This latest study also presents a solution to a long-standing mystery about the river Nile: why it hasn't shifted in position and changed course over time, as is typically the case with rivers of this size and heritage.

The new study argues that there's a conveyor-belt-like section of mantle rotating underneath the Nile – a convection cell – which has kept the river in place, and shows that it's been running through the same region for tens of millions of years.

"One of the big questions about the Nile is when it originated and why it has persisted for so long," says geologist Claudio Faccenna, from the University of Texas at Austin. "Our solution is actually quite exciting."

The competing hypothesis is that the Nile was first formed almost 6 million years ago, when a drainage basin shift linked to the East African rift occurred.

While the 30-million-year hypothesis isn't a completely new one, this latest study does present new evidence for it, including new geological modelling, and the matching of volcanic rock from the highlands of Ethiopia to the rest of the Nile delta.

Shifts in the Earth's mantle are responsible for the Ethiopian highlands emerging, the study proposes. This gentle drop of 1.5 kilometres (almost a mile) in height by the time the Nile reaches the Mediterranean has kept the river on course for so long.

Without this geological setup, the Nile would've long ago shifted direction towards the west. That of course would have had a profound effect on the history of human civilisation as well - so important has the Nile been to North Africa and the Middle East.

Besides offering up evidence that the Nile has been flowing in its current direction for 30 million years, the new study also serves up new information on how the movement of the deep mantle under the Earth's surface can affect what happens on the ground, and how rivers could give us clues to what's happening way down underneath.

Now the researchers want to apply the same type of analysis to the other major rivers in the world, including the Yangtze and Congo rivers. For the time being though, we may have ticked off one more of the mysteries around the great Nile.

"Despite the multiple small-scale modifications that the Nile drainage must have undergone during the last 30 million years, the river has existed without interruption, continuously connecting the Ethiopian topographic swell to the Mediterranean Sea," conclude the researchers in their published paper.

The research has been published in Nature Geoscience.

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