The missing apostrophe is a serious issue on many courses. We hope that the new web design has eliminated that but it may not affect pre-existing sentences.

It would be a help if you reported it directly since the moderators do not have the means to alter this:

https://support.duolingo.com/hc/en-us

Yes, I noticed it too. You're quite right. The way it's given means 'Our antecedent ancestors'.

The possessive with "of" is mostly for inanimate objects but I've included it here. It's a phrase I wouldn't use preferring the more natural "Our ancestors' past."

If you look at my comment on this page you'll see that we are aware of the issue and despite our reports to the technical staff it still hasn't been corrected. We'll keep trying.