The brain's ability to make fast decisions in threatening situations may sometimes literally make the difference between life and death. In our more primitive past this would have translated into fast decisions destined to help us escape from the claws of a predator; the situation is analogue even today, even if the nature of the threat might have changed in time.

Most researchers believe that the brain of mammals contains two distinct regions in charge with the decisional process, each operating at different speeds. Scientists of the University of Bristol have recently confirmed this theory by showing that the evolution of the faster, less accurate decisional system has been greatly influenced by the evolution of the slower, more accurate region, characteristic to humans and several other mammals.

"If we compare the brain of a human with that of a reptile, we find they are very similar except that mammals have a large 'outer cortex' around the outside of the existing 'sub-cortical' brain, that is common to other vertebrates. The fact that lizards make decisions indicates that the sub-cortical brain in humans is also likely to be used in decision-making. However, fMRI scans now reveal that parts of the outer cortex (which developed more recently in our evolutionary past) are also used when making decisions," said Pete Trimmer, lead author of the study.

There are several questions related to why the brain of mammals has two decisional centers, involving the size of the brain, the energy the extra systems consume, as well as the current role of the fast decisional system in humans and how it will evolve in the future. To answer them, the research team created models of how the two systems interact with each other in different situations.

It appears that when in immediate danger, the fast decisional system is much more useful, although in complex social situations the slower, more accurate cortical system in used more than the sub-cortical one.

"As life became more complex, the benefit of gathering information before making a decision put an evolutionary pressure on the early brain. This may have led to the rapid development of the cortex in mammals. So if humans continue to live in a world of dangers such as wild animals or fast-moving cars, there will still be an evolutionary benefit to maintaining the sub-cortical system, and it is unlikely to atrophy in future humans," Trimmer added.