David Reich and the effect of DNA research on Archeology

In his 2018 book, David Reich makes an important point when talking about the origins of the Indo-European language:

By tracing possible migration paths and ruling out others, ancient DNA has ended a decades old stalemate in the controversy regarding the origins of Indo-European languages.  

This is then the contribution that the DNA gene tracing can make. First, it can identify possible scenarios because of the markers. At the same time it also shows some paths to be impossible. 

In the particular case of the Indo-European languages, DNA analysis of ancient skeletons turned out to be lucky; the paths that Reich et al were able to rule out (prune in the search space view of the problem) axed all paths for a specific theory.

Notice that this does not say what happened. Reich and colleagues can say that the most likely remaining hypothesis points to south of the Caucasus, either Iran or Armenia, because the population DNAs there are the most similar to what they ended up being in the European heartlands:

... ancient DNA from people who lived there [i.e. south of the Caucasus Mountains, perhaps in present day Iran or Armenia, RCK] matches what we would expect for a source population both for the Yamnaya and for ancient Anatolians.

But as Reich had argued with respect to Colin Renfrew's agricultural hypothesis of language spread, that the populations who brought the agricultural revolution had brought Indo-European with them as well, 

... theory is always trumped by data ...

by which he means that new data will rule out previous paths. Data is the way to prune the search space.

And this pruning works both way, because it is a recognition function. It recognizes what it can accept (the prediction view) and it therefore knows what to filter. Sometimes these functions are couched positively and sometimes they will be couched negatively and that can muddle up the issue too.