Photosynthesis is the process by which plants, algae and some bacteria make their own food using only sunlight and compounds found in the surrounding air and water. With this development of autotrophism , nutritional sources could then increase or decrease with the needs of the cells, dependent only on basic environmental factors. All life forms are dependent on the nutrition derived from plants and algae in some way.
All life is dependent on photosynthetic organisms for the continued release of free oxygen into the air. When life began, the atmosphere did not contain free oxygen. Photosynthesis releases oxygen into the air through the dark reactions and makes it available for other life to use. As autotrophs multiplied in numbers, more oxygen was released. They continue to sustain the atmosphere we breathe now. Bands of the oxidized mineral iron found in rocks that date back to 3 billion years ago serve as proof for scientists to know that oxygen was developing in the air from photosynthesis.
The first living cells probably existed alone for 2 billion years in an aquatic environment. Photosynthesis developed in these aquatic organisms during this time, first using the pigments chlorophyll a and phycobillins to produce their nutrients, and then developing accessory pigments such as chlorophyll b. Aquatic autotrophs continued to diversify and develop into algae and bacteria with better ways of using sunlight filtered through the water.
Organisms were able to grow from single cells with no distinct nucleus into organized bodies through the process of endosymbiosis, which is when a bigger heterotrophic cell engulfs a small, autotrophic bacterial cell. The heterotrophic cell did not digest the bacteria, but instead used the nutrients made by photosynthesis to sustain a symbiotic relationship. After this first engulfment occurred, the big cell may then be invaginated by an even bigger cell, creating the mitochondria, or the powerhouses of cells. Scientists believe this to be the mechanism by which all life originally evolved from unicellularity to become multicellular with specialized organization of tissues.
The move of autotrophs to land, which occurred approximately about 425 million years ago, involved both benefits and risks. This move would not have been possible if not for the free oxygen products emitted from the original aquatic autotrophs, which produced a protective ozone layer (O3) to filter out the dangerous ultraviolet radiation. Plants evolved very specialized organs, such as leaves, to condense the chlorophylls and transport carbohydrates. Unfiltered sunlight also necessitated the development of mechanisms for filtering sunlight, such as the carotenoid pigments and the tissues for regulating the uptake and release of oxygen and carbon dioxide.