The onset and progression of the salivary and gut microbiota, the transmission and the impact of the salivary microbiota on the development of early fecal microbial communities was herein explored. We characterized the microbiota of 82 faecal and 80 salivary samples, collected from 82 healty newborns at birth, 7, 15, 30, 90 and 180 days of life, by 16S rRNA targeted-metagenomics approach. Correlation heat-maps and co-occurrence networks were used to investigate microbial taxa relationship in saliva, gut and between the two ecosystems. In saliva microbiota, Streptococcus and Staphylococcus appeared as early commensals, dominating this ecosystem through the time, while Fusobacterium, Prevotella, Porphyromonas, Granulicatella and Veillonella were late colonizers. anaerobes as Enterobacteriace, Staphylococcus and Streptococcus, were gut microbiota pioneers, followed by the anaerobic Bifidobacterium, Veillonella, Eggerthella and Bacteroides. Streptococcus, Staphylococcus and Veillonella were shared by gut and saliva ecosystems (core microbiota). Early saliva and gut microbiota seem to evolve independently driven by local adaptation strategies, with the only exception for the oral Streptococcus and Veillonella genera, involved in gut microbiota development as seeding species. A more comprehensive knowledge of how oral microbiota may impact pathophysiological conditions of gut microbiota may open new avenues on the design of postbiotics.