**5. Evolution in a gravitational environment or gravity as an evolutionary force**

The gravity of the Earth refers to the *acceleration* that the Earth imparts to any given body on its surface by virtue of its *mass*; this acceleration is directed toward the center of the Earth and is approximately 9,81 m/sec2 . More precisely, according to the law of gravitation formulated by Isaac Newton to 1665, the attractive force between two bodies, in this case the earth and any object, is directly proportional to the product of the masses and inversely proportional to the square of their distance. The *mass* is an intrinsic property of any body and is a constant in classical physics, on the contrary the *weight* varies and *on Earth* is the result of *mass* for *Earth's gravitational acceleration*. Gravity or, better, the Earth's *gravitational field* hasn't significantly changed since the origins and is considered a *constant* although there are some differences in

The main routes used for the administration of hematopoietic precursors. Left drawing, 1) direct injection in the in‐ farcted area; 2) direct coronary injection and 3) peripheral injection. Right drawing, peripheral injection in vivo in the femoral vein in rat.

**Figure 9.** Administration of adult stem cells in experimental model of myocardial injury

its distribution since the Earth is not a perfect sphere with a constant density and, due to its rotation, is subject to the *inertial force*.

By virtue of the gravity of the Earth, also living organisms have a weight that affects every aspect of life including biological ones. The life originated from the sea, landing on the ground of certain organisms and the experience of gravity has, therefore, required the development of specific mechanisms to counteract the effects of gravitational acceleration. Subsequently for some species the transition from the horizontal position to the vertical one has required the adoption of *sensors for the gravity* and of systems to allow the movement and the fine adjustment of the displacement of body fluids. (Table 2).

These postural changes from the horizontal position to the vertical one are cyclical and mainly related to the rhythms of activity and rest that are typical of each species. Furthermore, especially in placental mammals such as humans, for *n* months, first the embryo and then the fetus, are immersed in a fluid and compressed by a capsule that exerts a counter-pressure that diminishes the direction and the force of gravity creating a *microgravity environment*. This exposure to microgravity cause changes in gene expression in a variety of developing organ systems in live embryos (Shimada et al., 2005) thus the meaning of *prenatal life* goes beyond


**Table 2.** Requirements for the evolution of a complex organism in a gravitational environment

the simple effect of *mechanical protection* played by amniotic fluid as it covers an essential role in initiating the processes of long-term expression of the genetic program of the embryo. These different developmental pathways are ultimately triggered by the *external environment*through the maternal mediation (Fligny et al., 2009); the shift from uterine life to extra uterine life is concurrent with the progressive decrease of plasticity in favor of adaptability (Figure 10).

The transition from a microgravity environment (uterus) to a gravitational one and the assumption of standing pos‐ ture in humans are potential stimuli shifting adaptive resources from plasticity to adaptability.

**Figure 10.** Gravity challenges and adaptive resources allocation during human life

its distribution since the Earth is not a perfect sphere with a constant density and, due to its

**Figure 9.** Administration of adult stem cells in experimental model of myocardial injury

The main routes used for the administration of hematopoietic precursors. Left drawing, 1) direct injection in the in‐ farcted area; 2) direct coronary injection and 3) peripheral injection. Right drawing, peripheral injection in vivo in the

By virtue of the gravity of the Earth, also living organisms have a weight that affects every aspect of life including biological ones. The life originated from the sea, landing on the ground of certain organisms and the experience of gravity has, therefore, required the development of specific mechanisms to counteract the effects of gravitational acceleration. Subsequently for some species the transition from the horizontal position to the vertical one has required the adoption of *sensors for the gravity* and of systems to allow the movement and the fine adjustment

These postural changes from the horizontal position to the vertical one are cyclical and mainly related to the rhythms of activity and rest that are typical of each species. Furthermore, especially in placental mammals such as humans, for *n* months, first the embryo and then the fetus, are immersed in a fluid and compressed by a capsule that exerts a counter-pressure that diminishes the direction and the force of gravity creating a *microgravity environment*. This exposure to microgravity cause changes in gene expression in a variety of developing organ systems in live embryos (Shimada et al., 2005) thus the meaning of *prenatal life* goes beyond

rotation, is subject to the *inertial force*.

660 Regenerative Medicine and Tissue Engineering

femoral vein in rat.

of the displacement of body fluids. (Table 2).

The effects of gravity on organisms can be studied easily by varying the *gravity level* by simulating microgravity on the earth or in flight. The *simulated microgravity* is commonly obtained on Earth by *head down tilt*, *prolonged bed rest* or *dry water immersion* (Magrini et al., 1992), in flight, during the fall phase of *parabolic flight in jet aircraft* or in orbit around the Earth with *spacecraft* (Table 3).


**Table 3.** Experimental approaches to simulate microgravity

Since these techniques counteract some of the effects of gravity, it is believed they can provide important information relevant to biology in a gravitational environment.

Unfortunately the precise role of gravity on evolution is difficult to determine in complex organisms because, until now, the stay in a *microgravity environment* has been too limited and no higher vertebrate has passed at least one life cycle in microgravity to see the effects on subsequent generations. These simulations, even if limited in time, may nevertheless provide useful information on *adaptive processes* to counteract the force of gravity in different organisms and, in such sense, the responses of the cardiovascular system are paradigmatic. Previous observations suggests that prolonged dry water immersion after birth in rats is able to dissociate the effects of body growth and aging on systolic blood pressure since the micro‐ gravity reduces the needing for load-bearing structures and then the body weight, temporarily blunting systolic blood pressure rise (Magrini et al., 1992).

It should be remarked that the ability to survive in a gravitational environment is inversely proportional to the size, in other words unicellular organisms such as bacteria are much less sensitive to gravity when compared to complex organisms with blood circulation and subject to cyclical postural changes. These observations are derived from the study of extremophiles, i.e organisms capable of surviving in extreme environments and similar to those that are supposed to exist on other planets (Rothschild and Mancinelli, 2001; Brack and Pillinger, 1998).

Due to this lower sensitivity to changes in gravity, as demonstrated also by the growth at hyper-accelerations (Deguchi et al., 2011) and also to the high level of resistance in all ecosys‐ tems, bacteria have indeed the potential to travel in space and colonize planets with different gravity levels. This theoretical possibility, and some studies on meteorites, have led to the suggestive hypothesis that life on Earth has an extraterrestrial origin thanks to space vectors such as meteorites or asteroids (Mautner, 2002). This kind of exogenesis, challenged at the theoretical level (Di Giulio, 2010), moves indeed elsewhere the problem of the origin of life but, at the same time, is in line with the concept of a solid common trunk for the tree of life which possibly consists in extremophile bacteria.

Since the bacteria are *single-celled organisms*, in assessing the effects of gravity a point that deserves emphasis is the impact of the microenvironment in which the cells are suspended which affects nutrient supply and disposal of waste with potential cumulative effects (Klaus et al., 1997).
