Early puberty may be being caused by abundance. Our abundance makes our biology 'think': "you've got all this new found stable energy! Why aren't you using it to maximize biological fitness! Have kids, have kids quicker! Have more of them!"

If we weren't bio-cultural creatures - that is exactly what would happen. Whenever a species experiences a random windfall of energy - they reproduce like crazy until their population crashes.

However, what we are seeing throughout the developed world is a decline in fertility. A tremendous decline. The world fertility rate is at or below 2.4. Over 90 countries are below 2 fertility rate (replacement level is 2.1). There are over 30 countries with below 1.5 fertility rate using 2013 statistics.

In my explanation, what is happening is that cultural reproduction is supplanting biological reproduction. We are making culturally mediated decisions to not have kids. If we take my modified version of life history theory to account for cultural evolution, this can be easily explained. The more abundance we have and the longer we live, the less likely we are to biologically reproduce. If we take that to its logical extreme, a world of energy abundance and a world of radical life extension, will be a world without very many - if any - biological offspring.