A race of giants may have sprung from a mutant gene that first emerged around 1,500 years ago and causes uncontrolled body growth, scientists believe.
The "gigantism gene" was identified in the DNA of an 18th-century man known as the Irish Giant, who was said to stand at almost 8ft tall.
Copies of the same mutation have been found in living patients suffering from gigantism and other symptoms of over-growth.
Scientists writing in the New England Journal of Medicine said they suspect all inherited the gene from the same common ancestor who lived up to 66 generations ago. Around 200 to 300 people may be carrying the same mutation today.
The Irish Giant was Charles Byrne -- also known by his stage name O'Brien -- born in Littlebridge, Northern Ireland, in 1761. In the 1780s he found fame exhibiting himself as a curiosity or "freak" in London. Despite claims that he was more than 8ft tall, skeletal evidence shows he measured just over 7ft 7in.
Celebrity life eventually got the better of Byrne, who took to drink and died at his home in Charing Cross aged just 22.
After his death, Byrne's body was acquired by the 18th-century surgeon John Hunter, and his skeleton remains at the Hunterian Museum in London.
British and German scientists conducting the new research extracted DNA from two of the Irish Giant's teeth.
They discovered a mutant version of the aryl hydrocarbon-interacting protein gene, which matched those found in living patients from four Northern Irish families.
The gene variant triggers tumour growth in the pituitary gland at the base of the brain. Among its many functions, the gland releases hormones that regulate growth.
Pituitary tumours can cause tissue to grow abnormally, which may lead to gigantism -- as suffered by Byrne -- or acromegaly. Symptoms of acromegaly include thickened skin, enlarged hands and feet, distorted facial features and overgrown organs.
Professor Marta Korbonits, who led the research, said: "The most important clinical aspect of our study is that it is now possible to trace down carriers of this gene in time and treat patients before they grow to be a giant."