When it comes to evolution, people tend to focus on the big driving force of natural selection, which latches on to helpful mutations while purging the harmful ones. But there are other processes that change the frequencies of mutations—everything from random drift to the founding of small isolated populations.
Looking at our own species' history, we would expect to see some of this in action. After modern humans established themselves in Africa, smaller populations branched out to establish footholds in Asia before spreading east, eventually reaching the Americas. At each step, a small group of migrants took a fraction of humanity's genetic diversity with it, creating a series of population bottlenecks.
This should be easy to see in our DNA, but so far it has turned out to be complicated. Different attempts using distinct populations and methods have come to mixed conclusions about whether a clear signal is there. Now, a large international team of researchers has gone and sequenced genomes from multiple populations along humanity's route out of Africa, and they found a signature of these bottlenecks both in terms of genetic variation and in terms of potentially harmful mutations.