Traveling to the past can bring your Sim a new acquaintance or relative in the present.
On one trip to the past, my Sim convinced a pioneer Sim to keep going instead of heading back. When she got home, she had a new acquaintance with the same last name as the pioneer, presumably the pioneer's descendant.
A second trip had more surprising results. My Sim encountered a quarreling couple and smoothed out their relationship problems on a trip to the recent past, only to notice they seemed familiar. Before she returned, I saw a new box on the side of the screen showing the members of the household, then something shimmered beside the time machine and a child appeared! He has the same last name as my Sim and looks like her, and is her son according to the family tree (no mention of the father). I'm just glad there's a SimBot in the household too, so Social Services doesn't get on my Sim for leaving a minor unattended if she's out when he's home.