This includes (i) the
formation of protobionts, (ii) the origin of living organisms from them and
their diversification.
(i)
The formation of Protobionts: From the complex organic
molecules, large colloidal aggregates called coacervates [bubble like droplets) were synthesized due to
intermolecular attractions. Some type of chemical organisers [considered free genes) developed to give these
droplets 'the ability to take-in molecules from the surroundings'. Later they
acquired lipid membranes. Some of the proteins inside them acquired the
property of enzymes resulting in the fast multiplication of molecules.
(ii)
The origin of living organisms: The 'Free Genes' started
absorbing organic molecules from the prebiotic soup and evolved into anaerobic heterotrophs approximately three to four
billion years ago. They obtained their energy by the fermentation of some organic molecules. The
earliest living organisms had clumps of nucleoproteins containing one or two
DNA molecules and were similar to the present day prokaryotes- During the course of evolution
these early prokaryotes acquired the carbohydrate-synthesis catalysing enzymes- Thus early chemo-autotrophic organisms [e.g. iron and sulphur
bacteria) which
can thrive at high temperatures evolved. These organisms could take carbon
dioxide into their bodies and used the chemical energy to create carbohydrates
and sugars. Meanwhile some bacteria synthesised bacterial chlorophyll [e.g. purple and green sulphur
bacteria) from
the magnesium
porphyrin of
ocean waters. This pigment trapped the solar energy and fixed the C02.
These were the anoxygenic photo-autotrophic organisms. Later the bacterial
chlorophyll evolved into true chlorophyll as in cyanobacteria and
plants. As a result, oxygenic photo-autotrophic organisms [like blue green algae) evolved. It resulted in the
increase of large quantity of oxygen in the atmosphere about two billion years
ago. These events transformed the reducing atmosphere into modern oxidising
atmosphere. With the availability of
free oxygen, finally aerobic mode of respiration
evolved. Later, eukaryotes evolved probably by two processes, a) Prokaryotes
lived in the ancestral eukaryotes symbiotically and evolved into organelles such as mitochondria and plastids. b) The endomembrane system of
eukaryotes might have evolved by the infolding of plasma membrane of the
ancestral prokaryotes.
