What is meant by Prokaryotic evolution?

The term evolution means the gradual change or a transformation from a simple or primitive state to a complex or advanced state. Prokaryotic evolution is a concept that involves the evolution of the form and function of most primitive organisms (prokaryotes) through several stages and changes to form diverse life as a part of organic evolution.

What are Prokaryotes?

Prokaryotes are the most primitive living organisms on Earth. Bacteria are one of the most well-known prokaryotic organisms. Because of the absence of internal membranes, prokaryotes lack a distinct nucleus and other organelles. Prokaryotes are distinguished from eukaryotes by the absence of membrane-bound internal organelles. The prokaryotic cell cytoplasm contains the protein-synthesizing ribosomes and a circular double-stranded deoxyribonucleic acid (DNA). Many prokaryotes also contain additional circular DNA molecules known as plasmids.

Prokaryotes include profoundly different types of organisms that share many characteristics, such as small size, absence of meiosis, formation of a distinctive cytoskeleton, and genome structure. Fast growth, reproduction, and high rates of evolution are the major characteristics of prokaryotes. Prokaryotes can grow faster and have shorter generation times due to three factors: a small genome, simplified morphology, and reproduction through binary fission.

Prokaryotic evolution

During the early Earth, before the evolution of eukaryotic cells, the most primitive prokaryotes and the basic cellular structure of life evolved. Modern prokaryotes are most presumably descended from about 100 lineages that existed on Earth two billion years ago. Although some lineages appear to be represented by only a few small clades, others have undergone significant diversification.

Prokaryotic evolution differs profoundly from that of eukaryotes. For example, mutations that are unusual in eukaryotic populations are prevalent in prokaryotic populations. As a result, the possibilities for diversification are proportionately greater. Prokaryotes are classified into two phylogenetic domains: Bacteria and archaea.

Origin of life and the first Prokaryotes

It is generally held that life appeared on Earth abiogenically nearly 3,500-3,600 million years ago. Russian biochemist Oparin put forward his influential theory in 1924. British biologist John Burdon Anderson Haldane published a hypothesis on the origin of life on Earth in 1929. It is now known as the Oparin-Haldane theory of the chemical evolution of life. 

Its major postulates are the following: 

• Formation of simple organic micro molecules from inorganic constituents. 
• Formation of complex macromolecules by the condensation polymerization of macromolecules.
• Organization, chemical modification, and functional transformation of macromolecules to form complex living molecular systems (coacervates).
• Transformation of living molecular systems to primitive living organisms (prokaryotes). 

Bacteria and archaea in prokaryote evolution

The evolution of prokaryotes has resulted in more complex living organisms. This evolution may have resulted in the great diversity of bacteria and archaebacteria. Bacteria evolved into relatively new species with diversified structures and functions as they altered their structures to enlarge their territory and tolerance. Bacteria are classified into kingdoms based on their distinct characteristics.

Bacteria and Cyanobacteria

Bacteria are one of the phylogenetic domains of prokaryotes. They are simpler than archaea due to the absence of advanced structures that are found in archaea (presence of introns and complex ribonucleic acid (RNA) polymerase).  However, bacteria have cell membranes and distinct life functions. They can exist in colonies or alone, in various shapes. Some bacteria form a protective endospore to withstand adverse conditions, allowing the cell to remain dormant and viable until ideal conditions appear.

Cyanobacteria (blue-green algae) are interesting organisms because they are photosynthetic and are considered responsible for the transformation of the primitive reduced environment into oxygen-containing atmosphere. Anabaena is a genus of cyanobacteria that blooms in nutrient-rich aquatic environments. Anabaena blooms are used as an indicator of environmental quality. 

Archaea

Archaebacteria are the ancient bacteria that probably represent primitive forms of life. They continue to live under the conditions of the primitive Earth. They include three groups: Methanogens, halophiles, and thermoacidophiles.

• Methanogens: These are found among the flora of the cattle rumen. These produce methane in biogas fermenters.
• Halophiles: Halophiles are organisms that grow in saltwater environments. They can utilize light energy to produce adenosine triphosphate (ATP).
• Thermoacidophiles: They occur in hot sulphur springs. At a temperature of around 80^oC and under aerobic conditions, these oxidize sulphur to sulphuric acid. 

Archaeal structures are more closely connected to eukaryotes than bacteria, such as RNA polymerase and tRNA (transfer ribonucleic acid) nucleotide sequences. They have evolved complex proteins, lipid molecules, and carbohydrates that allow them to survive and reproduce in extreme environments where no other organisms can survive.

Evolution of Prokaryotes into Eukaryotes

Eukaryotic cells are more complex than the first formed prokaryotic cells. Fossil records indicate that prokaryotes evolved more than 3,500 million years ago, while eukaryotes first appeared late in the Pre-Cambrian period, about 1500 million years ago. During the interval between the appearance of prokaryotes and eukaryote cells, close contact between different prokaryotes might have taken place frequently.

