David Jacobs, a nutjob of a professor, asserts that the 'grey' aliens' appearance is a result of their abduction of humans and their use of their DNA.

The professor of history at Temple University who specializes in Ufology argues that his peculiar idea on grey aliens boils down to their having a slit for a mouth.

After emphasizing the similarities in look between grey aliens and humans, the professor is persuaded that they were produced using the DNA of abducted humans.

According to the Daily Star, there has never been any scientific proof to prove the existence of grey aliens, despite the fact that they have been popularized in television series, films, and the media.

When people say they've been abducted, they virtually invariably mention grey aliens, according to the professor. (Image: Wikipedia)

In spite of this, the professor has often asserted that aliens are among us on Earth and are plotting a "complete invasion."

Now, on the Richard Dolan Show, he's offered his idea on the origin of extraterrestrial greys, and he claims it all boils down to the mouth slit.

Every individual who claims to have been abducted invariably mentions grey aliens, thus he asserts that they were manufactured using DNA from human abductees.

Professor David Jacobs stated, "When persons claim to have been abducted, grey aliens are nearly always mentioned."

"Since we were taken, they took a small portion of our DNA to create grey aliens.

"The reason I believe grey aliens may have human DNA is because grey aliens have two enormous black eyes."

His idea is based on the fact that grey aliens have a slit for a mouth, which he argues they do not require (Image: Getty Images/iStock).

He went on to explain that grey aliens had two ear openings on the sides of their skulls in order to hear.

In support of his idea, he admitted that they are characterized by having two openings for a nose, similar to humans.

But, the peculiarity he emphasized was that they are thought to have a slit for a mouth, which he said is illogical.

"The question is why they have a slit for a mouth," he said. They do not breathe, and there is no proof that they do, and they approach individuals quite closely, sometimes almost touching their foreheads."

"I've inquired of individuals, 'Do you feel their breath on your face?' Always, the answer is no. They lack lungs, have extremely slender bodies, and do not extend and contract their lungs.