The discoveries, made by two separate teams, date from the Lower Cambrian era. This is 50 million years before the current estimate of when fish evolved, at about 530 million years ago.

This period is very early in the evolution of life from simple creatures to much more complex ones.

The fish belong to two new and quite separate species and are similar to primitive types of fish which do not have jaws. They are identifiable, say the scientists, because of their gills and a zigzag arrangement of muscles called myotomes, which are only found in fish.

One of the creatures, Haikouichthys ercaicunensis, has gills which are supported by gill bars and the second, Myllokunmingia fengjiaoa, has a more primitive arrangement of gills, but with a series of identifiable pouch-like structures.

Myllokunmingia fengjiaoa: About 28 mm in length

Professor Simon Conway Morris, of the Department of Earth Sciences, University of Cambridge, and Professor Degan Shu, of Northwest University, in Xi'an, China, have been analysing the fossils, along with eight other scientists from China.

Their research is published in the journal Nature. They say the discoveries will force scientists to reconsider the rates of evolution in the oceans during the Cambrian age.

Professor Conway Morris said: "These two specimens are of the greatest scientific importance. Humans are vertebrates - as are rabbits, eagles and frogs - and as such we all evolved from the fish.

"Until now, the early history of the fish has been extremely sporadic and sometimes difficult to interpret. This discovery shows that fish evolved much earlier than was thought.

Protective skeletons

"It indicates also that the rates of evolution in the oceans during the Cambrian period must have been exceptionally fast. Not only do we see the appearance of the fish, but also a whole range of different animal types."

Professor Conway Morris explained that a rapidly changing ecology was probably responsible for the rate of evolution.

"Animals were moving faster and hunting more effectively, and correspondingly other animals were busy developing protective skeletons. At the same time, there were probably significant changes in gene architecture," he said.

The fish were recovered from what has become known as the Chengjiang fauna - an astonishing series of sites near Kunming, in Yunnan Province, China, where thousands of exquisitely preserved soft-bodied fossils have already been found.

Chengjiang has revealed much about Earth's living past