It is still a challenge for scientists and philosophers to define life in unequivocal terms. Defining life is difficult—in part—because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from life on Earth. Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes (i.e., living organisms) from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate. Biology is the science concerned with the study of life.

Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms (life forms, approximtely 8.4 million forms of life) can be found in the biosphere on Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. In philosophy and religion, the conception of life and its nature varies. Both offer interpretations as to how life relates to existence and consciousness, and both touch on many related issues, including life stance, purpose, conception of a God or Gods, a Soul or an afterlife.

There are three kinds of Souls: the “vegetative soul” of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the “animal soul” which causes animals to move and feel;  and the “rational soul” which is the source of consciousness and reasoning which is found only in humans. Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.


Evidence suggests that life on Earth has existed for about 3.7 billion years, with the oldest traces of life found in fossils dating back 3.4 billion years. All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to proto-cells and metabolism. Many models fall into the “genes-first” category or the “metabolism-first” category, but a recent trend is the emergence of hybrid models that combine both categories.

There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses: Life as we know it today synthesizes proteins, which are polymers of amino acids using instructions encoded by cellular genes—which are polymers of deoxyribonucleic acid (DNA). Protein synthesis also entails intermediary ribonucleic acid (RNA) polymers.

Recent findings by NASA, based on studies with meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules) may also be formed extraterrestrially in outer space..


The meaning of life is a concept that provides an answer to the philosophical question concerning the purpose and significance of life or existence in general. It can be expressed through answering a variety of related questions, such as “Why are we here?” “What is life all about?” and “What is the meaning of it all?” It has been the subject of much philosophical, scientific, and theological speculation throughout history. There have been a large number of theories to these questions from many different cultural and ideological backgrounds. Even so, the meaning of life could manifest that question itself: “what is the meaning of life,” or life seeking the meaning of itself.

The meaning of life is deeply entrenched in the philosophical and religious conceptions of existence, social ties, consciousness, and happiness, and borders on many other issues, such as symbolic meaning, ontology, value, purpose, ethics, good and evil, free will, conceptions of God, the existence of God, the soul, and the afterlife. Scientific contributions focus primarily on describing related empirical facts about the universe, exploring the context and parameters concerning the ‘how’ of life.