## 3 Microbiome Evolution throughout Life  
During pregnancy and up to the moment of birth, the human being is composed only of its somatic cells and is kept in a sterile environment. However, after birth, the newborn is exposed to a wide variety of microorganisms such as bacteria, fungi, and viruses, among others, many of which are provided by the mother during and after passage through the vaginal canal, an ecosystem heavily colonized by a relatively limited set of bacteria, or shortly after a cesarean section by microorganisms from the mother's skin. Thus, early on, the construction of a positive microbiome for future life begins [1, 19, 45, 68]. A new microbial ecosystem begins to form in the body of the newborn, developing from the first days of life and becoming essential in the processes of immune recognition and tolerance. Its profile depends on the type of delivery (vaginal or cesarean) and the feeding method (breastfeeding or formula feeding), creating a wide range of microbiome compositions. Therefore, the interindividual variation in intestinal microbial diversity is greater among infants than among adults [64, 68]. The beginning of human microbiome development starts during birth, with the colonization of microorganisms from the environment. However, there are recent studies defending that colonization starts before birth [93, 94]. In the first hours of life, the microorganisms present in the mother's vaginal and fecal microbiota are typically the most-important sources of inoculation. The type of delivery can lead to differences in microbiome development, which may later contribute to variations in the host's normal physiology or even predisposition to diseases [68]. When a normal delivery occurs, the newborn will be colonized by bacteria present in the mother's vaginal canal, such as _Lactobacillus_, _Prerotella_, and _Sneathia_. On the other hand, in the case of a cesarean section, the microorganisms that will colonize the newborn will be similar to those found on the mother's skin, namely _Staphylococcus_, _Corynebacterium_, and _Cutibacterium_. As a result, bacterial communities only differentiate over the years with the emergence of new species and their evolution, making them distinct from one another [95, 96]. During the first months of a lactose-based diet, different formula feedings will influence the quantity and diversity of microorganisms found in the newborn's microbiome. Breakfeeding predominantly fosters bacteria such as _Staphylococcus_, _Streptococcus_, _Constridium_, _Bacteroides_, and _Bifidobacterium_. The latter is responsible for protecting the baby, as they are highly adapted to process the milk's oligosaccharides, metabolizing sugars into acids and contributing to competitive inhibition with other bacteria by adhering to the intestinal mucosa. This protects against intestinal pathogens [97]. On the other hand, the number of _Bifidobacterium_ is significantly lower in infants fed with formula, whereas other bacteria such as _Escherichia coli_ and _Bacteroides_ predominate. Thus, it can be said that breast milk is fundamental in preventing dysbiosis and potentially more-complicated diseases in the future [77]. Not only do these two factors influence the type of microbiome found in newborns, but depending on the geographical region where a certain baby is located, they will present a completely different microbiome, reflecting the environmental impact and sanitary conditions of the area. The use of antibiotics also heavily influences the infant's microbiome, creating significant disparities among infants [75; 98]. The introduction of solid foods and weaning signify a shift in the ecosystem of microorganisms, leading to an increasing resemblance to the microbiome of an adult. Thus, in the first year of life, the gastrointestinal tract progresses from a nearly sterile state to an extremely dense colonization, ending with a community very similar to that found in the adult gastrointestinal tract.