A better way to word this question would probably be “what happened by 11,000 BC?” That is because this book is not trying to rehearse all of the things that happened before that date. Instead, it is saying that a few important things happened by that date. This makes 11,000 BC, as the book says on p. 35 of the paperback edition (the beginning of Chapter 1)
A suitable starting point from which to compare historical developments on the different continents…
By 11,000 BC, Diamond says that two important things had happened. First, all of the continents of the world (with the exception of Antarctica, which is still not really inhabited) had been colonized by human beings. He recognizes that there is some dispute over the date by which people had come to the Americas, but he says that it is undisputed that they were peopled by 11,000 BC. Therefore, he uses that date. Second, as he says, “village life” had begun in a few parts of the world and agriculture would begin in some parts of the world “within a few thousand years.”
In this book, Diamond is interested in why some continents became more developed than others. He argues that agriculture was the main force that led some continents to develop faster than others. Since continents were populated and agriculture was going to begin relatively soon, 11,000 BC is a good point to start his discussion.
So, what happened by 11,000 BC is that there were people on all the continents of the world and agriculture was soon to begin.