How humans became so intelligent is a question that has plagued scientists for years.

However, they believe that they have answered the question.

Researchers have pinpointed the exact time in history when they believe that human ancestors evolved the capacity for intelligence.

The historical moment took place roughly 550 million years ago, the researchers say in two different papers published in Nature Neuroscience.

The answer lies in a period far earlier than when humans roamed the earth. The researchers believe that the answer lies in an invertebrate animal living millions of years ago.

Seth Grant from the University of Edinburgh and his colleagues suggest that this animal experienced a genetic accident. This accident resulted in an extra number of brain genes being created.

The accident benefited the animal's descendants, including humans.

Grant and his colleagues came to this conclusion after conducting a series of experiments that studied the mental abilities of humans and of mice.

The experiments asked participants to compare and identify objects on a touch-screen computer.

The researchers used these tests and combined them with various species' genetic codes in an effort to find out when certain behaviors evolved.

They found that higher-level brain functions in both mice and in humans were controlled by the same genes.

As exciting as the study is, it also has a dark side.

The study found that the same genes responsible for human intelligence are also responsible for the development of mental illness and psychiatric disorders.

When these higher-level genes were damaged or mutated, they damaged higher mental functions.

"Our work shows that the price of higher intelligence and more complex behaviours is more mental illness," Seth Grant said in a statement.

Researchers had previously found that gene mutations were responsible for over 100 childhood and adult brain illnesses.

They hope that doctors can use a combination of genetic and behavioral testing to help these individuals.