This paper is an introduction to gravitational and space life sciences and a summary of key achievements in the field. Current global research is focused on understanding the effects of gravity/microgravity on microbes, cells, plants, animals and humans.

It is now established that many plants and animals can progress through several generations in microgravity. Astrobiology is emerging as an exciting field promoting research in biospherics and fabrication of controlled environmental life support systems.

India is one of the 14-nation International Space Exploration Coordination Group (2007) that hopes that someday humans may live and work on other planets within the Solar System. The vision statement of the Indian Space Research Organization (ISRO) includes planetary exploration and human spaceflight.

While a leader in several fields of space science, India is yet to initiate serious research in gravitational and life sciences. Suggestions are made here for establishing a full-fledged Indian space life sciences programme.

Life has evolved in the past four billion years under the influence of the Earth's massive field of gravity. Several features of life were shaped by gravity as organisms adapted to living in water, air, land and on trees.

Gravitational biology explores how organisms perceive and respond to gravity and how gravity influences the structure, development, function, evolution and behav- iour of organisms (Morey-Holton 2003).

The space age allowed for experiments in microgravity environment and the emergence of space life sciences including fields such as human space physiology and medicine.

'Space biology' is often used as an alternate expression by researchers and journals (e.g. Gravitational and Space Biology) to describe the functioning of all life processes in space environment.

Gravitational biology and space exploration have stimulated enduring interest in astrobiology, which addresses questions such as how does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond (Des Marais et al. 2008; Lucas 2009).

Gravity and life
Of the four fundamental forces, only electromagnetism is directly involved in biological processes. No direct role is known either for the strong or weak nuclear forces or gravity in any biochemical process.

Gravity influences living organisms indirectly. The gravitational force is 1038 times weaker than the strong nuclear force. However, gravitational force is of infinite range, and positively acts on all particles with mass/energy.

Any object with a mass at the surface of the Earth accelerates towards Earth's centre at approximately 9.81 m/s2. This acceleration due to gravity at the Earth's surface is generally treated as 1g (one Earth gravity).

The dilemma of growing in a 1g environment and responding to it can be seen in nearly 400-million-year-old fossil vascular plants and their descendants. On land, roots became positively gravitropic to obtain water and nutrients from the soil.

The negative gravitropism of shoots and various branching angles and the characteristic tree architectural patterns evolved for efficient access to carbon dioxide and light and strategic placement of reproductive structures for pollination and dispersal.

The mechanical load due to gravity is about a thousand times larger for land-living organisms than for those living in water. In response to this increased load, plants might have evolved 'antigravitational' substances such as lignin, cellulose and pectin, while animals strengthened their bones with hydroxyapatite mineral form of calcium associated with collagen (Volkmann and Baluska 2006).

Plant gravitropism has been studied for the past 200 years (Kiss 2006; Hasenstein 2009; Moulia and Fournier 2009). In the animal kingdom, response to gravity has been studied in all major groups. Organs such as the antennal sensilla of Johnston's organ in insects and the vestibular apparatus of higher animals detect gravity for proper orientation and gravitaxis (=geotaxis) movements.

The musculo-skeletal system evolved to support the body mass and provide structural and postural stability to land animals as they moved about in search of food. The sensory-motor system evolved so that organisms could recognize the gravity vector and orient themselves and move about. A vestibular system evolved in fish for efficient swimming.

The versatile design of this system, with minor modification in nerve-motor connections, was retained by amphibians, reptiles, birds and mammals for navigation in water and air and on trees and land. Human beings have inherited many of these evolutionary adaptations (Highstein et al. 2004).

The vestibular system is the key to the human senses of balance, motion, and body position. The otolith organs allow humans to sense the direction and speed of linear acceleration and the position (tilt) of the head. The semicircular canals help sense the direction and speed of angular acceleration (Coulter and Vogt 2004).

As a bipedal erect animal, the genus Homo had to adapt to gravity for at least 2.4 million years. Their ancestral bipedal hominins faced this dilemma nearly 7 million years ago. The cardiovascular system helped maintain adequate pressure and supply of blood to various parts of the body, especially the head.

The normal and healthy life of humans is conditioned at least in three different ways by the 1g environment of Earth: (a) by experiencing a pull of the g-force in the head-to-toe direction, (b) through various normal physical activities of exertion against the g-force, and (c) through changes experienced during oriented movements/ postural changes (Legner 2003).

Read the complete science at IAS