Evolution (Gk. e out, volvere to roll) is unrolling or unfolding of nature that brings about an orderly change from one form or condition to another resulting in descendents becoming different from ancestors. Evolutionary biology is the study of the history of development of newer forms of life from the pre-existing ones in various periods of time on earth. Life has evolved into many forms since its origin on earth.
ORIGIN OF LIFE
Origin of life or biopoiesis is the development of living matter from complex organic molecules that are themselves non-living but self replicating. Before the origin of life, the origin of the universe must be considered. Most accepted theory to explain the origin of the universe is the big-bang theory which was proposed by Abbe Lemaitre in 1931. According to this theory, the universe had an explosive beginning. Universe originated about 20 billion years ago by a big bang (thermonuclear explosion) of a dense entity. Then the universe expanded and hence the temperature came down. Hydrogen the elementary atom fused into progressively heavier atoms of different kinds of elements found today. To explain the origin of life, carlier it was believed that some special power created life. It is called the theory of special creation.
According to another hypothesis called Nebular hypothesis which was given by Kant (1755)-Laplace (1796), our solar system was probably created about 4.5 to 5 billion years ago when the gaseous cloud called solar nebula was formed
Another theory termed as cosmozoic theory was also formulated. According to it, primitive life or panspermia came to earth from some other planet in the form of microbes or spores. But the extreme environment of interstellar space must have killed these panspermia before reaching earth. Hence this theory was discarded. Theory of spontaneous generation believes that under certain conditions nonliving substances give rise to living beings spontaneously. It was supported by many old scholars 1like Thales, Anaximander, Aristotle, Plato, Epicurus, etc.
However, living beings are neither produced spontaneously nor created. Instead, life comes from pre-existing life or omne vivum vivo. The phenomenon is called biogenesis (Gk. bios-life, genesis birth). Biogenesis was proved by the work of three scientists Redi (1668), Spallanzani (1767) and Pasteur (1867).
Louis Pasteur (in l1864) used swan-neck flasks and prepared a meat broth in these flasks, and boiled them for several hours.
He then left the flask unsealed on a laboratory bench and there was a free exchange of air with the environment, so the system did not lack oxygen Still, the flask remained free of microbial contamination for months.
This experiment thus disproved the concept of spontaneous generation completely.The modern hypothesis of origin of life was formulated by Haeckel. This idea was elaborated in the chemie theory (in 1920s) by two workers independently: a Russian biochemist A.l. Oparin and an English biologist J.B.S Haldane. It was summarized by Oparin in his book" The Origin of Life" published as an English edition 1938 .
Oparin and Haldane state that
•Spontaneous generation of life under the present environmental conditions is not possible.
• Earth's surface and atmosphere during the first billion years of its existence were radically different from today's conditions.
Earth's initial atmosphere was a reducing one Abiogenesis occurred about 3.7 billion years ago.The Oparin-Haldane theory (also called Protobiogenesis was experimentally supported by Stanley Miller in 195 and separated the entire process into the following steps: origin of Earth and its primitive atmosphere; chemical evolution (chemogeny) and biological evolution. The conditions on earth were high temperature
volcanic storms, reducing atmosphere containing C NH, etc. In 1953, S.L. Miller, an American scientist student of Harold C.Urey created similar conditions on a laboratory scale. He created an electric discharge 1 closed flask containing CH, H, NH, and water vapor at 800°C. He observed formation of amino acids Similar experiments others observed, formation Sugars, nitrogen bases, pigment and fats. Analysis meteorite content also revealed similar compound indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of conjectured story, i.e., chemical evolution was more less accepted.
The synthesis of carbohydrates, fats and amino acids and other complex organic substances probably occurred in the sea which had been described by Haldane as' The hot dilute soup'. The formation of protein molecules is considered a landmark in the origin of life.
PROTOCELL MODELS
Proteinoids These are polypeptides synthesized by heating a mixture of aminoacids from 160 to 210°C for several hours.
The large organic molecules which were synthesized abiotically on primitive earth later came together, and due to intermolecular attraction, they formed large colloidal aggregates. Such water bound aggregates have been named microspheres by Sydney Fox.
Coacervate is an aggregate of macromolecules, such as proteins, lipids, and nucleic acids, that form a stable colloid unit with properties that resemble living matter. Many are coated with a lipid membrane and contain enzymes that are capable of converting such substance as glucose into
more complex molecules, such as starch.
Biogenesis: The principle that a living organism can only arise from other living organisms similar to itself i.e. that like gives rise to like) and can never originate from nonliving material. After the origin of life in the form of giant molecules, self replicating aggregates of macromolecules like nucleic acids and proteins, biological evolution started. It resulted in formation of various complex biological forms that we can see today.