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It is believed that the first cellular life forms were anaerobic heterotrophs because they lacked fully functional mitochondria. For example, majority of parabasalids and diplomonads are found in surroundings that lack oxygen. These protists do not contain plastid. In addition, their mitochondria are short of DNA and enzymes required for the tricarboxylic acid cycle. They also lack an electron transport chain. In other species, the mitochondria are extremely tiny. In these species, the cytosol contains cofactors that take part in production of ATP.

A common example of a diplomonad is Girdia intestinalis got by taking contaminated water. Girdia thrives in mammals’ intestines where there is inadequate oxygen. Parabasalids include trichomonads such as Trichomonas vaginalis, which live in the vagina of female human beings (an anaerobic environment). Trichomonas vaginalis outgrow other beneficial bacteria and infect the vaginal lining under suitable conditions of acidity. Protists generally carry out three main metabolic activities, which are photosynthesis, nitrogen fixation, and respiration.

Metabolic paths in Protists involve many materials and pathways, some of which are unique to this group only. Blue-green algae or the cyanobacteria use chlorophyll a and other pigments known as phycobilins for photosynthesis. Phycobilins have two colors, phycocyanin (blue) and phycoerythrin (red). Thylakoid-like membranes hold chlorophyll a just like in plants. Purple and green bacteria are anaerobes that utilize sulphur compounds, fatty acids, and alcohols as electron donors during the energy generating processes.

Photosystem 2 in plants appeared to have evolved from photosynthesis in purple bacteria whereas, in bacteria, this system is a predecessor of photosystem 1. Prokaryotes are the only living things that have the ability to fix free nitrogen from the atmosphere. Endosymbiosis played an extremely large role in the evolution of prokaryotic and eukaryotic protists. Endosymbiosis involved unicellular organisms engulfing other cells. These cells in return evolved into organelles in the hosts’ cells.

For example, the original eukaryotes obtained mitochondria as a result of swallowing up alpha proteobacteria. Studies of all protists indicate that they all have mitochondria, or at least previously contained them. Later on, a descendant of heterotrophic eukaryotes obtained a photosynthetic cyanobacterium as an endosymbiont. This photosynthetic cyanobacterium later changed to a plastid. Red and green algae arose from this lineage. Scientific evidence of this phenomenon is the similarity in the DNA sequences of cyanobacteria and plastids in red and green algae.

These algae also went through secondary endosymbioisis through the process of evolution on a number of incidences. They were swallowed in a heterotrophic eukaryotes’ food vacuole and became endosymbionts. Chlorarachniophytes is a good example of algae that progressed from such a process. Fossils of protists and other microscopic organisms are contained in microfossils. Microfossil records indicate that prokaryotes were the earliest forms of being in the world for a long time (about 3.5 billion years ago).

Protists came after the prokaryotes at some 1.5 billion years ago. Fossil records are able to give distinctions between prokaryotic and eukaryotic cells in terms of the internal cell compositions. Zonation on a Local and Global Scale Zones are biological phenomena that