Formation of the Earliest Cells

Formation of the Earliest Cells

(i) The first living organisms originated among organic molecules and in oxygen free atmosphere (reducing atmosphere). They presumably obtained energy by the fermentation of some of these organic molecules. They were anaerobes, capable of respiration in the absence of oxygen. They depended on the existing organic molecules for their nutrition and thus they were heterotrophs.
(ii) When the supply of existing organic molecules was exhausted, some of the het- erotrophs might have evolved into autotrophs. These organisms were capable of producing their own organic molecules by chemosynthesis or photosynthesis.
 (a) Chemosynthesis. The organisms performing chemosynthesis are called chemoauto- trophs. They were anaerobic. Chemoautotrophs developed the ability to synthesise organic molecules from inorganic raw materials. Such a mode of nutrition is present even now in some bacteria, e.g., sulphur bacteria, iron bacteria, nitrifying bacteria.
(b) Photosynthesis. The photosynthetic organisms, the photoautotrophs, developed the pigment chlorophyll by combination of simple chemicals. They prepared organic food by using solar energy captured with the help of chlorophyll. They lacked the biochemical pathways to produce oxygen. They were still anaerobic and utilized hydrogen from sources other than water. At later stage, oxygen releasing photosynthetic organisms developed. These were similar to the existing blue green algae (cyanobacteria). They used water to get hydrogen and released oxygen. Addition of 02 to the atmosphere started oxidising the methane and ammonia, which began to disappear.