Scientists say a jawbone discovered in Taiwan may provide clues to a prehistoric human that was not previously known to exist.

The fossilised bone could be nearly 200,000 years old, and may have come from a fourth type of ancient man. 

Fishermen discovered the human jawbone in the Penghu Channel, and sold it to an antique shop. Researchers recovered the rare specimen, and have been studying it for five years. 

The teeth and jawbones of humans were believed to have grown smaller over the centuries, but the newly found bone is thick, with large molars. Scientists believe the fossil belongs to a previously undiscovered type of human that they named "Penghu 1."

"The new Taiwan mandible is clearly different than the known Homo erectus populations from northern China and Java, and likely represents a group that has been unrecognised so far," researcher Dr Yousuke Kaifu told CNN.

"It's only one piece, but the significance is huge."

The previously known ancient humans are Homo erectus, who lived in China and the Indonesian island of Java, Homo floresiensis of Indonesia, and the Neanderthals of the Altai Mountains in eastern and central Asia. 

Analysis of the bone was difficult because it was found in the sea, and researchers are still working to get a more specific age and other information on Penghu 1.

"The findings reminded me that our knowledge is always very limited, and we have very limited fossil records," Dr Kaifu admitted. "That's a great lesson for me."