It is no wonder that the composition of blood of a rat and a person coincides, the structure of body tissues also coincides; moreover, experiments have shown that rats, like humans, have abstract thinking, at least, in a number of situations. About it in more detail you can read on Internet.
All these data are not so striking, so how mammals evolved from Theriodont (medium-sized lizard) about two hundred million years ago, and the branches of the rodents and the primates diverged about 30 million years ago.
The advantage of the rat is the fact that it has evolved about three times faster than a human.
Grey rats can run with speed up to 9.6 km/h, jump up to the height of 80 cm, swim 2 km, dig the long burrows, arrange the refuge practically in any place. They are unusually adaptive, unpretentious and capable to give birth up to 23 cubs. In the wild the rats seldom live till 1 year, but they live in house conditions about 3 years.
Rats communicate among themselves with very large number of various signals in the ultrasonic range.
Rats are capable to think, as has shown a number of experiments, no worse than primates.
Rats are capable to understand value of a number of words and to use them on sense according to a situation. Rats are also capable to copy the human speech.
Rats can generalize a number of situations and use the abstract forms, matured in their heads.
Rats are also capable to deduce the general rules of behavior on the basis of the accumulated life experience and to use them for the solution of the tasks with which they weren't encounter yet.
Rats even possess rudiments of self-awareness, in particular, being able to estimate adequately correctness of own decisions made in ambiguous situations, increasing thereby probability of correctness of own actions further.
On the other hand, the rat brain increases in a size quicker than new neurons are got. Therefore, the larger individuals possess smaller number of neurons on gram of neural fabric, than small individuals while one gram of neural fabric of the large primates and the small primates contains identical amount of neurons. In other words, the brain of primates is constructed on the most economic use of space.
As a result, for example, the brain of a rat with 86 billion neurons (as at the person) would weigh 35 kg.