Complete the summary using the list of words below.

early

easy

large

late

lengthy

new

recently

small

quick

Long-term memory

Various researchers have examined the way lasting memories are formed. Laboratory experiments usually involve teaching subjects to do something [1], and treating them with mild electric shocks or drugs. Other studies monitor behaviour after a learning experience, or use sophisticated equipment to observe brain activity.
The results are generally consistent: they show that lasting memories are the result of a [2] and complex biological process.
The fact that humans share this trait with other species, including animals with [3] brains, suggests that it developed [4] in our evolutionary history.