Scientists have examined the genes of "whale lice" to track whale evolution.
The small parasitic crustaceans were taken off right whales, which have been driven to the brink of extinction in some waters by commercial hunting.
The genetics of the lice reveal their hosts split into three species 5-6 million years ago, and these were all equally abundant before whaling began.
The study, carried out by University of Utah scientists, is reported in the journal Molecular Ecology.
'Right' for hunting
Right whales were the first whale to be commercially hunted, 1,000 years ago.
They were so named because they were the "right" whale to kill; their blubber made their carcasses float for easy recovery.
They can reach a massive 18m and 70 tonnes.
Two of the three species are on the brink of extinction. Only about 200 survive in the North Pacific, 350 in the North Atlantic, whilst the Southern Hemisphere population numbers around 8,000 to 10,000.
The new research studied the genes of so-called whale lice - not lice as we would normally think of them, but harmless, small crustacean parasites that live on the surface of the marine mammals.
The idea was to understand the evolution of these giants by getting at the genetics of creatures that have spent most time with them.
It is an approach that has been used before with other animals - but this is a first for whales.
"Whale researchers have dreamed of being able to ride with the whales and see the world they experience," says study co-author Dr Vicky Rowntree.
"Whale lice have been doing it for millions of years, and can tell us things about the whales we can't learn any other way."
The whale lice - they are properly called cyamids - look like miniature crabs and are between 0.5-1.5cm in length. About 7,500 live on the surface of a single whale, feeding on sloughed skin.
As they spend their entire lives on the whales, both species share a common history. And in certain respects, the parasite's genes actually tell scientists more about the whale's history than its own genes.
The lice are more genetically diverse and have a greater number of mutations ("spelling mistakes") in their DNA for scientists to track. This is because there are many more of them, and they reproduce much more often.
In other words, the shared history of both animals is easier to read in the lice.
Right whales are particularly good for this kind of study, as they have three different species of lice.
The Utah biologists collected the parasites from beached right whales all around the world - 8 Southern Hemisphere, 4 North Atlantic and 1 North Pacific.
They extracted DNA and analysed certain genes found in the mitochondria, the tiny "power-stations" of cells.
This mtDNA, as it is known, has become a useful tool for studying evolution. It mutates - errors appear - at a steady rate, meaning it can be used as a "molecular clock" to investigate the history of a species.
Using this information, they built family trees to show the relationships among whale lice - and hence the right whales.
The study gives an estimate of when a single species of right whale diverged into three different species.
"The genetics of whale lice show conclusively that the three species of right whales have been isolated in the North Pacific, North Atlantic and Southern Hemisphere for about 5-6 million years," says Professor Seger, who led the study.
This time-scale is consistent with a previous study of the whales' own genes.
"This puts an end to the long debate about whether there are three species of right whale. They really are separate beyond a doubt," he told the BBC News website.
North and South America were separated by seas at that time, but these were so shallow that whales could not swim between the North Pacific and North Atlantic.
Isolation by physical barriers allows separate species to form.
Warm currents also kept right whales from moving between southern and northern oceans.
The lice were recovered from beached whales
"Right whales have such thick blubber they can't cross the equator," says Dr Rowntree. "The waters are too warm. They can't shed heat."
However, the new study also showed that at least one southern right whale did manage to cross the equator 1-2 million years ago.
"There were probably times when the equatorial waters weren't as hot as they usually are, and some adventurous juvenile male crossed the equator," says Dr Rowntree.
It is known that North Atlantic right whales have a lower genetic diversity than those in the Southern Hemisphere.
This could be a result of whaling activities, or the pre-whaling population may simply have started out smaller.
But the new study showed "the genetic diversity of the whale lice is virtually as great for the North Atlantic right whale as for the southern right whale", says Professor Seger.
"This suggests that the reduced genetic diversity of North Atlantic right whales happened recently, possibly due to whaling." Limited data from North Pacific whale lice suggested a similar picture.
Since this rules out a longer-term deadlier cause, it raises hope for the ultimate recovery of the North Atlantic and North Pacific species.
People are also using the whale lice approach to study other endangered species, including Pacific grey whales.