Regardless of the exact mechanism involved, the emergence of eukaryotic cells led to a dramatic increase in the complexity and diversity of life on Earth. At first, organisms were capable of existing only as independent single cells. Later, some evolved into multicellular organisms in which various cells became specialized for different functions. These multicellular forms became adapted to life in a great variety of environments.

Endosymbiotic hypothesis

The endosymbiotic hypothesis was proposed by Lynn Margulis. This theory explains the evolution of eukaryotes and their evolutionary relationship with prokaryotes. Symbiosis is an intimate and long-term association between two different species of organisms. It is argued that natural selection might have favored such associations early in the history of life. Thus, the mitochondria and chloroplasts of the present day might have been the endosymbionts (internal symbionts) of yesterday. If this is true, it becomes all the more clear that endosymbiosis resulted in the evolution of membrane-bound cell organelles and this, in turn, paved the way for the evolution of eukaryotes.

The first step is thought to have occurred when a large anaerobic amoeboid prokaryote ingested a small aerobic bacterium and stabilized its prey as an endosymbiont rather than digesting it. This aerobic bacterium became a mitochondrion, the site of aerobic respiration in eukaryotic cells. The formation of mitochondria helped the host to become aerobic. Flagella might have arisen through the ingestion of spirochete-like prokaryotes. Ingestion of prokaryotes which somewhat resemble the present-day cyanobacteria or blue-green algae, could have resulted in an endosymbiotic origin of chloroplasts in a fashion similar to the formation of the mitochondrion. 

The suggested pieces of evidence in support of the hypothesis are:

• Mitochondria and chloroplasts are similar to bacteria and cyanobacteria in morphology and size.
• Chloroplasts and mitochondrion are self-replicating organisms with DNA that is more similar to that of prokaryotic cells than eukaryotic cells.
• Ribosomes in these organelles are smaller than those in the cytoplasm of eukaryotic cells and are similar in size to the ribosomes of prokaryotic cells.

The DNA of mitochondrion and chloroplasts is much smaller than bacterial DNA, and some organellar proteins of mitochondria and chloroplasts are coded by nuclear genes. This also suggests that the endosymbionts must have transferred, over time, some of their genes to the host nucleus and thus relinquished their independence for the sake of the symbiotic relationship.

Horizontal gene transfer

Horizontal gene transfer is the process of transmission of genetic material into another cell. It is a major mechanism in the evolution of prokaryotes (bacteria, archaea, and unicellular eukaryotes) as it can cause the replacement of a gene or introduce a new gene in the genome. It acts as a component of any evolutionary synthesis. It is the transfer of genetic material from one organism to another, and it serves as a source of genetic changes within the recipient organism. Gene transfer takes place among and between domains: from archaea to bacteria, bacteria to archaea, bacteria to eukarya, and archaea to eukarya, which causes their evolution by genetic changes. The study of horizontal gene transfer using different gene trees is a way of explaining the evolutionary relationship between organisms, especially between eukaryotes and prokaryotes.

Context and Applications

This topic is significant in the exams at school level, graduate, and post-graduate levels, especially for

• Bachelor of Science in Zoology/Botany.
• Master of Science in Zoology/Botany.

Practice Problems

Question 1: The organelles _____ and ___ are endosymbionts.

1. Mitochondrion and flagella
2. Chloroplast and mitochondrion
3. Endoplasmic reticulum and flagella
4. None of the above

Answer: Option 2 is correct.

Explanation: The organelles chloroplasts and mitochondrion are endosymbionts. These organelles in eukaryotic cells show a symbiotic relationship.

Question 2: The endosymbiotic hypothesis explains _____.

1. Evolution of eukaryotes
2. Evolutionary relationship of eukaryotes with prokaryotes.
3. Both 1 and 2
4. None of the above

Answer: Option 3 is correct.

Explanation: Endosymbiotic hypothesis explains both the evolution of eukaryotes and the evolutionary relationship of prokaryotes.

Question 3: The study of horizontal gene transfer using different gene trees is a way of explaining ______.

1. Natural selection
2. Evolutionary relationships
3. Endosymbiosis
4. None of the above

Answer: Option 2 is correct.

Explanation: The study of horizontal gene transfer using different gene trees is a way of explaining the evolutionary relationships between organisms.

Question 4: The two domains into which prokaryotes are categorized are _____.

1. Bacteria and eukarya
2. Archaea and eukarya
3. Bacteria and archaea
4. None of the above

Answer: Option 3 is correct.

Explanation: Prokaryotes are classified into two domains: Bacteria and archaea.

Question 5: Gene transfer can occur between _____.

1. From archaea to eukarya
2. From bacteria to eukarya
3. From archaea to bacteria
4. All of the above

Answer: Option 4 is correct.

Explanation: Gene transfer takes place among and between organisms of different domains.