By Bradley Stockwell
My two favorite arenas of academia are science and history, and the more I study the two, the more I see how interwoven they really are. There’s no greater example of this than something called the “bomb pulse”. Whether you know it or not, lurking inside of you is a piece of Cold War history—even if you weren’t alive at the time—and it is this little memento that finally solved one of biology’s most elusive secrets: How old are you? And I don’t mean how many times has the cellular clump of mass known as you swung around the sun, but how old are the individual cells that make up that mass? Your skin cells, heart cells, neurons—your body is constantly renewing itself with new cells and it is only as of 2002 that we began to have a definitive answer for how old each one was. With this post, my intentions are twofold. One: I want to tell you about one of the greatest scientific discoveries of the 21st century, and two: I’m hoping that by wrapping them in this titillating story, I can also slip in a few basic principles of nuclear chemistry. With that said, let’s begin!
Between 1945 and 1963, over four hundred nuclear bombs were detonated, unleashing an untold number of extra neutrons into the atmosphere. Some of these neutrons found their way into nitrogen atoms, causing them to eject a proton. If you’re familiar with some basic chemistry, when a seven-proton nitrogen atom loses a proton, it becomes a six-proton carbon atom. However, because these carbon atoms still have two extra neutrons from when they were nitrogen, they become something called an isotope, a variant of an element which differs in neutrons, but has the same amount of protons. In this case, these slightly more massive and radioactive isotopes become an isotope of carbon called carbon-14.
When I say radioactive, all I mean is that the atom’s nucleus is unstable; that it is emitting energy in the form of ejected subatomic particles or energetic light waves to stabilize itself until it becomes a stable isotope, or a completely new element altogether. This radioactive decay comes in three main forms: alpha, beta and gamma. Alpha decay—which only happens with heavy elements like uranium—is the ejection of something physicists call an alpha particle, but chemists just call it a helium atom, a bundle of two protons and two neutrons. In fact, almost all the helium here on Earth came from this type of decay. Think about that the next time your sucking down a helium balloon; you’re inhaling the atomic leftovers of uranium, thorium and other heavy, radioactive elements. Beta minus decay is the ejection of an electron and beta plus decay is the ejection of the electron’s antiparticle, the positron. Gamma decay is the emission of an extremely energetic light wave called a gamma ray and it is often emitted in conjunction with alpha and beta decay.
The time it can take for a radioactive element to reach a stable form can be anywhere from instantaneous to far longer than the age of the universe. Because individual atoms decay unpredictably, the way in which we measure this loss is through probability, or something called a half-life. This is the time it takes for half a quantity of radioactive material to decay into a more stable form. This is not to say if you have four radioactive atoms, in x amount of time you’ll have two necessarily, but more that each individual radioactive atom has a fifty percent chance that it will decay to a more stable form in x amount of time. For example, carbon-14, the star of our story, has a half-life of 5,730 years. This means if you had a pound of it, after 5,730 years you’d have a half pound of carbon-14 and half a pound of nitrogen-14, carbon-14’s more stable form. Then after another 5,730 years you’d have a quarter pound of carbon-14 and three-quarters pound of nitrogen-14, and so forth. This is how carbon dating works; by measuring the relative portions of carbon-12 and carbon-14 in a sample of organic matter, archeologists are able to determine its age.
