The brain is the material mechanism for intelligent response to internal and external stimuli.  Mind, while not a physical evolution, is wholly dependent on the brain capacity afforded by purely physical and evolutionary developments.  The stoppage of life destroys the physical brain patterns for mind endowment, and the disruption of mind terminates mortal consciousness. 
The one physical uniformity of mortals is the brain and nervous system; nevertheless, there are three basic organizations of the brain mechanism: the one-, the two-, and the three-brained types. Urantians are of the two-brained type, somewhat more imaginative, adventurous, and philosophical than the one-brained mortals but somewhat less spiritual, ethical, and worshipful than the three-brained orders.  Adjusters gain better contact with three-brained mortals. 
The intellectual and spiritual differential between the three brain types characterizes individuals who are otherwise quite alike in mind endowment and spiritual potential, being greatest in the temporal life and tending to diminish as the mansion worlds are traversed one by one.  These three types of physical-brain endowment are not factors in Adjuster bestowal, in seraphic service, or in any other phase of spirit ministry.  Senses are sharper in three-brained mortals. 
Dinosaurs and sea serpents perished because they did not have brains large enough to afford protection for their immense bodies.  Early mammals possessed large brains in comparison to body size.  Evolution followed growth of brain, not physical bulk.  Lemurs had the largest brains for their size of any animal that had theretofore existed on earth.  Mid-mammals had large brains.  Primates’ increased use of hands helped develop inherent brain power.