Being human is a gut feeling.

Some metagenomic studies have suggested that less than 10% of the cells that comprise our bodies are Homo sapiens cells. The remaining 90% are bacterial cells. The description of this so-called human microbiome is of great interest and importance for several reasons. For one, it helps us redefine what a biological individual is. We suggest that a human individual is now best described as a super-individual in which a large number of different species (including Homo sapiens) coexist. New concepts of biological individuality must extend beyond the traditional limitations of our own skin to include our resident microbes. Besides its important contributions to science, microbiome research raises philosophical questions that strike close to home. What is left of Homo sapiens? If most of our cells are not Homo sapiens cells, what does it mean to be an individual human being? In this paper, we argue that the biological individual is determined by the amount of functional integration among its constitutive parts, a definition that applies perfectly to Homo sapiens and its microbiome.