The period between 1945 and 1963 in which all this atomic testing was happening is now called the “bomb pulse” by the scientific community. It was called this because the amount of carbon-14 in the atmosphere was doubled during this period from all those free neutrons crashing into nitrogen. In 1963, when the Soviet Union, the U.K. and the U.S. agreed to the Limited Test Ban Treaty which prohibited all above-ground detonations, the amount of carbon-14 began to decrease by half every eleven years and will eventually be depleted somewhere around 2030 to 2050. This isn’t because the carbon-14 is decaying into nitrogen-14 (remember the half-life of carbon-14 is 5,730 years), but because it is being absorbed by the life inhabiting our planet, which includes us. Although carbon-14 is an altered carbon atom—a carbon isotope, it still behaves like a carbon atom because it is the number of protons in an atom that determines its chemical behavior, while the number of neutrons determines its mass; and like a regular carbon atom, these carbon-14 atoms have been binding to oxygen, forming CO2, which is sucked up by plants during photosynthesis and then fed to the rest of us through the food chain. Like the plants, our bodies too can’t tell the difference between carbon and carbon-14, so for the last seventy-plus years all this extra carbon-14 has been used by every living creature to build new cells, proteins and DNA.
While our bodies can’t tell the difference between carbon and carbon-14 (because they have the same amount of protons), scientists can because of their slight difference in mass (remember carbon-14 has two extra neutrons). The difference in mass is measureable through a technique called mass spectrometry, which sorts atoms by weight. Without getting too technical, an instrument called a mass spectrometer strips atoms of some of their electrons and launches them into a magnetic field, which alters the atoms’ course, and because of inertia, heavier atoms take a wider path than lighter ones. By measuring how many atoms travel along certain paths, scientists can determine how much of a specific atom—in this case a carbon-14 atom—is in a sample.
So what does this have to do with determining a cell’s age? Well for a long time nothing. But somewhere around 2002, Krista Spalding, a postdoc at the Karolinska Institute in Stockholm, Sweden, wanted to challenge the longtime doctrine that said the human brain couldn’t create any new neurons after the age of four. There had been growing evidence that the adult hippocampus—a seahorse-shaped region deep in the brain that is important for memory and learning—could regenerate neurons, but no one knew for sure. Spalding and her postdoc advisor, Jonas Frisén, had a hunch that the “bomb pulse” period could somehow offer a solution and it did, culminating in a paper by Spalding, Frisén and their team published in June 2013, which conclusively found that the hippocampus did produce approximately 700 to 1,400 new neurons per day, and these neurons last twenty to thirty years. How you ask? Well there’s an episode of Radiolab (a wonderful science podcast I recommend you all listen to) that has a much more colorful version of Spalding and Frisén’s journey here, but because I know I’m probably already pushing your attention spans, I’ll just give a brief overview. You see, atmospheric scientists have been measuring the amount of carbon-14 and other elements in the atmosphere every two weeks since the late 1950s, giving us an extremely accurate timetable of how much carbon-14 is and was in the atmosphere at any given time after. By correlating this data to the amount of carbon-14 found in a cell’s DNA (while other molecules are regularly refreshed throughout a cell’s life, DNA remains constant), researchers can determine not just the age of a hippocampal neuron, but any cell. So by accident, the nuclear age finally shed light on when tissues form, how long they last and how quickly they’re replaced.
You—and every other living organism—are continually creating new cells. Cells that make up your skin, hair and the lining of your gut are constantly being replaced, while others, like cells that make up the lens of your eye, the muscles of your heart and the neurons of the cerebral cortex, have been with you since birth and will stay with you until you die.
So why is this so important? Well firstly, it gives us a key insight into the mechanisms behind many neurodegenerative diseases such as amyotrophic lateral sclerosis (Lou Gehrig’s disease), Parkinson’s, Alzheimer’s, Huntington’s and many more. Really we’ve only just begun to dig into this Pandora’s box so to speak, and unfortunately time is limited (well unless we start blowing up a bunch of atomic weapons again, but let’s hope humanity has moved past this) because, as I said, this measureable spike of carbon-14 in our atmosphere from the “bomb pulse” will eventually be depleted somewhere between 2030 and 2050.
Despite what the “bomb pulse” is and will offer to scientific research, isn’t it cool just knowing which cells have been at the party of you the longest? Or that like the rings of a tree, or the sedimentary layers of rock, our bodies too tell the story of our times? With that, until next time, stay curious my friends.
The Party Trick Physicist 