Yeast is easy. You can make bread just by leaving the grain mush out in the air; sourdough. 

The ethics of the anthropologist are questionable but the age of the girl is only questionable by someone with puritanical ethics like most Americans. Everyone in the tribe found it not only OK but massive strange that he would even consider saying no to the young woman! Ethics and imposing your ethics on another culture is a REALLY hard thing to do correctly. I still have problems in Germany and they are almost the same as us in the USA. Imagine how complicated and difficult it would be on an isolated other world!

Here in Germany my Turkish friends would think nothing of a 14 year old being married to a 40 year old without even asking the girl. I have met three German woman that were all married for more than 15 years, have kids and met their husbands at the age of 14. The men were between 2 and 8 years older than the woman. No one here thinks twice about it other than to think it is strange that they did not, "try out", a bunch of men first; they missed out on a lot of fun and perspective building experiences.