**2. Microbiome's evolution**

Microbiome are the home tract of wide range of microorganisms that can be commensal, symbiotic, or toxic to all multicellular organisms, including plants. The microbiota includes bacteria, archaea, protists, fungi, and viruses, all of which have been shown to be vital for their host's immunologic, hormonal, and metabolic balance [15]. Microbial communities live in multiple body sites in humans and animals (including the stomach, oral cavity, esophagus, skin, and vagina) and interact with and influence their hosts' immune system and metabolism. In addition, microbes have developed alongside humans and are now an essential component of life, performing a variety of essential roles. Due to changes in environmental parameters such as temperature, pH, oxygen, and nutrition availability, their composition varies greatly between body locales and specific biogeography. Although much has been done to explore its diversity, a full understanding of our microbiomes demands an evolutionary perspective. At the strain level, microbial evolution may occur (e.g., when advantageous mutations in specific genes drive adaptation to new selection pressures) selection may also enhance the frequency of a specific microbial taxon, causing the adaptive microbiome's microbial taxa to be lost [16]. The microbiome can evolve at two levels: first, each individual microbe is subjected to evolutionary processes (mutation, selection, migration, drift, speciation, etc.), and second, a host species' microbiome can evolve by incorporation and elimination of microbial taxa, or by changes in their relative abundances as a consequence of these evolutionary processes [17].

Interestingly, mammals that have independently evolved on herbivorous diet often exhibit similar microbiomes [18]; however, this is not the case of panda bears, whose microbiome resembles that of their carnivorous and omnivorous close relatives, despite the panda's herbivorous diet, probably due to phylogenetic constraints [19]. The compositional overlap between the gut microbiota of species populations in the western hemisphere correlates with their geographic proximity in most mammals, and each geographic location has a distinct microbiome composition that is not attributable to the diets or evolutionary histories of the mammals living there, suggesting that horizontal transmission also shapes the microbiome [20]. Because one species and its associated microbiome serve as the meal for the paired predator, this link is most visible in sympatric predator-prey groups. The structure of the relationships in primate species is unknown, but they are likely to follow some of the same patterns.

It is important to remember that the microbiome is a complex and dynamic ecosystem and multiple overlapping factors shape the microbiome composition and it is unique in each individual, and the differences among individuals are largely compared to the typical biochemical differences within a person over time. The gut microbiota is shaped by a variety of factors, including genotype,

dietary composition and mode of delivery, recreational drugs, antibiotic therapy, pre and probiotic treatment, lifestyle (e.g., smoking and physical activity), social interactions, and environmental exposure to various xenobiotics. In addition, several other factors are also involved including (i) Diet. The types of food that a person consumes can have a significant impact on gut microbiota. (ii) Exposure to pathogens, (iii) Age, (iv) Psychological Stress/Anxiety, (vi) Medication/Drug Use, (vii) Tobacco Use, and Alcohol Consumption (vii) Physical Activity [21]. One important factor emerging from the research advances is the importance of microbial diversity. In healthy settings, an individual's microbiota is more diverse than in sickness, when diversity is diminished. Low microbiome diversity has been linked to metabolic inefficiency, skin issues, gastrointestinal problems, and low-level inflammation.

Because of the biological interaction of the organisms with the immune system throughout time, the indigenous organisms in the human body are well adapted to the immune system. A shift in the gut microbial flora plays a crucial impact in human health and disease pathogenesis. These changes are caused by a combination of factors, including lifestyle and the existence of an underlying disease. Dysbiosis makes the host more susceptible to infection, the type of which varies depending on the anatomical place. The precise metabolic activities and functions of these microorganisms within each bodily location are accounted for by the inherent diversity of the human microbiota. As a result, it's critical to comprehend the human microbiome's microbial composition and behaviors as they relate to health and disease. The microbiome can affect many physiological processes in our body, including immune system development, the ability to process dietary polysaccharides, vitamin and hormone production, pH regulation, processing and detoxification of environmental chemicals and maintenance of the skin and mucosal barrier function [22, 23]. There has been a boom of research into how the microbiota of the gastrointestinal system affects human health and disease, and what treatments might be made, particularly in the last decade (**Table 1**).


#### **Table 1.**

*In the human body, the most common bacterial phylum [24].*
