Many fish species have hard structures that record growth over time. Scientists can examine annual growth bands in certain bones or other structures, much as they count rings in a tree trunk.
Greenland sharks present a different problem.
Their skeletons are largely cartilaginous, and their vertebrae do not provide the clear, dependable growth bands that researchers can use to age many other fish. Their slow growth offered an important clue, but it could not establish whether an individual was 80, 150, or 300 years old.
The research team therefore turned to an unusual source of evidence: the sharks' eyes.
More specifically, the scientists studied the nuclei of their eye lenses.
The central proteins in these lenses are formed before birth and remain remarkably stable throughout the animal's life. Because the proteins do not continually renew themselves in the same way as many other tissues, their chemical composition can preserve information about the period when they formed.
Researchers used radiocarbon dating to analyze this material.
Radiocarbon dating measures the amount of carbon-14, a radioactive form of carbon, in biological material. Carbon-14 enters living organisms through the food web and other natural processes. Its abundance can help scientists estimate when biological material was formed.
The method became particularly useful because atmospheric nuclear weapons testing during the mid-20th century dramatically increased the amount of carbon-14 in the environment. This created a recognizable signal, often called the bomb pulse, that can help researchers identify material formed during or after that period.
In the Greenland shark study, this signal helped the team interpret the youngest animals. For older sharks, the researchers used radiocarbon measurements and statistical models to estimate when the eye-lens proteins had formed.
This was not a matter of looking at the shark and counting its years. It was a sophisticated reconstruction based on chemical evidence.
The results suggested that the largest shark in the sample, measuring approximately 16.5 feet, or 5.02 meters, was 392 years old at the central estimate.
However, the researchers did not claim to know its exact birth year.
The estimate had a broad uncertainty range, reflecting the limitations of the method and the difficulty of reconstructing the history of carbon in deep-ocean food webs.
The U.S. National Oceanic and Atmospheric Administration (NOAA) explains that Greenland sharks are estimated to live for at least 250 years and may survive for more than 500 years.
Source: NOAA Ocean Service — How long do Greenland sharks live?.
This distinction matters. The discovery does not mean scientists have identified a shark's exact birthday or proved that every Greenland shark lives for centuries. Instead, it provides compelling evidence that the species can reach ages far beyond those of most other vertebrates.
The uncertainty makes the result less like a precise birthday certificate and more like a remarkably well-informed estimate of an animal's place in history.
And even the conservative interpretation is astonishing.
Older Than the United States
To understand the scale of the discovery, consider what was happening on Earth around the time this shark may have been born.
The early 17th century was a very different world.
In 1616, William Shakespeare died in England. European powers were expanding their overseas influence, while Indigenous societies across North America had already developed complex cultures, communities, and systems of trade long before European colonization.
The English colonies that would eventually become part of the United States were only beginning to take shape.
The Declaration of Independence would not be adopted until 1776, approximately 160 years after the shark's estimated birth under the central age estimate.
The first successful powered airplane flight, achieved by the Wright brothers in 1903, lay nearly three centuries in the future.