A living organism, whether it is a microscopic bacteria or a giant animal, is a very complex and highly organized entity. The living cell exists at a far more organized level as compared to its surroundings. It is stable as well as delicate at the same time. It is able to maintain itself by a constant expenditure of energy. The energy is obtained from its surroundings. So it is required to keep unhindered communication with its environment which is at a lower level of organization. So, (while keeping active communication with the environment,) its organization is constantly under stress.

Environment may be considered as indispensible to the idea of life as the body of the organism. An individual may be visualized as body-environment complex.

Living beings are programmed to maintain and perpetuate themselves. Elementary awareness of the life cycles of plants and animals makes you realize that life is defined by the functions performed by an organism. Such functions are broadly divided into two categories. First, metabolic functions keep the individual alive. Second, reproductive function perpetuate the species to which it belongs, by leaving behind progeny after its death It is as if it is entrusted with the task of perpetuating itself both as an individual and as a species.

Celebrated author William Leonnordo observed, “An individual’s motivation is to maximize its own reproductive success. Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass on those genes. On a broader canvass reproductive success includes the social and cultural traits besides intrinsic biological potentiality of the individual and its species. Reproduction as is well known involves sex. It is safe and logical to conclude that sex as a means to reproduction has arisen by the process of natural selection during evolution. Sex originated and evolved so that the act of reproduction can be accomplished efficiently. Instances of reproduction without involving sex are observed in many primitive organisms, which indicate sex being an outcome of the process of natural selection.

This article attempts to draw a hypothetical account of the origin and evolution of sex.

A survey of the plants that appeared early during the process of evolution provides clue to a hypothesis of the origin and evolution of sex.

The story of origin and evolution of sex is a classical story of the ways natural selection operates in the process of evolution. Variations are commonplace and random too in any population. They provide material to natural selection to act upon. Such variations are favoured and selected, which provide the organism better capacity to survive i.e. have survival value. In other words, these are adapted best to the surroundings.

Fluidity is found in plant kingdom with regard to sexuality. So we may look here for clues for origin and evolution of sex. The earliest organisms lived in water and multiplied by fragmentation; the plant broke into several fragments and each fragment grew to become a whole individual. There was occasional scarcity of water leading to remarkable retardation of metabolic activities accompanied with structural sluggishness. On return of favourable conditions it regains its original structure and germinates into a new organism. This is a sort of rejuvenation. The individuals in the next generation have enhanced vigour. Thus the experience gives advantage to the species. Evolution preserves and acts upon such advantages.

These reproductive units swim/ float in the water prior to germination, In course of random movements they collide among themselves. The collisions sometimes result in fusion of two or more cells (gametes). The resulting fusion product of two units proves to be stable and fertile, while those between more than two are unstable and sterile. So natural selection favours the zygote (duplicate product). The next generation arising out of the germination of the zygote is more robust. So it is favoured by natural selection.. It involves the fusion of two units in the act of reproduction. The whole scheme may be called incipient sexual reproduction.

It is observed that fusing gametes may be similar or dissimilar. In case they are dissimilar, the larger one has less motility as compared to the smaller one; this difference increases the probability of fusion between two gametes. Such dissimilarities are raw materials of natural selection. It follows a simple law of nature. Let us consider hypothetical options. One, two persons move randomly to meet each other. Second, one of the two is stationery while the other one moves randomly to seek it. The probability of meeting is greater in the latter event. Natural selection acts on this premise.

A hypothetical model of the evolution of sex may be suggested. Difference of size as well as motility between the two fusing gametes gets pronounced. The larger gamete eventually becomes completely immobile while the smaller one gets still smaller retains mobility accompanied with increase in numbers. Ultimately they are completely different in morphology as well as physiology. Now the larger one is recognizable as ovum or egg while the smaller one is sperm or male gamete. Retention of the ovum and formation and liberation of sperms in large numbers is a mechanism evolved by natural selection. The sperms seek the ovum which waits passively but eagerly for one of them; several may be able to be there, though only one is lucky enough to fertilize the ovum. The ovum bearing organ develops certain features that attract sperms towards it. The large number of sperms is ensures the probability of the sperm fertilizing the ovum. Retention of ovum and its fertilization in situ ensures protection and nutrition to the developing embryo. The ovum bearing female sex organ and the sperm bearing male sex organ were in due course located on separate individual bodies recognised as female and male individuals respectively. The segregation ensured greater variability leading to better adaptability.

With advance in civilization and culture sex got delinked with reproduction in human beings. Concept of gender is a prominent feature of modern societies. Gender describes the characteristics that a society or culture delineates as masculine or feminine while sex refers to biological differences; internal and external sex organs. So while your sex as male or female is a biological fact that is the same in any culture, what that sex means in terms of your gender role as a 'man' or a 'woman' in society can be quite different cross culturally. These 'gender roles' have an impact on the health of the individual.

In sociological terms 'gender role' refers to the characteristics and behaviours that different cultures attribute to the sexes. What it means to be a 'real man' in any culture requires male sex plus what our various cultures define as masculine characteristics and behaviours, likewise a 'real woman' needs female sex and feminine characteristics.