where does our first normal flora come from
Gut flora In humans, a gut flora similar to an adult's is formed within one to two years of birth.[4] The gastrointestinal tract of a normal fetus is considered sterile, but microbial colonisation may occur in the fetus[41] and Lactobacillus and Bifidobacterium species were present in placental biopsies in one study.[42] During birth and rapidly thereafter, bacteria from the mother and the surrounding environment colonize the infant's gut.[4] As of 2013, it was unclear whether most of colonizing arise from the mother or not.[4] Infants born by caesarean section may also be exposed to their mothers' microflora, but the initial exposure is most likely to be from the surrounding environment such as the air, other infants, and the nursing staff, which serve as vectors for transfer.[41] During the first year of life, the composition of the gut flora is generally simple and it changes a great deal with time and is not the same across individuals.[4]
Great Oxygenation Event Oceanic cyanobacteria, which evolved into coordinated (but not multicellular or even colonial) macroscopic forms more than 2.3 billion years ago (approximately 200 million years before the GOE),[7] are believed to have become the first microbes to produce oxygen by photosynthesis.[8] Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE started when these oxygen sinks became saturated, at which point oxygen produced by the cyanobacteria was free to escape into the atmosphere.