Origin & Evolution of Life in Earth

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Biogenesis. ... Conceptually, biogenesis is primarily attributed to Louis Pasteur and encompasses the belief that complex living things come only from other living things, by means of reproduction. That is, modern life does not arise from non-living material, which was the position held by spontaneous generation.

The theory of recapitulation, also called the biogenetic law or embryological parallelism— often expressed in Ernst Haeckel's phrase as "ontogeny recapitulates phylogeny"—is a largely discredited biological hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages in the evolution of their remote ancestors. With different formulations, such ideas have been applied and extended to several fields and areas, including the origin of language, religion, biology, cognition and mental activities,anthropology,education theory and developmental psychology.[4] While examples of embryonic stages show that molecular features of ancestral organisms exist, the theory of recapitulation itself has been viewed within the field of developmental biology as a historical side-note rather than as dogma.

In contrast, there is no consensus against the validity of the theory outside biology. Recapitulation theory is still considered plausible and is applied by some researchers in fields such as the study of the origin of language,cognitive development, behavioral development in animal species.
The Haeckelian form of recapitulation theory is considered defunct.However, embryos do undergo a period where their morphology is strongly shaped by their phylogenetic position, rather than selective pressures.

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THEORY OF ATAVISM:
Early embryos of various species display some ancestral feature, like the tail on this human fetus. These features normally disappear in later development, but it may not happen if the animal has an atavism.
Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways. One way is when genes for previously existing phenotypical features are preserved in DNA, and these become expressed through a mutation that either knock out the overriding genes for the new traits or make the old traits override the new one. A number of traits can vary as a result of shortening of the fetal development of a trait (neoteny) or by prolongation of the same. In such a case, a shift in the time a trait is allowed to develop before it is fixed can bring forth an ancestral phenotype.

In the social sciences, atavism is a cultural tendency—for example, people in the modern era reverting to the ways of thinking and acting of a former time. The word atavism is derived from the Latin atavus. An atavus is a great-great-great-grandfather or, more generally, an ancestor.

Homology is a relationship defined between structures or DNA derived from a common ancestor. Homologous[Etymology 1] traits of organisms are therefore explained by descent from a common ancestor. The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in the last common ancestor but rather evolved separately. An example of an analogous trait would be the wings of bats and birds, which evolved independently in each lineage separately after diverging from ancestors with forelimbs not used as wings (terrestrial mammals and theropod dinosaurs, respectively).

It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons. In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods. Homology can also be described at the level of the gene. In genetics homology can refer to both the gene (DNA) and the corresponding protein product. It has been hypothesized that some behaviors might be homologous, based on either shared behavior across related taxa or common origins of the behavior in an individual’s development, though this remains controversial.
Anatomical homology:

The wings of pterosaurs , bats and birds are analogous as wings, but homologous as forelimbs.
Evolutionary ancestry means that structures evolved from some structure in a common ancestor; for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development; the ovaries of female humans and the testicles of male humans are homologous in this sense.

Homology is different from analogy, which describes the relation between characters that are apparently similar yet phylogenetically independent. The wings of a maple seed and the wings of an albatross are analogous but not homologous (they both allow the organism to travel on the wind, but they didn't both develop from the same structure). Analogy is commonly also referred to as homoplasy, which is further distinguished into parallelism, reversal, and convergence.

From the point of view of evolutionary developmental biology (evo-devo) where evolution is seen as the evolution of the development of organisms, Rolf Sattler emphasized that homology can also be partial. New structures can evolve through the combination of developmental pathways or parts of them. As a result, hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and shoots.

A Giant Feroceous Elephant attacked on Primitive men,but they fought together with that with their own created Stone Arms and Fire..

With Stone Primitive men created a lot of sharp arms for save themselves and also stop the feroceous animals' attack..these all were created with full of hard stone,and its front side was made like too sharp..for this reason animals easyly wounded and leave them..

INDUSTRIAL MELANISM HYPOTHYSIS : - The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon. However, because of widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees that peppered moths rested on became blackened by soot, causing most of the light-coloured moths, or typica, to die off from predation. At the same time, the dark-coloured, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.

Since then, with improved environmental standards, light-coloured peppered moths have again become common, but the dramatic change in the peppered moth's population has remained a subject of much interest and study, and has led to the coining of the term industrial melanism to refer to the genetic darkening of species in response to pollutants. As a result of the relatively simple and easy-to-understand circumstances of the adaptation, the peppered moth has become a common example used in explaining or demonstrating natural selection.

These all stones were the arms of Primitive men in their early life...then they couldn't know how were the arms created(by those stones/other things,like metal)...???
So then they used just scatter stones for defeated those ferocious animals,who were attacked on....

"Fight of Primitive men for livelihood"--
Primitive men fought with ferocious animals for their own safety & also lived in their living place with completely unworried...
Here some dangerous animals attacked on those primitive men..but they(men) also fought with those with their arms so bravefully...