How far back can we research our family tree? It's a question that comes up periodically, especially from beginners who are sometimes overwhelmed with finding other people's trees going back very far. In practice, the answer will vary greatly depending on your tree. One branch might dead end in the 18th century, another might go back to the 16th century, and another yet might link to royalty and date back to Charlemagne (8th-9th century). But how far back is it plausible or realistic? At what point exactly do all these trees that date back to ancient times, mythical figures, Adam and Eve, etc become impossible?
In general, the simplest answer for European research that has no known connection to royalty or nobility is that the 1500s is the end of the line. Like I say, not all your branches will likely even go back that far. Many times, the trail simply runs cold well before that point. For example, if you're American, you may never be able to find the specific origins of an immigrant ancestor. But if you're lucky, you may find a few branches here and there that go back to the 1500s.
Why the 1500s? Because that's when parish records began to be mandated in Europe. England was among the first to do so. In 1534, England separated from the Catholic Church and formed the Church of England, a protestant church, all so that Henry VIII could divorce his wife and marry his mistress. A mere 4 years later, England required that their brand new church begin keeping parish records of baptisms, marriages, and deaths/burials in 1538. Around the same time in 1540, the Lutheran Church also started requiring parish records be kept throughout their rapidly expanding churches in central Europe. In 1563 at the Council of Trent, the Catholic Church ordered that parishes keep baptism records, and in 1579, another order required marriages and deaths/burials. This meant essentially most churches in Catholic or Protestant Europe were expected to keep parish records from that point onward.
However, not all churches began adhering to these requirements right away. Many were slow to start keeping records, so depending on the location, you may not find parish records going back quite this far. In England, only 14.8% of parishes were keeping records by 1555, and that had risen to 54% by 1600. Most parishes in Italy didn't start keeping records until about 1595, but at the same time, a few Italian parishes (namely Palermo and Firenze) had taken it upon themselves to keep records long before the Catholic Church mandated it, sometimes going back as far as the 14th century! In France, general compliance wasn't until around the mid-1600s, and most Reformed churches were keeping records by 1650 as well. So in many places, you may only be able to go back to the 1600s.
Additionally, even when records were kept from this early on, not all have not survived to today. Many were damaged, lost, or destroyed over time, through natural disasters or war, or simply deteriorated over time. Some from the 16th century may have survived but there might be large gaps, making it impossible to connect the dots.
So, how far back parish records go, and whether they've survived to today or not really depends on the specific location in Europe, but in general, it's safe to say the 1500s are the furthest plausible cut off point. Unless a branch has genuine links to royalty or nobility (and there's a lot of false links out there, so be careful), or you're among one of those rare exceptions of parish records going back to the 14th century, a tree extending beyond the 1500s is probably not accurate or reliable.
That doesn't mean every tree going back to the 1500s is reliable though, just that you would have to look more deeply to determine that. As I mentioned earlier, in some cases, your trail may dead end with your immigrant ancestor. If you can't find the specific origins of your immigrant European ancestor, it doesn't even matter how far back European parish records might go. And just because parish records may go back this far doesn't necessarily mean you can use them to reliably trace your lineage. Parish records are notoriously vague, containing very little information that can often make it impossible to say for sure if the records you're looking at are for the right person you're researching. Especially when you only have access to an index and not the original documents (which is common for early parish records like this). All it takes is more than one person with the same name born around the same time and location to completely throw their identity into question. Or one ancestor moving across the country with no record of it, and having no idea where to find them. Records can be so scarce, it's safe to say that if you're not descended from a somewhat notable lineage that was better documented, like wealthy land owners, merchants, or holding some sort of position like a sheriff (not necessarily nobility), there's a very good chance you'll never be able to reliably research back as far as the 1500s, even if the parish records exist.
Now, I keep saying "unless you have a genuine connection to royalty or nobility". So what if you do? Despite the amount of false links out there to royalty, some of them are genuine, and in those cases, it is possible (likely, even) to go back much further than the 1500s. Most royal and noble lines are well documented even before parish records were kept, because their titles were inherited, so documenting their lineages, especially male lineages, was very important. How far back they go depends entirely on the lineage, but many royal lines go back to Charlemagne, who ruled much of Western Europe in the late 8th century and early 9th century. Charlemagne's ancestry has also been traced back to his 5th great grandfather, a 6th century nobleman named Ansbert, whose wife, Blithilde (or alternate spellings), has been claimed to be the daughter of Chlothar I, but this is highly debatable. Ansbert is generally considered the end of the royal line, and not all lines will go back that far.
As you can see, even royal lines only go back to about the 6th century at the most, so proving European descent from BC is just not possible. There are many theories out there, but none are proven. Any tree that goes back to BC is highly speculative at best. That's not to say the family trees of people who lived in ancient history can not be traced within Antiquity, just that there's a known genealogical gap in between Antiquity and the Middle Ages.