"Rewilding" the modern, industrialized human gut microbiota to improve health without careful consideration of microbial evolution could be risky, argue Rachel Carmody and colleagues in a Perspective. The human gut microbiota has co-evolved with humans over millions of years. However, there is growing evidence that humans' shift to industrialized ways of life has resulted in rapid, health-diminishing changes to the microbiome. To reverse this, some have proposed restoring aspects of the gut microbiota to its ancestral, preindustrial state through interventions such as replacing lost gut microbial taxa, engineering microbes to perform depleted functions or transplanting whole gut microbial communities from donors in nonindustrial societies. However, according to Carmody et al., while an altered microbiota may foster disease, it does not necessarily mean that health will improve upon restoring it to a preindustrial or ancestral microbial state; even problematic host-microbiota interactions could reflect important adaptive responses we've made to our rapidly changing industrialized society, they say. Here, the authors discuss how restoring the preindustrialized microbiome could be ineffectual or problematic and suggest the need for a deeper understanding of what defines a health-promoting gut microbiota that is matched to the modern lifestyle and environment of industrialized peoples. "To most effectively manipulate the gut microbiota in the service of health, the challenge is to disentangle which aspects of health are promoted by matching the microbiota more closely with the host, to the environment, or, to a lesser extent, to both," write Carmody et al. "It is clear that restoration will require a scalpel, not sledgehammer approach."