Gabrielle Kendall Di Salvo Family trees merge because we can usually trace a branch of our family tree to noble families and royal family, as their descendants eventually become commoners.
Many historical people who we know existed have tales of their ancestry, to link them to biblical characters or mythological characters.
It's your choice to decide if you're going to add those branches to your personal family tree or not. As you get to choose which story you tell about your ancestry.
Some of us enjoy adding these tales, as it's the ancestry our ancestors believed in. Be it believing they were descendant of the Gods, or descendant of Adam & Eve. It's all fun.
In the end, we all know we are descendants of an ape-like hominoid that lived over 7 million years ago. We also know from DNA, all modern humans are descendant from a DNA Eve who lived around 150,000 years ago, and a DNA Adam who lived around 275,000 years ago.
Personal Family Trees should be fun in my opinion. We don't use them to make claims on land ownership like Dukes, Counts, and Kings use to do. So have fun with it. Decide how far back you want your tree to go.