In the dim past of Earth’s long, 4.5 billion year history, perhaps 2.5 billion years ago, single celled organisms reproduced, but had no sex. Reproduction, and evolutionary change, the main function of sex, occurred only by simple cell division. In this process each cell when it reached “maturity” and with no further need for stimulation, simply divided into two equal parts…equally sharing the parent cell’s genetic material to create two almost physically identical offspring cells.
This reproduction process has been variously termed “budding”, “cell division”’ “sporulation”, etc. The end products of the process are two new growing daughter cells with little change in their physical properties (which are the expression of their genetic material). In simple cell division the only genetic change is that caused by random and probably rare DNA mutations…perhaps as a result of ionizing radiation, chemical exposure, or other random alterations to DNA during the physical division (mitosis) of the parent cell. Ionizing radiation was likely much more common on the surface of ancient Earth.
Darwinian Natural Selection depends on minor variations in the physical characteristics of each species. For species to evolve, physical variations must exist in a species, these are exposed to a trial for adaptation in the existing environment. Those that are adaptive or provide some advantage for survival are conserved while other manifestation of those variations are lost. The physical variations of species permit the potential of selection to take place. Few variations in a species tends to slow or retard speciation (formation of a new more adapted species) —and adaptation of these new species to a modified environment. If species have little physical (genetic) variation natural selection is limited. Those species best adapted to the environment survive and others perish.
As a result, the slow rate of mutagenic change or alteration of the genetic material, as in asexually reproducing species—must have resulted in corresponding slow rates of evolutionary change. Speciation -the evolution of new species—slowed down. As the Earth evolved—its physical environment changed—those species formed in earlier times, were not variable enough to adapt and thus perished. Asexual reproduction simply did not produce enough variations in physical form for biological evolution to keep pace with the slow physical alteration of the planet upon which these organisms lived.
Our planet Earth is in constant flux undergoing continual slow change in temperature, atmosphere and oceans, even the locations and positioning of the continents themselves on the Earth’s globe are in constant change. These early sexless simple organisms were likely less competent to adapt to the new physical conditions of Earth as continents moved over its surface, rove into and under each other , produced mountain ranges and continents split apart to form ocean basins. Inability to adapt resulted in death and die off of these early “sex starved” species. In this early sexless world organisms evolved only slowly as they struggled to adapt to a constantly altering planetary environment. But competion was coming.
THE DEVELOPMENT OF SEX
About two billion years ago, during the Proterozoic Era, some single celled organisms which reproduced had reprduced asexually by simple “budding” or fission evolved a more costly and complex process known as sexual reproduction. Sexual reproduction provided a survival benefit for these new cells.
In sexual reproduction the cell nucleus undergoes a complex intracellular process called a“reduction division”. The nucleus of these cells divide in a special way (meiosis) to form offspring cells or “gamete” cells which have only one-half of the genetic material of the parent cell. To form a new individual (offspring cell) these (gamete) cells must combine with other gametes of the same species (each with only half of the genetic material of a parent cell) to form a new offspring.
This sexual process is more costly to the individual, requires more time and more energy, but it provides an evolutionary advantage, by producing greater genetic variation in the gamete offspring cells (more variation ) as a result of the more complex, (more “accident prone”?) reduction division, and then the random recombination of the often free moving gametes, which often must combine in an alien environment (as in marine organisms). Sexual reproduction provides many more chances for gene alteration, intracellularly during the more complex formation of gametes, and then during the process of sexual combination of the gametes often in an alien chemically challenging extracellular environment.
All of these circumstances results in greater genetic variations and higher levels of physical diversity of offspring. As the result of these developments of the process of sexual reproduction increased the rate at which evolution progressed. Such a process favors the evolutionary selection and adaptation to a rapidly changing environment like that of planet Earth. Perhaps without sex, Earth as a planet may have altered physically over time but, with only a very primitive level of biosphere and little or no advanced living organisms. Humans may have never evolved on such a planet. The difficulty of this transmission from asexual to sexual reproduction may be one reason that our near-by known habitable planets remain radio silent and leave us seemingly very much alone in the near by universe.
That is why all paramecia and pachyderms vary both genetically and in their physical manifestation (termed the phenotype). All humans too! Diversity is a natural manifestation of almost every living organism. Its benefit lies in the necessity of evolution to operate by moulding each species to evolve into specimens better adapted to the environment in which it lives.
Sexual reproduction provided offspring cells which had greater variation than sexless cells. That greater ability to adapt to continuously changing environments gave “sexy cells” a great advantage for survival.
Thus by nature and for over two billion years all life is by natural design, unequal, and diverse. We can not change that.
So what is sex good for? Evolution. Sexual reproduction permitted evolutionary change which was rapid enough to keep pace with a rapidly changing Earth. It permitted the living earth (its biosphere) to continue to survive and to become more complex and better adapted to its changing surface.
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