Volcanoes were active on the Moon's surface soon after it was formed, a new study in the journal Nature suggests.
The dark areas are the great basalt plains formed by eruptions
Precision dating of a lunar rock that fell to Earth shows our satellite must have had lava erupting across its vast plains 4.35 billion years ago.
This is hundreds of millions of years earlier than had been indicated by the rocks collected by Apollo astronauts.
Scientists say the information will help us better understand the beginnings of the Solar System.
And they urge future Moon missions to try to obtain more of these most ancient rocks.
"We want to understand how the Solar System formed, how the planets formed," said Mahesh Anand from the UK's Open University.
"The Moon is the only place where you can go to find the first 500 million years of geological history, because these old rocks have been lost on Earth," he told BBC News.
According to the favoured theory, the Moon was created some 4.5 billion years ago in a smash-up between the Earth and a Mars-sized body.
Material thrown into space is believed to have coalesced to become our satellite.
Volcanism on this new object would not have started until its surface had cooled to form a crust and its insides had become separated into a mantle and a core. Quite when this might have happened has been hard to pin down.
Kalahari 009 is the biggest of all the known lunar meteorites
Virtually none of the basaltic rocks collected by moonwalkers are older than 3.9 billion years; but with less than 400kg of lunar material returned to Earth, many scientists suspected Apollo would not be the last word on the subject.
Now, Dr Anand - working with Dr Kentaro Terada, from Hiroshima University, Japan, and other colleagues - has put a new date on a lunar meteorite known as Kalahari 009.
Sometime in the past, this 13.5kg volcanic rock was blasted off the Moon by the impact of an asteroid or comet and fell to Earth in what is now Botswana.
Scientists know it comes from the Moon because of the type of oxygen atoms it contains.
And by looking closely at the ratio of uranium and lead atoms in the rock's phosphate minerals, the team has also been able to say when the basalt was ejected - 4.35 billion years ago, give or take 150 million years.
"The age of the phosphate is the age of the rock," said Dr Anand, "because the rock solidifies when the magma cools, and when the magma cools the mineral forms.
"Volcanic activity is a secondary process. A planet has first to form, solidify, and separate into layers; and then there is melting of that solidified mantle to produce volcanism.
"So we are pushing all of this further back in time; and [our research] suggests these processes took place over a much shorter timescale than had previously been thought."
Space agencies have renewed their interest in the Moon three decades after the Apollo landings.
Europe's recent Smart-1 orbiter has been swiftly followed by Japanese and Chinese spacecraft.
India will be next; and in the coming decade we should see robots land on the lunar surface, with astronauts set to return by 2020.
Scientists say the Moon has much to tell us about the early Earth. The surface rocks on our planet are relatively young because they are constantly recycled into the interior.
The not-so-dynamic Moon, on the other hand, has an abundance of early material on its surface. Researchers think it should even harbour ancient Earth meteorites - rocks that travelled in the opposite direction to Kalahari 009.