Well,I recently read a theory about the origin of the Titans and it made me think their origin too.After some thoughts here's my theory:

What if Shingeki no Kyojin is in fact several thousand years in the future and during some time between now and then,for reasons unknown,they switched to biological tech,thus leading to the creation of Titans to serve as workers,becoming the new machinery,kind off like the Wraith in Stargate Atlantis.

The fact that most titans are active only during the day could mean that their skin is in fact coated with foto-voltaic cells(solar panels) and the energy is stored somewhere inside the body,possibly in the chest cavity.Also,as it was stated by Hanji Zoe,if I am not mistaken,Titans are too light for their size.A possible explanation for that would be the use of extremely advanced materials,possibly even nano machines to strengthen the molecular bonds,which could explain their regeneration capabilities.

It is quite a coincidence that where Titan "pilots" are and where they need to hit a Titan to kill it are the same.That could mean that the control node for all a Titan's functions are there and that there is its "cockpit",thus reinforcing the idea that Titans are infact human machines,so to speak,along with the fact that certain humans can transform into a Titan.

The technological gap between when the Titans where created and the present in the manga/anime,could be explained in the following way:

Sometime after their creation,Titan software(or A.I.) somehow got corrupted and made them think the humans are both enemies and source of energy(which could also mean that Titans had an alternative source of energy besides sunlight)at the same.Humans were unequipped to deal with Titans and lost the war with them,forcing humanity to run and eventually losing most of their advanced technology.

I will make a theory about Titan shifters in the future