Overview of Origin and Evolution of Life

Key Points: Origin of life

Living organisms show basic characteristics such as metabolism, growth, reproduction, energy transformation and responsiveness.

The origin of life is still a mystery and many scientific fields have tried to explain how life began on Earth.

The theory of special creation states that all life was created by a supernatural power, but it has no scientific evidence.
The cosmozoic (panspermia) theory suggests that life came to Earth from outer space in the form of spores or microorganisms, but it does not explain the origin of life elsewhere.
The theory of spontaneous generation (abiogenesis) proposed that life arose from non-living matter, but this idea was disproved by Louis Pasteur.
The theory of biogenesis states that life arises only from pre-existing life, explaining continuity of life but not the origin of the first living organism.

Key Points: Chemical Evolution of Life

Stage	Process	Main Events	Key Scientists / Terms
Chemical evolution	Origin of life by chemical reactions	Life arose from non-living chemicals through gradual reactions over long time	Haeckel, Oparin, Haldane
Origin of Earth & atmosphere	Formation of primitive Earth	Earth formed ~4.6 billion years ago; atmosphere was reducing and lacked free oxygen	Big Bang – Lemaitre
Formation of simple gases	Chemical combination of elements	Formation of CH₄, NH₃, H₂O, H₂S from H, C, N, S	Reducing atmosphere
Simple organic molecules	Abiotic synthesis	Amino acids, sugars, fatty acids formed using UV rays, lightning, heat	“Primordial soup” – Haldane
Complex organic molecules	Polymerisation	Formation of proteins, fats, polysaccharides, nucleotides	Proteins acted as enzymes
Nucleic acids	Self-replication	RNA and DNA formed; gained ability to replicate	Basis of heredity
Protobionts / Procells	Pre-cellular structures	Coacervates and microspheres formed showing growth and division	Oparin (coacervates), Fox (microspheres)
First living cell	Origin of true cells	First cells were anaerobic, heterotrophic and chemo-heterotrophic	Protocells / Eobionts

Key Points: Organic Evolution

Meaning of evolution – Evolution means gradual and irreversible change in organisms, resulting in present-day complex forms from simpler ancestral forms.
Organic evolution – Organic evolution is a slow, continuous process through which descendants become different from their ancestors over generations.
Darwin’s view – According to Charles Darwin, evolution is “descent with modification”, where variations are inherited and lead to formation of new species.
Weismann’s contribution – August Weismann disproved inheritance of acquired characters and proposed the theory of continuity of germplasm, stating that only germ cell variations are inherited.

Key Points: Modern Synthetic Theory of Evolution

Meaning – Modern Synthetic Theory of Evolution is a combined explanation of evolution based on genetics, ecology, paleontology, anatomy and natural selection.
Major contributors – Important contributors include R. A. Fisher, J. B. S. Haldane, T. Dobzhansky, Julian Huxley, Ernst Mayr, G. G. Simpson and Stebbins.
Population concept – Evolution occurs in populations; a small interbreeding group within a species is called a Mendelian population.
Gene pool and gene frequency – The total genes present in a population constitute the gene pool, and the proportion of an allele in the gene pool is called gene frequency.
Factors causing evolution – Gene mutations, chromosomal changes, genetic recombination, migration and natural selection alter the gene pool.
Speciation – Reproductive isolation along with genetic variation and natural selection leads to the formation of new species.

Key Points: Genetic variations

Genetic variation refers to changes in genes and gene frequencies within a population, leading to differences among individuals.
Gene mutation is a sudden, permanent and heritable change in a gene or chromosome, producing new traits and phenotypic variation.
Genetic recombination occurs during sexual reproduction due to crossing over and fertilization, resulting in new gene combinations.
Gene flow is the movement of genes into or out of a population through migration, gamete transfer or DNA transfer, altering gene frequencies.
Genetic drift is a random change in allele frequencies due to chance events, especially significant in small populations.
Chromosomal aberrations involve changes in chromosome number or structure (deletion, duplication, inversion and translocation), causing genetic variation and evolutionary change.

Key Points: Origin of life