Archaeologists have discovered prehistoric fragments that could date back 5,500 years on land in Truro where a supermarket and homes are to be built.
Excavations are taking place on Truro Eastern District Centre (TEDC) land as a planning requirement for the development agreed by Waitrose, the Duchy of Cornwall, Cornwall Council and The Taste of Cornwall.
The site is on Duchy-owned land between the A39 Newquay Road and the A390 Union Hill.
Academics believe the remains are likely to have come from a "causewayed" – a large circular or oval area enclosed by a bank and ditch.
Initial findings from the excavations, led by the council's Historic Environment Service (HES), suggest the eastern end of the site may have been a causewayed enclosure dating from the early Neolithic period (3,800BC to 3,600BC).
Dan Ratcliffe from HES said the team would catalogue findings, take samples and carefully re-bury the site to protect it for future generations.
He said: "It is important that we take the opportunity to learn more about our findings now.
"But best practice is for the site to be preserved for future generations of archaeologists who will have better technologies to understand it than we do today.
"Scientific analysis of evidence recovered during the excavations is expected to take some years after the sample excavation has concluded."
Initial surveys of the site were carried out in 2009, with a condition of the planning approval being to carry out further archaeological research.
The work was commissioned by the council's Transportation Service. Tim Wood, the council's assistant head of transportation, said the proposed development had "sufficient flexibility" in the design to ensure construction above did not interfere with the archaeological remains.
He said: "Following recommendations from the council's advisor, we will reflect the archaeological significance of the site including installing interpretation boards."
Mr Ratcliffe said the Neolithic period saw the first introduction of agriculture to Britain, the domestication of animals and the manufacture of pottery.
He added that the first appearance of large ceremonial monuments, which were built and used communally happened during the period.
He said: "Both the construction of the site and the activities within and around it probably served to bring communities together."
Bert Biscoe, the council's portfolio holder for transportation and highways, said: "I am very pleased that Cornwall Council has some of the best archaeologists in the UK today who have unearthed such an important find."
In May this year the go-ahead was given to build the £40 million development that includes shops and homes. It is expected to create 120 jobs.
Opponents penned 250 letters of objection claiming the scheme would affect the landscape, cause traffic problems and draw money from the city centre.
Archaeological investigations at the site are expected to be completed at the end of November.