ThruLines vs DNA Circles

The most common question I see about ThruLines is whether it uses DNA, or family trees, or both, and there seems to be a good deal of misunderstanding and confusion about ThruLines, especially in regards to how it compares to the now retired DNA Circles. The best thing I can say is that ThruLines does things with trees that DNA Circles didn't do, and DNA Circles did things with DNA that ThruLines doesn't do.

I'll start with what DNA Circles used to do.

A screenshot from Ancestry's blog of how DNA Circles
showed everyone in the group shared DNA with each other 

DNA Circles would first look for a group of people who mostly all shared DNA with each other. So let's say you shared DNA with A, B, and C and on top of that, A, B, and C all shared DNA with each other too. In addition, you did not share DNA with another person called D, but D did shared DNA with A, B, and C, and so was included in the group. So not everyone in the group had to match every single other person, but had to match enough people in the group to justify including them. Once this DNA group (or circle) was established, the system would then look for a common ancestor among your trees - and only among the trees of the people in the group. Once the common ancestor was identified, people in the group who didn't have this ancestor in their tree yet (or didn't even have a tree to begin with) would get DNA Circles in the form of "New Ancestor Discoveries". Within the tools of DNA Circles, you could see who all was in the group and who shared DNA (and who didn't) with whoever else (see above right). (Note: I think the minimum for a Circle to be created was 7 people not too closely related to each other, not 5 like the example I'm using, but I reduced it for the sake of ease).

So, DNA Circles was primarily looking at the shared group DNA and only using trees to identify the source of all that shared DNA (which is actually much how Genetic Communities work too, but I digress).

ThruLines works sort of in the completely opposite way.

ThruLines is only showing these matches descend from the
same ancestor based on trees - it does not tell you whether
they share DNA with each other or not

ThruLines looks for a common ancestor between you and an individual DNA match by looking at the entire database of (searchable) family trees. This is something DNA Circles didn't do, because it only looked at the trees of those in the Circle. So ThruLines is taking match A and looking for a common ancestor by trying to compile all the data from available trees (not just the tree of you and match A). It does this for each DNA match individually, so it then separately finds match B and C are also supposedly descended from the same ancestor and it groups A, B, and C based on what the trees say is their shared ancestor, regardless of whether these matches also share DNA with each other or not (and it doesn't even tell you if they do or not). ThruLines does not check to see if A, B, and C also share DNA with each other like DNA Circles did, and it doesn't even involve D because you don't match D. You can sometimes check to see if they share DNA with each other yourself by using the Shared Matches feature, but remember only estimated 4th cousins or closer are included in the Shared Matches list. If you share less than 20 cM with A, B, and C, you can't see whether they share DNA with each other or not.

What does this mean? It means that beyond the fact that ThruLines is only looking for tree connections with your DNA matches, DNA really isn't a part of ThruLines. The groupings are not based on DNA like DNA Circles were because ThruLines doesn't even know, much less show you if the people in the group share DNA with the others in the group or not. Knowing which of your matches also match each other is important for establishing a connection to an ancestor because remember, family trees are fallible and you can't rely on them alone, especially when there's also no known paper trail to confirm it. You need those DNA groups/circles to tie those alleged descendants together and confirm what the trees say, but ThruLines doesn't do this.

Another screenshot from Ancestry blog showing the now
retired New Ancestor Discoveries.

Another thing ThruLines doesn't do, because it's primarily working off trees instead of primarily working off DNA like Circles did, is provide you with Circles, or New Ancestor Discoveries, even if you don't have a tree attached to your test results (shown right). Because ThruLines is working off trees, it needs a starting point in your tree to connect it to other people's trees. Without at least a basic tree with your parents in it (and ideally they want at least 4 generations), you will not get any ThruLines at all. But because DNA Circles was firstly looking at groups of shared DNA, it didn't matter what your tree said or whether you even had a tree, it could tell you that you fit into that established DNA group based purely on DNA. While ThruLines does include "Potential Ancestors" not yet in your tree, this is again based on what trees say and you will not get any if you don't have a tree attached to your results. This is a drawback for adoptees and people looking for unknown biological family since they have no biological tree to add.

Hopefully that helps clarify the main differences between these two tools. They are/were both useful in their own ways, but they are different and understanding their differences is important so you're not making assumptions about ThruLines and letting it lead you astray.