How far does the kissing cousin branch extend? What about parents whose adult kids will never have kids? Or their only child died, and with that loss, their branch on the family tree ends.

An curved arrow pointing right. Figuring out how exactly you're related to the people in your extended family can be surprisingly tricky. Follow BI Video: On Twitter More from Lifestyle Figuring ...

And yet, this entire branch of the human family tree disappeared 40,000 years ago. Who were these cousin species of us and why did they (or could they) not survive? And most importantly ...