Image caption Eyeless and colourless, Madagascan cave goby relatives were living "down-under".

A pair of eyeless, cave-dwelling fish species, separated millions of years ago, have turned up on either side of the Indian Ocean.

A study in PLoS One showed Madagascan and Australian cave fish inherited their blindness from a common ancestor.

Their forebears probably lived in caves on the prehistoric southern super-continent Gondwanaland.

Then continental drift tore this family apart - transporting them to their current locations.

On his voyages of discovery, Darwin noted eyeless, colourless cave-dwelling creatures whose appearance was so bizarre and primitive he thought they were "wrecks of ancient life".

Prosanta Chakrabarty, assistant professor and curator at the Louisiana State University Museum of Natural Science, and his colleagues were particularly interested in two populations of goby fish living in Madagascar and north-western Australia.

Gobies form one of the largest families of fish, containing about 2,000 different species.

Though living in different parts of the world the cave fish shared important features: they were small - under 10cm in length - eyeless, colourless and lived in freshwater, limestone caves.

How such similar fishes came to be living on different sides of the world was the question the researchers wanted to answer.

In the genes

One possibility was that the cave fishes had evolved independently, from terrestrial counterparts.

That they're 6,000 km apart in Madagascar and Australia is pretty remarkable Prosanta Chakrabarty, Louisiana State University Museum of Natural Science

Species adapt to environmental challenges and opportunities and - through a process of natural selection - only the fittest survive.

When separate species are exposed to the same selective pressures they often come up with the same solutions - a process known as convergent evolution.

For example, the elongated fingers of both the aye aye lemur of Madagascar and the striped opossum of Australia evolved so they could probe nooks and crannies for their favourite insect food.

In the case of the cave fish, an alternative possibility was that their odd features - or traits - were adaptations inherited from an ancestor common to both.

To determine which scenario was most likely, Dr Chakrabarty and his team used phylogenetic analysis - a study of the historical record held deep within their DNA sequences - to rebuild the goby family tree.

The researchers compared DNA sequences from more than 100 different goby species, including those from Australian and Madagascan cave fish that the researchers had collected.

Though separated by thousands of kilometres of ocean, the cave-dwelling fish of Madagascar and north-western Australia were genetically more similar to each other than to any other goby: they inherited their unusual suite of characteristics from a common ancestor.

"That they're 6,000 km apart in Madagascar and Australia is pretty remarkable," observed Dr Chakrabarty.

Lost continents

But when and where did their ancestors live?

To answer this, the researchers used goby fossil records to calibrate their family trees.

The newly chronicled evolutionary history of the fish showed that their shared ancestor was swimming in subterranean pools 45 - 110 million years ago, when the Earth looked very much different to how it looks today.

Image caption Underground caves on either side of the Indian Ocean hide a fishy secret

Australia and Madagascar formed part of a super-continent, called Gondwanaland, which also included what are now Antarctica, Africa, South America and India.

Around 180 million years ago, America and Africa broke away from this super-continent. Then, about 60 million years later in the early Cretaceous period, Australia-Antarctica started to split from India-Madagascar.

It was this separation that ultimately led to the descendants of the Gondwanaland cave fish living on opposite sides of the Indian Ocean.

Dr John Sparks, a co-author from the American Museum of Natural History explained: "The sister-group relationship between obligate cave fish from Madagascar and Australia is a remarkable example of Gondwanan vicariance - a split dating back to the Late Cretaceous."

But the study threw up some anomalies.

The authors believe that the common ancestor was dispersed throughout eastern Gondwanaland - which would include India - yet this country is devoid of similar gobies.

"Antarctica could have provided the connection [between Gondwanaland-locked Madagascar and Australia], or gobies could have been present in India but became extinct," suggests Dr Chakrabarty.

A more controversial possibility is that our knowledge of Gondwanaland might need updating - the kind of change to geology that very recently saw the presumed history of the isthmus of Panama adjusted by millions of years.

The study also threw up another surprise: one of the Australian caves explored by the team contained a fish that had developed pigment.

John Sparks, another curator at the museum, observed: "Our results, and the fact that we have recently discovered new cave fish species in both Madagascar and Australia belonging to these genera, in particular a fully pigmented form from Madagascar, are intriguing from another perspective - they show that caves are not evolutionary dead-ends."