Complex sentence: Many of these contemporaries had direct descendants, but not all of them left an unbroken link of descendants all the way down to today's population: in other words, some of these contemporaries are ancestors of no one in the current population.
Split into simpler sentences: Many of these contemporaries had direct descendants, but not all of them left an unbroken link of descendants all the way down to today's population. In other words, some of these contemporaries are ancestors of no one in the current population.