 MacLeans model is based on the idea of the larger the brain size, the higher the order of thinking, thus, an increased cognitive ability.The neomammalian brain is in charge of all ‘rational thinking’, his model follows Charles Darwin's natural selection idea of ’survival of the fittest’, where those mammals that developed characteristics of the neomammalian brain survived and then passed this trait onto their offspring, until a stage was reached where the majority of the population of higher order mammals attained the survival trait, a process that occurred over millions and millions of years.Archaeologists have discovered and are still discovering fossil records that allow comparative anatomy to occur between the modern-day Homo Sapien and primate ancestors.