There are a few examples of adaptive addons that we've integrated to the point that we almost don't think of them as separate anymore. Going back really far, mi

There are a few examples of adaptive addons that we've integrated to the point that we almost don't think of them as separate anymore. Going back really far, mitochondria are eukaryotic organisms that are now just part of our cells. Gut bacteria are likewise pretty vital, especially in ruminant species. It's just comparatively rare because it's easier to benefit off of an environment non-sustainably (at least in the short term).