Carl Zimmer in The New York Times:
In the pageant of life, we are genetically bloated. The human genome contains around 20,000 protein-coding genes. Many other species get by with a lot less. The gut microbe Escherichia coli, for example, has just 4,100 genes.
Scientists have long wondered how much further life can be stripped down and still remain alive. Is there a genetic essence of life? The answer seems to be that the true essence of life is not some handful of genes, but coexistence. E. coli has fewer genes than we do, in part because it has a lot fewer things to do. It doesn’t have to build a brain or a stomach, for example. But E. coli is a versatile organism in its own right, with genes allowing it to feed on many different kinds of sugar, as well as to withstand stresses like starvation and heat. In recent years, scientists have systematically shut down each of E. coli’s genes to see which it can live without. Most of its genes turn out to be dispensable. Only 302 have proved to be absolutely essential. Those essential genes carry out the same fundamental tasks that take place in our own cells, like copying DNA and building proteins from genes. And yet the 302 genes that are essential to E. coli turn out not to be life’s minimal genome. Scientists have come up with lists of essential genes in other microbes, and while the lists overlap, they are not identical.