Final answer:
In the late 1970s, a group of Harvard physicians determined that the activity of the brain should be used to define death.
Step-by-step explanation:
When the brain ceases to function, it is an indication that the person has died. The brain controls vital functions such as breathing, heart rate, and consciousness. Without brain activity, the body cannot survive.
This determination of brain death allowed for more accurate determination of death in cases where the heart may still be beating due to artificial life support.