By Dr David Whitehouse - BBC News Online science editor
An ancient supercontinent, far older than anything proposed before, has been pieced together by an international team of geologists.
The giant landmass, which has been dubbed Columbia, would have spread across the face of the Earth more than one and a half billion years ago.
The theory is that it then fragmented into smaller pieces before reassembling into another supercontinent called Rodinia; and, later still, a huge landmass scientists now refer to as Pangea.
The researchers base their conclusions on rocks from India, East Africa and Saudi Arabia. The specimens were collected during a joint Indian and US study of the evolution of southern India.
Lead researcher Professor John Rogers, of the University of North Carolina, US, suggested the name for the newly proposed, ancient landmass.
"I named the supercontinent Columbia because some of the best evidence for its existence is in the Columbia River region of western North America," he said.
The researchers describe their ideas in the latest issue of Gondwana Research, a quarterly journal devoted to studies of Earth's early landmasses.
Professor Rogers said: "Starting at about 1.8 billion years ago, all of the continents existing at that time began to collide into a single land area."
Based on magnetic and geologic evidence, it is believed that the east coast of India became attached to western North America, with southern Australia pushed up against western Canada.
Most of present South America rotated so that the western edge of Brazil aligned with eastern North America, forming a continental margin that extended into the southern edge of what is now Scandinavia.
"This formed an area that stretched about 12,900 kilometres (8,000 miles) from southern South America to northern Canada and was about 4,830 km (3,000 miles) across at its widest part," the professor said.
The researchers say the evidence they have collected suggests Columbia began to break up about 1.5 billion years ago, and its fragments then moved around the Earth independently for several hundred million years.
About a billion years ago, however, the fragments came together again to form a new supercontinent, dubbed Rodinia.
This lasted until about 700 million years ago, before it too broke into several fragments.
Scientists think these chunks moved independently until about 250 million years ago, when yet another supercontinent emerged, now called Pangea.
This began to break up almost immediately to form the world's present array of continents.
In 1912, German meteorologist Alfred Wegener first put forward the theory of continental drift to describe the movement of major landmasses across the surface of the planet.
Initially, the theory was widely criticised but then later absorbed into the current, accepted model of continental dynamics known as plate tectonics.
Continents move at slower than a snail's pace, like pieces of a puzzle, squeezing together and pulling apart to form oceans and landmasses of various sizes.
Movements deep within the Earth are thought to drive the whole process, although the exact mechanism is still being investigated.