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They may not appear to be related, but hippos have long been thought to share an ancient ancestor with whales.

But up until now, a huge age gap between in the fossil record has prevented scientists from confirming their theory.

Now researchers have uncovered 28 million-year-old molars of a great-great grandfather of the hippo which has finally provided direct proof of their link with whales.

A great-great grandfather of the hippopotamus likely swam from Asia to Africa some 35 million years ago, long before the arrival of the lion, rhino, zebra and giraffe Pictured is a view of one of the fossils that confirmed this. It shows an upper molar of a species known as Epirigenys lokonensis

The previously unknown, long-extinct relative also confirmed that cetaceans - the group to which whales, dolphins and porpoises belong - are in fact the hippo's closest living cousins.

'The origins of the hippopotamus have been a mystery until now,' Fabrice Lihoreau, a palaeontologist at France's University of Montpellier.

'Now we can say that hippos came from anthracothere' - an extinct group of plant-eating, semi-aquatic mammals with even-toed hooves.

Until now, the oldest known fossil of a hippo ancestor dated from about 20 million years ago, while cetacean remains aged 53 million years have been found.

The remains of a 28-million-year-old animal discovered in Kenya's Lokone Hills has provided an important piece of the puzzle, according to a study in the journal Nature Communications.

It has been name Epirigenys lokonensis - 'epiri' means hippo in the Turkana language and Lokone after the discovery site.

The species was about the size of a sheep, weighing in at 100kg (220lbs), which is about a twentieth the size of today's 'common hippopotamus', a sub-Saharan giant.

It may have spent much of its time immersed in water.

E. lokonensis was not a direct forefather of today's hippo, belonging instead to a side branch.

But it lived much closer in time to the ancestor from which they both branched off, allowing for inferences to be drawn about the ancient animal.

The anthrocothere, shown here, is believed to be a common ancestor to hippos. These were an extinct group of plant-eating, semi-aquatic mammals with even-toed hooves

Pictured is a simplified family tree showing the relation between hippos, anthracotheres and cetaceans and the key position of the new species from Kenya

This image shows the evolutionnary transition of the upper molar from an anthracothere (left), Epirigenys (middle), and a primitive hippo (right). Black circle are the cusps, black line are the crests and in orange the styles show enamel

Dental analysis led the team to conclude that E. lokonensis and the hippo both came from an anthracothere forefather, which migrated from Asia to Africa about 35 million years go.

As Africa was then an island surrounded by water, it likely swam there.

This means the ancestors of hippos 'were among the first large mammals to colonise the African continent, long before those of any of the large carnivores, giraffes or bovines,' all of which arrived only about 18 million years ago.

The modern-day hippo evolved independently in Africa, and is a creature truly endemic to the continent, according to the research paper.

'We filled a gap in the evolutionary history of the hippo, bringing us closer to the point of divergence from their modern-day sister group of cetaceans,' said Professor Lihoreau. 

The right image shows a Epirigenys lokonensis fossil with molars (right) compared to the left of a hippopotamid fossil. The left image shows a hippopotamus at the chobe National Park, Botswana, Africa

The previously unknown, long-extinct relative also confirmed that cetaceans - the group to which whales (pictured), dolphins and porpoises belong - are in fact the hippo's closest living cousins

The remains of a 28-million-year-old animal discovered in Kenya's Lokone Hills. It has been name Epirigenys lokonensis - 'epiri' means hippo in the Turkana language and Lokone after the discovery site