When do livings acquire the ability to think?

[scilover89]

If I am not wrong, the most primitive life is in molecular form. Somehow, they acquire the ability to duplicate themselves, and get better and better through evolution.
But, how, and when do livings acquire the ability to think?(In other words, acquire a brain)

[iansmith]

Nervous cell have started to evolved as early as in the Medusa but it is mostly for sensory reason rather than any complex behavior.

The simplest central nervous system started to appear in worms. The brain is involved in some complex behavior; however, the brain is not required for most of the daily activities.

Most complex brain appeared in insects and it has allowed insect to have complex social behaviors. The brain of invertebrates and vertabrates are quite distinct as they do not have the same evolutionnary path. Insect brain appeared before the vertebrate brain. So we may assume that insect were probably the first to have a thinking brain.

We should also try to define what is thinking.