Darwin’s unanswered question
Darwin’s unanswered question is: when did life begin on earth? One thing is for sure, life could not start earlier than the formation of earth. Earth was formed about 4.5 billion years ago. Darwin discussed origin of species, but he was not sure about origin of life. Fossils are undetected for first 3.8-4 billion years. The oldest stromatolite containing microorganisms (photosynthetic bacteria) determined to be ~ 3.5 billion years old. Life ends with fossil, not start with. So when did life start?
The idea of proto-life
If we assume life did not result from a seeding event from an asteroid or other interstellar body, then life had to have developed from inorganic materials. However, spontaneous animation to form life from non-life does not happen today. This could be due to the atmosphere environment is very different. Urey & Miller designed an experimental model of early earth atmosphere, in which organic molecules can be synthesized from inorganic molecules. With pre-biotic conditions set, Proto-life may occur according to many hypotheses: Organic compounds form polymers of repeating units, lipids & others can and do organize into spheres based on inherent properties. The lipid spheres can encapsulate organic molecules; the encapsulated organic molecules such as RNA can then self-replicate, are autocatalytic, and can act as rudimentary templates for protein synthesis. Competition for scarce resources such as RNA monomers leads to natural selection of “fittest” RNA molecule, resulting in reproduction of that RNA molecule.
When did life start?
Currently nobody knows how life starts from proto-life or when proto-life was evolved into life. Standard answer depends on fossil record as noted earlier. More specific date depends on answers to following theoretical questions: 1) When transition from proto-life to life occurred; 2) Definition of life used to determine transition.
80% of the history of life on earth concerns prokaryotes (Monera). Prokaryotes evolved in isolation for ~ 2 billion years. Most Prokaryotic adaptations revolve around metabolic capacities as opposed to phenotypic changes. The major metabolic adaptations include glycolytic pathway, oxidative phosphorylation, photosynthetic enzymes, and each new metabolic capacity enabled bacteria to occupy new niches. For prokaryotes, most important speciation event is between archaebacteria & eubacteria.
Major groups of protists include protozoan – for example, amoebas, algae and slime molds. Major evolutionary acquisition of protists is through endosymbiosis which results in eukaryotic organelles, and colony behavior which results in multicellular organisms. Endosymbiotic hypothesis is an attempt to account for the major evolutionary leap from prokaryotes to protists, the first eukaryotes. The hypothesis proposes a larger prokaryote ingested a smaller prokaryote such as a nutshell, organelles such as mitochondria and chloroplasts therefore arose. Much evidence comparing similarities of mitochondria to prokaryotes supports this idea.
All plants are multicellular eukaryotes with the capacity for photosynthesis. Plants are evolved from aquatic protists. The steps of adaptation include cuticles to prevent drying, developing system of specialized cells such as vascular conduits for more efficient water transport. These provide new niches further away from water sources. Early terrestrial plants such as ferns were seedless, gametes were dispersed via spores. One reproductive adaptation is evolving of seeds which carry plant gametes further from parents and therefore increased opportunities for uncontested resources. Evolution of flower (a specialized leave structure) enables more efficient seed dispersal and pollination secondary to pollinators.
Fungi Evolved from protists via unknown mechanisms. Traits of fungi include branched hyphae, cell wall if chitin, dual reproduction modes, and absorption of food.
Animals are evolved from ancestral protists. Following the evolutionary history come Eumetazoa, Bilateria, Coelomates, Deuterosomes, Chordates, and Vertebrates, all of which have a major body plan difference. One major cause of body plan change is paedogenesis, which is the act of reproduction by an organism that has not achieved physical maturity. The hypothesis is that paedogenesis of early larval urochordates leads to maintenance of motile organism with all chordate features which caused major change in body plan. Selective pressures acting on paedogenic larval urochordates lead to modifications of chordate traits. Other events during evolution: 1) Extinction events: There have been several in the triassic period alone, not the least of which being the dinosaur extinction. Some estimate that 90% of all animal species were lost during this time period with an unknown reason. 2) Super continent Pangaea forms and breaks-up ~300 – 180 million years ago, this lead to geological isolation of gene pool.