Is there any role of bacteria in evolution of animals and plant?

Is there any role of bacteria in evolution of animals and plant? If micro-organisms play important role in survival of host, why would they haven't play a key role in the evolution of animals and plants?

This is very interesting question.....right? But It's difficult too, as the answer deserves enough scientific explanation. So, today we'll answer this question with solid scientific proofs.

The present universe has its own history, thus the evolved plants and animals. Evolution is the lengthy process of change, scientific evidence shows that the physical and behavioral traits shared by all organisms originated from ancestors and evolved over a large period of time. There are numerous theories talking about the evolution of Homo-sapiens and plants from their ancestors, but these theories didn't take account of micro-organisms who might have played a key role during this evolution. 

The process of evolution involves a series of natural changes that cause species to arise, adapt to the environment, and either reproduce or extinct, in all these evolutionary processes microbes have helped the hosts by providing  their genomic capabilities.

Today I am trying to explain 'Hologenome Theory Of Evolution' given by Ilana Zilber-Rosenberg & Eugene Rosenberg in 2008. http://www.ncbi.nlm.nih.gov/pubmed/18549407). Which describes possibilities that enables us to believe,  "YES, micro-organisms have played and still playing a vital role in the evolution and development of animals and plants"

 Basically hologenome is the combined genome of host and micro-organisms (symbiont) and this combination gives unique properties to the super-organism (host + symbiont) that are far better than host and symbiont (microbes) alone.

The 'fact' or 'belief' is accepted only when it passes through criticism and cross validation, so lets start criticism with hologenome theory. 

If we consider 'hologenome theory is invalid and thus Animals and Plants evolved without aid of Microorganisms, then as per mutation rate and long generation time, animals and plants would have taken a longer period of time to be evolved. Evolution of any species depends on the availability of new genes, generation time, and inheritance of new characters to the off-springs. These all characteristics can be accomplished by the host only when it is associated with micro-organisms as microbes possesses high mutation rate, short generation time and transferability between generations.

The most important feature of this symbiosis could be the diversity of micro-organisms that introduces host with novel genes, it ultimately bestows super-organism with novel characteristics, this might have helped the host during adaptation (evolution).

It is mentioned in the literatures, the number of bacterial cells and their sum of genome often exceeds that of their hosts. Particularly in case of Homo sapience, microbes adds 5,00,0000 genes to the host and out these, 250,000 are the unique genes.  Now one can guess, lack of this symbiont can make a species become extinct. Actually under fluctuating environmental conditions, microbes provide 'insurance policy' i.e. enough time and essential nutrients to the host to adjust with the changing environment. (Proc Natl Acad Sci USA 96: 1463–1468).

Below I am elaborating Principles of hologenome theory of evolution and beautiful examples of symbiosis that may help to get the concept more clear.

Human-microbes symbiosis: As the partnership between host and symbiont is established it must be transferred to the next generation in order to get the benefits. In case of humans, mother before and during pregnancy transfers the gut and skin microbiota to fetus, also during breast-milk feeding gut microbiota of fetus takes shape and helps in the milk digestion. Further throughout the life these bacteria plays a key role in digestion and maintaining healthy state of the individual.

The Aphid-Buchnera symbiosis: Aphids are small sap-sucking insects and passively feeds on the plant sap. This diet is unbalanced and lacks essential amino acids. Like all the animals Aphids are unable to synthesize amino acids and here's the role of microbes called Buchnera comes. This bacteria is present within the cells of Aphids and synthesize essential amino acids, in return asks for shelter and food. During reproduction Aphids lays the egg and transfer this symbiont to the offspring (egg).  

Termites-strategy:   Termites use 'Fecal Microbiota Transplant' technique (FMT) to transfer gut microbes to the juveniles. The worker Termite community fed feces of adult termites to the new born to continue the symbiotic benefits.

Ruminant and rumen microflora: Vertebrates like cows, buffalo lack the ability to hydrolyse the beta 1-4 glycosidic bond of plant cellulose due to the lack of the enzyme cellulase. In the rumen, where microbial flora is abundant, ferment cellulose and other metabolites into short chain fatty acids (SCFA), these SCFA are absorbed into the blood stream and circulates to the whole body. Some of the bacteria and undigested food are passed to lower digestive system where hosts enzymes degrade bacteria and food material. These dead microbes are good source of essential amino acids and vitamins. The offspring (bovine) of these herbivores inherits microbiota during passage through birth canal and feeding on grass contaminated by the feces or saliva of the parents.

Plants: During vegetative reproduction the fragment of the plant which falls on the earth to start a new journey, it falls by having its microbiota share from the mother plant. Needless to mention all of us knows the symbiosis of plant-root system and nitrogen fixers.

Squids and Vibrio fischeri:  Vibrio fischeri microorganism considered as a light organ of the squid. Squid is a deep-ocean dwelling organism and to hunt the food it uses the fluorescence produced by these bacteria as a light source. Everyday at the dawn time squid releases large amount of Vibrio fischeri to ensure the availability of this bacteria to the juveniles.

I think these pretty examples are sufficient to believe hologenome theory and thus the fact that microbes indeed have played a key role in the evolution of humans and plant.

In addition to this bacterial ability of horizontal gene transfer and host ability to introduce variation into holobiont are the major points to be discuss.