this attribute is the main influence of the geomagnetic field on life. The real question then becomes “is it the only role” and if so, why and how did life forms develop systems that use electrical signaling with concomitant magnetic field generation within a powerful magnetic field? Such an environment may have led to development of a bioelectric code early in the evolution of simple and then more complex life forms [
7], and such a code would also have an electromagnetic component. Thus, such a system may respond to exogenous magnetic fields as well [
8] It would be intuitive to conclude that commitment to such systems, such as the brain and neural systems, as well as the heart and cardiovascular system could have evolved approaches to either negate the geomagnetic field or embrace it. Furthermore, as the distribution of elements on Earth that could lead to local magnetic fields is not uniform (i.e. large deposits of Fe containing hematite and taconite non-uniformly concentrated in various locals), the evolved adaptations to the geomagnetic field must account for the exposure to such local concentrations or suffer the consequences