Growing to more than five metres (16 feet) in length and weighing more than 1,000 kilograms (2,205 pounds), the Greenland shark is one of the largest predatory shark species in the world. But when Bushnell first encountered these giants, it was not just their size that struck him. ‘I was amazed at how large they were – and even more amazed at how lethargic they were. They just lay there. When we caught them on a long-line, it was sometimes difficult to tell a live one from a dead one.’ It is not hard to imagine why scientists named the shark Somniosus microcephalus, meaning ‘sleepy little brain’.

Their lack of speed notwithstanding, most Greenland sharks have hitchhikers that dangle from the surface of their eyeballs: parasitic copepods that attach themselves to the cornea. Scientists suspect that the presence of these seven-centimetre (three-inch) hangers-on may render the sharks partially blind, but as they exist in such a deep, dark world, it is doubtful that they have much use for sight anyway. Greenland sharks are usually found between 300 and 600 metres (1,000 and 2,000 feet) deep but have been tracked to beyond 1,800 metres (6,000 feet). The frigid water temperature at such depths ranges between -1 °C and 6 °C (30 °F and 43 °F) , which makes it possible for the species’ metabolic rate to be very low. This, in turn, leads to one of the most startling of all this enigmatic shark’s mysteries: its incredible lifespan.

Ironically, the key to this secret lies in the lens of the Greenland shark’s parasite-infected eyeballs. With the help of a group of international collaborators, Bushnell, Steffensen and Julius Nielsen (a PhD student from the University of Copenhagen) applied a radiocarbon dating technique to the shark’s eyeballs that has never been used with animals before. In doing so, they have dragged the Greenland shark from relative obscurity to the heights of biological fame as the oldest known vertebrate species on the planet. A crucial element of Bushnell’s study is the atmospheric bomb pulse – a radical injection of carbon-14 (a kind of radioactive carbon) into the atmosphere – that resulted from the testing of hydrogen bombs in the mid-1960s. This sudden and dramatic increase in carbon-14 (bomb-carbon) was quickly incorporated into both terrestrial and marine environments, making all creatures living at the time part of the world’s largest chemical-tagging experiment. The higher levels of carbon were incorporated into the marine food web over the next decade, and the fact that they can still be identified in organic tissue enables scientists to determine the age of that tissue.

During field trips, the team spends weeks on research vessels off the coast of Greenland setting long-lines to catch sharks. ‘There’s nothing like bouncing about in a small rubber boat, hands numb from the cold water, feeling dwarfed by icebergs as they float by, and with an extremely sulky, large and powerful shark 20 centimetres (eight inches) from your face to remind you that this is its world and you are nothing more than an unwelcome intruder,’ reflects Bushnell.

‘Our biggest challenges are icebergs and cold. You put your hands in the water and you lose feeling after 15 seconds, so it’s difficult to do anything. I’m not scared of the sharks themselves, although their skin is incredibly rough and can tear you up like a wood rasp. What I am concerned about is that they get hopelessly tangled in the gear and while I’m untangling them I might get caught up in the line and that will drag me down. Death would be a very slow and unpleasant process.’

In order to estimate the ages of individual sharks, the team has examined the eyeballs of 28 Greenland sharks caught in fjords and offshore waters. The animals ranged from 80 centimetres (32 inches) to 5.02 metres (16.5 feet) long. After testing the nucleus of each eye lens, the researchers discovered that only three sharks, all smaller than 2.2 metres (seven feet), displayed signs of the bomb pulse. This meant that the 25 larger animals had to be more than 50 years old. To age the other sharks, the researchers needed to use carbon dating in another way that would enable them to see further into the past.

Despite numerous expeditions to Greenland and having caught and tagged more than 60 sharks in the past five years, Bushnell and his team have never captured a pregnant female. In fact, there is only one scientific report of a pregnant female; her pups were near term and measured 40 centimetres (16 inches) long. Tag-and-recapture experiments performed between 1936 and 1952 suggested that Greenland sharks only grow 5–10 millimetres (0.2–0.4 inch) per year and female Greenland sharks only become sexually mature when they are 4–4.5 metres (13–15 feet) long. The research indicates that at this size they would be about 150 years old; a very long time to wait before being able to reproduce. This means that although the sharks may seem common throughout the Arctic – at least common enough to be considered a source of biofuel – very few of them are actually big enough to breed. And with such slow growth rates, it will be decades before the younger generation, in turn, is ready to procreate.

Moreover, although the largest shark on record only measured 5.5 metres (18 feet), Bushnell has heard stories of animals as long as 6–7 metres (20–23 feet). Perhaps the liver oil fishery took its toll on these enormous and very old sharks or perhaps, as he points out, the tales of seven-metre giants are just ‘fishing stories’.

The study published in Science is really just the first step in learning how to age such long-lived elasmobranchs. With more samples, especially from very big and very small sharks, researchers may be able to narrow down their estimates and achieve more accurate age measurements. In the meantime, though, it seems safe to say that some of the individual Greenland sharks that are still swimming through the waters of the icy north were born long before Charles Darwin. These sharks are such masters of evolution that some of them have been alive longer than the theory itself.