The answer can be anywhere, according to this exchange on The Open Notebook, partly funded by NASW. Nature features editor Brendan Maher: "It will rarely be one single piece of evidence, but rather one or two things heard in passing (i.e. reading a paper, or talking with a trusted regular source at a meeting, or having a random conversation on a plane, or seeing a single line in a news story that makes you go, “Huh. I wonder if there’s something more to that!”).
.png)
