![]() ![]() The term microbiome has been used to refer to the collection of genes contained within a community of microbes today, it is also used to refer to the organisms themselves. THE EARLY DEVELOPMENT OF THE MICROBIOME FIELD These advances have opened up a panoramic vista of how microbes change over space and time and how they relate to processes ranging from the physiological to the psychological. In just a decade, a typical study has advanced from collecting a few dozen sequences for each sample to collecting a few hundred million. Our ability to read out these complex microbial communities and understand their impact on human biology has been transformed by advances in technology, especially DNA sequencing and computational methods. Furthermore, unlike the fixed human genomes, these microbial gene repertoires are highly malleable, offering exciting prospects for novel therapies. ![]() The impact of this enormous number of microbial genes and cells on human biology must be profound. Microbial cells even outnumber human cells, although the early and widely reproduced estimates of a 10:1 ratio of microbial cells to human cells are overstated the most detailed report to date suggests that we are only approximately 47% human on average by cell count ( 139) (of course, because microbial cells are much smaller, this corresponds to only a couple of kilograms of microbial biomass in a typical adult). Estimates of the gene content of the microbiome from either back-of-the-envelope calculations ( 155) or empirical observations ( 64, 127) place the number of microbial genes associated with the human body at 2–20 million, exceeding the ∼20,000 human genes by at least a factor of 100. Yet the human genome, which is essentially fixed at birth, represents only a small fraction of the genetic diversity associated with the human body. The Human Genome Project was a remarkable success in government-funded “big science”: In President Obama's 2013 State of the Union address, he estimated the cost of the Human Genome Project at US$3.8 billion, with an incredible 140:1 return on investment. When we think about what defines us as a species, our thoughts naturally turn to the human genome. ![]()
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