Radioactive fallout from nuclear tests serves as measuring stick.
If wisdom comes with age, then brain cells are some of the wisest in the body: researchers have applied carbon dating to DNA to confirm that cells in the brain live longer than most others.
This is a new application for the technique, which is traditionally used in archaeology and palaeoanthropology to pinpoint the age of fossils.
Carbon dating looks at the ratio of radioactive carbon, which is naturally present at low levels in the atmosphere and food, to normal carbon within an organism. While a creature lives, eats and breathes, its ratio of radioactive to normal carbon will equal that of its environment. But when it dies, this ratio will fall, as the carbon-14 decays.
Radioactive carbon decays slowly, such that a given amount of carbon-14 halves every 6,000 years. So detecting the subtle change in the ratio of normal to naturally occurring radioactive carbon over just a few years is incredibly hard.
But Jonas Frisén of the Karolinska Institute in Stockholm, Sweden, says it can be done if one takes advantage of the signal left by nuclear testing, which spewed high levels of carbon-14 into the air during the Cold War.
By the time a halt was called to aboveground nuclear testing in 1963, levels of carbon-14 in the atmosphere had doubled beyond natural background levels, says Frisén. Since the halt, this has halved every 11 years. By taking this into account, one can see detectable changes in levels of carbon-14 in modern DNA, he says.
"Most molecules of the cell will turn over all the time. But DNA is a material that does not exchange carbon after cell division, so it serves as a time capsule for carbon," Frisén says.
Other approaches have tried to assess cell age in the past, and have included looking at the length of DNA 'caps' known as telomeres. But we currenty know too little about telomeres for this to be much use. "Shortened telomeres may mean the cell was generated more recently, but how recently we don't know because we don't have a timescale for it," says Frisén.
Frisén and his team looked at tissue samples from more than a dozen deceased subjects, about half of whom were born after the mid-1960s. By measuring carbon-14 in their DNA, they say they can pinpoint individual cells' birth dates to within two years.
They found that all of the samples taken from the visual cortex, the region of the brain responsible for processing sight, were as old as the subjects themselves, supporting the idea that these cells do not regenerate. "The reason these cells live so long is probably that they need to be wired in a very stable way," Frisén speculates.
"This could help to address long-standing questions of just how rigid or just how flexible our brains are at the cellular level," says Jeffrey Macklis of Harvard Medical School in Boston, Massachusetts.
Other brain cells are more short-lived, the team reports in Cell1. In an average person of some 30 years of age, intestinal cells are about a decade old and skeletal cells a bit older than that. Cells that endure a great deal of physical stress, such as red blood cells, are known to turn over every few months
The research team believes that dating cells using carbon-14 will shed light on the role of cell death in cognitive disorders. Previous studies have suggested that a lack of proper regeneration of some brain cells can cause depression, they note.
SpaldingK., BhardwajR., BuchholzB., DruidH. & FrisenJ. et al. J. Cell, 122. 133 - 143 (2005).