The appendix does have a use - re-booting the gut

For generations medical orthodoxy has maintained that the appendix is useless, warranting attention only for its tendency to become painfully inflamed and requiring swift removal. But now the reputation of this cul-de-sac in the human gut has been rehabilitated by a theory from a team of immunologists .

The US scientists found that the appendix acted as a "good safe house" for bacteria essential for healthy digestion, in effect re-booting the digestive system after the host has contracted diseases such as amoebic dysentery or cholera, which kill off helpful germs and purge the gut.

This function has been made obsolete by modern, industrialised society; populations are now so dense that people pick up essential bacteria from each other, allowing gut organisms to regrow without help from the appendix, the researchers said.

But in earlier centuries, when vast tracts of land were more sparsely populated and whole regions could be wiped out by an epidemic of cholera, the appendix provided survivors with a vital individual stockpile of suitable bacteria.

"The function of the appendix seems related to the massive amount of bacteria that populates the human digestive system," said Bill Parker, a professor of surgery and one of the scientists responsible for establishing its status as a useful organ. "The location of the appendix, just below the normal one-way flow of food and germs in the large intestine, helps support the theory."

Other studies had shown that, in less-developed countries where the appendix may still be useful, the rate of appendicitis was lower than in the US, he said.

The theory, developed by a team from Duke University Medical School, North Carolina, and published in the Journal of Theoretical Biology, has sent ripples through the scientific community. It "seems by far the most likely" explanation for the function of the appendix and "makes evolutionary sense", said Douglas Theobald, a professor of biochemistry at Brandeis University in Boston.

"I'll bet we'll eventually find the same sort of thing with the tonsils," said Gary Huffnagle, a professor of internal medicine and microbiology at the University of Michigan.

The appendix is a worm-like tube between two and four inches long protruding from the start of the large intestine. Doctors routinely remove it should it become infected and inflamed – a burst appendix can cause peritonitis, which can kill.

In the UK, one in six people will have their appendix out at some point in their lives, although appendicitis is most common between the ages of eight and 14. NHS surgeons perform about 39,000 appendectomies a year.

And other 'spare parts'

* Male nipples

Men have nipples and mammary tissue which can be stimulated to produce milk. They can also get breast cancer.

* Wisdom teeth

Early humans had an extra row of molars to help with the vast quantity of vegetation they had to chew .

* Coccyx

The remains of a tail lost long before man began to walk upright six million years ago.

* Spare ribs

Humans have 12 ribs but about eight per cent of people have an extra pair, as do chimps and gorillas.