what are some scientific theories of life?

The Wikipedia article on Abiogenesis is a good starting point: http://en.wikipedia.org/wiki/Abiogenesis

Although Earth was created around 4.5 billion years ago, life began to exist not long after. Our solar system was still young, and the sun was still cooling down after its formation. The circumstances of our solar system and our planet gave rise to life, the eagerness of life to exist was apparent from the beginning.

Due to the volatile and volcanic nature of the young Earth, various chemical reactions began to take place which would result in the creation of new compounds and elements. One of the family of compounds created over time were the amino acids, organic compounds which are the building blocks of life.

Earth today bears little resemblance to the early Earth, in which life developed from it’s raw ingredients: volatiles and organics. Under­standing the mechanisms and the prebiotic chemical processes that lead to life, however, is complicated.

The first origions of life would have been caused by the conditions of the enviornment, and the organic chemistry. The earliest life may have derived from self-replicating RNA molecules, which began to develope inside enclosing membranes which provided a stable physical and chemical environment conducive to their replication: proto-cells.

It is suggested that heterotrophs were the beginnings of life on Earth, inhabiting the sea within organic soup they developed in, it also being an energy source. Around 3 billion years ago, autotrophic cells had evolved. Autotrophs are capable of synthesising energy from inorganic material, the sun and elements on the earth.

Around 2 billion years ago protists began to emerge. These organisms were single celled like the other organisms but they were notably bigger, probably taking advantage of an ecological niche. Protists obtain nutients or engolf other organisms. Unlike the other organisms, the protists contained cell organelles, which meant that a fundamental difference in the way life operated had evolved. Previous species were more simple in their nature, and did not possess nuclei or such a complex cellular structure.

Eukaryots (complex cells) evolved-> that evolved into multicellular life-> that evolved into simple metazoa -> that evolved into complex metazoa Fish evolved -> that evolved into amphibians -> that evolved into reptiles Reptiles evolved into dinosaurs -> small tree dwelling dinosaur being a common ancestor of birds Reptiles evolved into mammals

3.8 billion years of simple cells (prokaryotes), 2 billion years of complex cells, 1 billion years since multicellular life, 600 million years since simple metazoa, 550 million years since complex metazoa, 500 million years since fish and proto-amphibians, 475 million years since land plants, 400 million years since insects, 360 million years since amphibians, 300 million years since reptiles, 232 million years ago dinosaurs evolved, 200 million years since mammals,
85 million years since primates, ? million years since birds, 65 million years dinosaurs died out, 2.5 million years since the appearance of the genus Homo, 250,000 years since Homo sapiens evolved

Evolution occurs throguh the cumulative changes over generations of a population of species. Evolution can happen through mutation, gene flow, genetic drift, coevolution, sexual selection and natural selection.

Evolution was slow at the beginning of life, but natural selection would give species’ a competitive advantage over others and with the arrival of prey/predator animals. Life adapted to its environment, diversified, and filled available niches, as opposed to competing with one another. The fittest survived and contributed their genes to their offspring, producing a population better adapted to the environment.

Homo sapiens evolved around 250,000 years ago. The earliest primates evolved around 85 million years ago, http://en.wikipedia.org/wiki/Human_evolution#Before_Homo. Some of our recent ancestors include Australopithecus, Homo habilis, and Homo erectus.

http://en.wikipedia.org/wiki/Timeline_of_evolution http://www.biology-online.org/tutorials/9_evolution_origins.htm