How to start:
After making your Adam and Eve sim, go into edit town and evict/delete all the other sims from their homes so they are no longer in your world (if you know a more effective way to get rid of all the other sims please let me know in the comments ) Then move your Adam and Eve sim's into an empty lot.

Goal: You are the only two sims alive (not counting NPC's) the only thing you can do now is to survive together and bring more sims into the earth. To complete this challenge you must have at least 20 kids to fill in more households (this may be done in multiple generations)

Rules:
Your may start with 100 simoleons
No cheating AT ALL! (well unless there's a glitch)
you may use the grocery store to buy produce for food and gardens
you may not buy a fridge until one of your sims gains at least 1 logic skill
your sims may not get a job (earn money by fishing, selling produce, etc.)
If you need to continue through more then one generation then the last baby born will continue the legacy
do not purposefully use NPC's for any reason
your sims must start on an empty lot
sims may not attend school (i suggest deleting the school overall in edit town mode)

If you have any questions, or suggestions please let me know hope you enjoy

You could always place Adam and Eve in an empty world from the exchange. And maybe combine this with the Nothing is Free challenge? For a harder challenge?

Irish, there is a challenge very similar to what you said also called the Adam and Eve Challenge where you unlock different lots. It's hosted on TS3 forums but also has a FB page.

That being said, I might try this one out and keep everyone living from the land. Though, very tempted to do it with only the founders.