Evolution is technically defined as "a gradual process in which something changes into a different and usually more complex or better form." As it is most famously used, "evolution" is the process by which an organism becomes more sophisticated over time and in response to its environment. The Theory of Evolution is currently the most popular concept of how life reached its current state.
The origin of life is a scientific problem which is not yet solved. There are plenty of ideas, but few clear facts.
It is generally agreed that all life today evolved by common descent from a single primitive life form. We do not know how this early form came about, but scientists think it was a natural process which took place perhaps 3,900 million years ago. This is in accord with a philosophy called naturalism: only natural causes are admitted.
We do not know whether metabolism or genetics comes earlier. The main hypothesis which supports genetics first is RNA world hypothesis, and the one which supports metabolism first is Protein world hypothesis.
Another big problem is how cells develop. All existing forms of life are built out of cells.
The Nobel Prize in Chemistry winner Melvin Calvin wrote a book on the subject, and so did Alexander Oparin. What links most of the early work on the origin of life was the idea that before biological evolution began there must have been a process of chemical evolution.
Another question which has been discussed by J.D. Bernal and others is the origin of the cell membrane. By concentrating the chemicals in one place, the cell membrane performs a vital function.
Current models: There is no "standard model" on how life started. Most accepted models are built on molecular biology and cell biology:
1. Because there are the right conditions, some basic small molecules are created. These are called monomers of life. Amino acids are one type of these molecules. This was proved by the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953, and we now know these basic building blocks are common throughout space. Early Earth would have had them all.
2. Phospholipids, which can form lipid bilayers, a main component of the cell membrane.
3. Nucleotides which might join up into random RNA molecules. This might have resulted in self-replicating ribozymes (RNA world hypothesis).
4. Competition for substrates would select mini-proteins into enzymes. The ribosome is critical to protein synthesis in present-day cells, but we have no idea as to how it evolved.
5. Early on, ribonucleic acids would have been catalysts, but later nucleic acids are specialised for genomic use.
The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life is thought to have formed are:
* Methane (CH4),
* Ammonia (NH3),
* Water (H2O),
* Hydrogen sulfide (H2S),
* Carbon dioxide (CO2) or carbon monoxide (CO), and
* Phosphate (PO43-).
Molecular oxygen (O2) and ozone (O3) were either rare or absent.
Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution from molecules to cells
Bernal suggested that evolution may have commenced early, some time between Stage 1 and 2.
a. Darwin's Theory of Evolution:
The Premise: Darwin’s Theory of Evolution is the widely held notion that all life is related and has descended from a common ancestor: the birds and the bananas, the fishes and the flowers -- all related. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival -- a process known as "natural selection." These beneficial mutations are passed on to the next generation. Over time, beneficial mutations accumulate and the result is an entirely different organism (not just a variation of the original, but an entirely different creature).
Natural Selection: While Darwin's Theory of Evolution is a relatively young archetype, the evolutionary worldview itself is as old as antiquity. Ancient Greek philosophers such as Anaximander postulated the development of life from non-life and the evolutionary descent of man from animal. Charles Darwin simply brought something new to the old philosophy -- a plausible mechanism called "natural selection." Natural selection acts to preserve and accumulate minor advantageous genetic mutations. Suppose a member of a species developed a functional advantage (it grew wings and learned to fly). Its offspring would inherit that advantage and pass it on to their offspring. The inferior (disadvantaged) members of the same species would gradually die out, leaving only the superior (advantaged) members of the species. Natural selection is the preservation of a functional advantage that enables a species to compete better in the wild. Natural selection is the naturalistic equivalent to domestic breeding. Over the centuries, human breeders have produced dramatic changes in domestic animal populations by selecting individuals to breed. Breeders eliminate undesirable traits gradually over time. Similarly, natural selection eliminates inferior species gradually over time.
Darwin's Theory of Evolution Slowly But Surely: Darwin’s Theory of Evolution is a slow gradual process. Darwin wrote, "Natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps. "Thus, Darwin conceded that, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." Such a complex organ would be known as an "irreducibly complex system". An irreducibly complex system is one composed of multiple parts, all of which are necessary for the system to function. If even one part is missing, the entire system will fail to function. Every individual part is integral. Thus, such a system could not have evolved slowly, piece by piece. The common mousetrap is an everyday non-biological example of irreducible complexity. It is composed of five basic parts: a catch (to hold the bait), a powerful spring, a thin rod called "the hammer," a holding bar to secure the hammer in place, and a platform to mount the trap. If any one of these parts is missing, the mechanism will not work. Each individual part is integral. The mousetrap is irreducibly complex.
Darwin's Theory of Evolution - A Theory in Crisis: Darwin's Theory of Evolution is a theory in crisis in light of the tremendous advances we've made in molecular biology, biochemistry and genetics over the past fifty years. We now know that there are in fact tens of thousands of irreducibly complex systems on the cellular level. Specified complexity pervades the microscopic biological world. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10 - 12 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand million atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world."
And we don't need a microscope to observe irreducible complexity. The eye, the ear and the heart are all examples of irreducible complexity, though they were not recognized as such in Darwin's day. Nevertheless, Darwin confessed, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection seems, I freely confess, absurd in the highest degree."
b. Theory of Germplasm :
Friedrich Leopold August Weismann was a German evolutionary biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin. Weismann became the Director of the Zoological Institute and the first Professor of Zoology at Freiburg.
His main contribution was the germ plasm theory, at one time also known as Weismannism, according to which (in a multicellular organism) inheritance only takes place by means of the germ cells the gametes such as egg cells and sperm cells. Other cells of the body somatic cells do not function as agents of heredity.
The effect is one-way: germ cells produce somatic cells and are not affected by anything the somatic cells learn or therefore any ability the body acquires during its life. Genetic information cannot pass from soma to germ plasm and on to the next generation. This is referred to as the Weismann barrier. This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck.
c. Mutation Theory of Evolution by Hugo Devries:
He conducted his experiments on Oenothera Lamarckian, (Evening Primrose) and found several aberrent types. When O. Lamarckian was self-pollinated and its seeds were allowed to grow, majority of F1 plants were similar to the parents, but a few were different plants.
The different plants were also self-pollinated and when their seeds were sown, the majority of the plants were similar to the parents while a few were still more different plants and this continued generation after generation. These plants appeared to be new species, Hugo de Vries suggested from his experiments that new types of inherited characteristics may appear suddenly without any previous indication of their presence in the race.
Hugo de Vries believed that mutation causes evolution and not the minor heritable variations which was mentioned by Darwin. Mutations are random and directionless while Darwin’s variations are small and directional. According to Darwin evolution is gradual while Hugo de Vries believed that mutation caused species formation and hence known as saltation (single step large mutation).