Describe the evolution of microbes.

Describe the evolution of microbes

When the earth formed some 4.6 billion years ago, it was a lifeless, inhospitable place. A billion years later it was teeming with organisms resembling blue-green algae. How did they get there? How did life begin?

Important dates in the history of life –

4.6 billion years ago – origin of the earth
3.5 – 4.0 billion years ago – origin of life on earth
3.5 – 2.0 billion years ago – prokaryotes dominate
2.7 billion years ago – oxygen accumulates in atmosphere
2.0 billion years ago – eukaryotic life begins
1.2 billion years ago – multicellular eukaryotes evolve

Earth was formed when a very large, hot star exploded thus creating a new star (the sun) and other components of the galaxy (Madigan, Martinko & Parker 2003). Millions of years passed before the first life forms evolved on the planet. During this period, conditions on earth were very different from that of today. A reducing atmosphere was present containing very little oxygen and it is thought that the environment had high levels of hydrogen, ammonia and methane present ().

In a reducing environment such as this, it is thought that the origin of life was most certainly possible. The joining of simple molecules through chemical bonds into more complex larger molecules can occur in a reducing environment. The joining of molecules would however require an intense energy source. The most important energy source available on primitive earth was probably UV radiation from the sun. Lightning, volcanic activity and meteorite bombardment was much more intense than today and would have also provided energy sources (Campbell (1996).

The prevailing view is that life developed from nonliving (abiotic) materials that became ordered into molecular aggregates that were eventually capable of self-replication & metabolism. The most widely accepted hypothetical scenario is that the first organisms were the products of a chemical evolution and occurred in four stages

Abiotic synthesis and accumulation of monomers such as nucleotides and amino acids (small organic molecules that are building blocks).
The joining of monomers into polymers e.g., amino acids → proteins and nucleotides → nucleic acids.
The formation of protobionts – aggregation of molecules into droplets. These droplets are stable and have chemical environments that differ from their surroundings.
Origin of heredity either before or after protobiont appearance.

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Protobionts can exist in various forms –

Microspheres – self assembled drops of proteinoids, coated by a selectively permeable protein membrane
Liposomes – contain lipids that ...

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Abiotic synthesis and accumulation of monomers such as nucleotides and amino acids (small organic molecules that are building blocks).
The joining of monomers into polymers e.g., amino acids → proteins and nucleotides → nucleic acids.
The formation of protobionts – aggregation of molecules into droplets. These droplets are stable and have chemical environments that differ from their surroundings.
Origin of heredity either before or after protobiont appearance.

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Protobionts can exist in various forms –

Microspheres – self assembled drops of proteinoids, coated by a selectively permeable protein membrane
Liposomes – contain lipids that form a lipid bi-layer similar to that found in cell membranes. Sometimes engulf smaller liposomes and then split.
Coarcervates – droplets that primarily contain polysaccharide, nucleic acids and polypeptides.

Laboratory experiments can be used to support evolutionary theories - they indicate that these larger molecules can form spontaneously under conditions that simulate the early environment on earth ().

It is speculated that as the earth cooled, water condensed to form lakes and oceans. The molecules were carried by water into these lakes to form a primordial soup – a breeding ground for the assembly of lipids, proteins and the nucleic acid RNA.

Archaea living in

Volcanic vent

Another theory is that the more complex molecules were formed not in lakes but in hydrothermal vents. Comparisons of selected molecules suggest an early split of prokaryotes into the archaebacteria (live in extreme environments) and all other prokaryotes, the eubacteria.

However, it is agreed that proteins, nucleic acids and lipids provided the raw materials for the first cells. Observations in laboratory experiments show that lipid molecules can join together forming spheres that represent structures similar to

plasma cell membranes. It is believed that after millions of years

of collisions between molecules, the lipid spheres had enclosed RNA molecules within. These would have been ancestors to the first prokaryotic cell (Campbell 1996).

Life could not exist until a collection of specific catalysts appeared that could promote the synthesis of more catalysts of the same kind. It is believed that early stages in the evolutionary pathway centred on RNA molecules. These present specific catalytic surfaces and also can potentially duplicate through the formation of a complementary RNA molecule. Therefore it was assumed that a small RNA molecule arose that would have been able to catalyse its own duplication. This kind of RNA duplication would have been far from perfect, so many variant RNA molecules would have arisen. At some point during the evolution of biological catalysts the first cells were formed ().

The oldest evidence for life comes from fossils of filamentous and spherical prokaryotes found in structures called stromatolites. These are made up of fossilised microbial mats built from filamentous prokaryotes. The layers are made up of sediment that sticks to the mucus like coats of the microbes. These then migrate out from the layer of sediment to form a new one and thus creating a banded structure (Campbell 1996).

Cross section of fossilised stromatolite living stromatolites in Western Australia

showing banded structure

The nature of these fossils and the composition of the rocks in which they are found indicate that fermentative modes of metabolism were the first to evolve in early prokaryotes, and that anoxygenic photosynthesis (non oxygen producing) developed later – around 3 billion years ago. Oxygenic photosynthesis (oxygen producing) arose in the cyanobacteria. The result was a gradual build up of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen for aerobic respiration, a more efficient ATP producing process than fermentation ().

These prokaryotic cells were the only form of life on earth for approximately 2 billion years, before much larger, more complicated eukaryotic cells evolved.

It is thought that eukaryotic cells may have evolved from primitive prokaryotes around 2 billion years ago through a process known as endosymbiosis, first proposed by Lynn Margulis in the 1960’s. Endosymbiosis is where one cell lives inside another cell, the host cell in a mutually beneficial relationship. Molecular studies of the bacteria like DNA and ribosomes in chloroplasts and mitochondria indicate that chloroplast and mitochondrion ancestors were once free living bacteria. Chloroplasts are probably the descendents of cyanobacteria – photosynthetic prokaryotes. Ancestors of mitochondria are thought to have been bacteria that were aerobic heterotrophs ().

(Campbell 1996)

They may have possibly gained access to the larger host cell in the form of undigested prey and then each performed mutually beneficial functions.

Measuring the evolutionary distance between each taxonomic group – prokarya archaea and eukarya can be achieved by analysis of the nucleotide sequences of ribosomal RNA’s. Ribosomal RNA is very useful in evolutionary studies for several reasons –

It is evolutionary ancient
No living organism lacks it
Plays the same role in translation in all organisms
It has evolved slowly enough that groups of organisms posses sequence similarities. (Purves, Sadava et al 2001)

It is the degree of similarity in ribosomal RNA sequences between two organisms that indicates an evolutionary relatedness. From this, phylogenetic trees are produced to show the most probable evolutionary position of organisms relative to one another.

From these studies, it is thought that there was an early split of the prokaryotes into the archae bacteria that live in conditions similar of primitive earth and all other prokaryotes. Some studies propose that archae bacteria may be more closely related to eukaryotes. (Madigan, Martinko & Parker 2003)

However, there is still controversy over prokaryotic phylogeny in that genes have been moving amongst prokaryotic species by a process called lateral gene transfer. In time the genes transferred become inherited by the recipient and will be recognised as part of the normal genome of the descendant. Therefore new phylogenetic patterns will continue to emerge.

The study of evolution is difficult because it started after an event that occurred several billion years ago. It is clear that life originated from non-living matter, but scientists can only really speculate what happened. Experiments in laboratory situations simulating primitive earth can give us clues as to what may have occurred. The study of evolution will continue to bring forward new ideas and theories as scientific knowledge is increased.