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![Figure 1 is an illustration that indicates that the outer layer is related to the inner layer and features a symbol of Earth and a stick figure of a human. The following information is given: Two nested layers of biodiversity: Outer layer dependent on the environment we live in (including air, soil, natural waters, plants, and animals). Inner layer inhabits the human body [internal (airways, gut) and external microbiota (skin)], dependent on colonization from the outer layer.](https://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=11218706_ehp13948_f1.jpg)
Layers of biodiversity.6
