Thirteen hominid teeth that were discovered in a cave on an island in the English Channel between 1910 and 1911 have been re-examined with CT technology by researchers from The Natural History Museum of London, UCL Institute of Archaeology, University of Kent and four other institutions. The results of that analysis have shown that the teeth may have come from two different individuals who may each have been of mixed heritage — part Homo sapiens and part Neanderthal. The news has caught widespread attention.
There is a kind of romance to the idea of early humans and their cousins living in community. Our fascination with them depends to a certain extent on our being able to estimate how long ago they lived. The teeth found in the cave seem be about 45,000 years old. How do scientists know? Reckoning the age of someone or something that has been dead for tens of thousands of years depends on a relatively simple technology called radiocarbon dating. Here are twenty facts to help make that technology more understandable.
1. Radiocarbon dating has been available since 1946 when it was invented in Novel Prize winning work by University of Chicago physical chemist Willard F. Libby. Also called “carbon dating,” it’s a method for calculating the age of anything that’s dead.
2. It depends on a discovery made in 1939 by New York University physicist Serge Korff. He found that, when cosmic rays bombard Earth, they often encounter nitrogen, which is the most abundant element in our atmosphere.
3. Cosmic rays are not really rays. They are subatomic particles that shoot at nearly the speed of light through and between galaxies. Many originate from supernova explosions. Most are protons. Sometimes they include neutrons, electrons, and neutrinos.
4. Some cosmic rays have about 40 times the energy as particles accelerated on Earth through the Large Hadron Collider — which, by the way, is the single largest machine ever made. (And, think about it. The largest machine is used to study the smallest particles.)
5. One cosmic ray, the Oh-My-God particle detected over Utah in 1991, was probably a single proton. The space and astronomy news website Space.com estimated that, “That single proton slammed into our atmosphere going roughly 99.99999999999999999999951 percent the speed of light. And no, all those nines aren’t just for dramatic effect to make the number look impressive — it really was that fast…. Due to relativistic time dilation, at that speed, the OMG particle could travel to our nearest neighbor star, Proxima Centauri, in 0.43 milliseconds of the particle’s own time. It could continue on to our galactic core by the time you’ve finished reading this sentence (from its own perspective). OMG, indeed.”
6. Most cosmic rays are considerably lower in energy than Oh-My-God. Still, when a cosmic ray neutron collides with nitrogen in Earth’s atmosphere … mon Dieu! The nitrogen instantly gets turned into carbon — the very sort of carbon that is referred to in the term “radiocarbon dating.” Here’s how that change happens:
7. First of all, you need to know that all elements are defined by their atomic number, which is the number of protons in the nucleus. Any atom with 6 protons is carbon. Any atom with 7 is nitrogen.
8. When a neutron hits a nitrogen atom at unfathomably high speed, it joins the nitrogen atom’s nucleus. At the same time, it pops one proton out of that nucleus. This creates from a nitrogen atom that had 7 protons and 7 neutrons a carbon atom that has 6 protons and 8 neutrons. Having more neutrons than protons, the new carbon atom is unstable. Over time, it will beta decay, gradually changing back into nitrogen in a radioactive process. Carbon atoms that have 6 protons and 8 neutrons for a total atomic weight of 14 are notated as “carbon-14.” Because they are radioactive, they are also called “radiocarbon.”
9. Plain ol’ carbon — carbon that has 6 protons and 6 neutrons for an atomic weight of 12 — is far more stable than radiocarbon, and therefore vastly more plentiful in Earth’s atmosphere. There are about 1 trillion times as many atmospheric carbon-12 atoms as radiocarbon atoms.
10. Both carbon-12 and radiocarbon can bind with oxygen to form carbon dioxide molecules.
11. How does carbon dioxide get into dead things so that scientists can determine how long ago those things died? When plants are alive, through photosynthesis they convert atmospheric carbon dioxide into organic tissue. Because every animal, insect, microbe, and human on Earth derives its nutrition either directly from plants or indirectly from animals that eat plants, all life on Earth contains both radiocarbon and carbon-12.
12. The ratio of radiocarbon to carbon-12 that is in the atmosphere is equal to the ratio that is in plants and plant products — and in human, insect, microbe, and animal bodies.
13. When a living thing dies, it takes in no more atmospheric or dietary radiocarbon. The radiocarbon begins to beta decay.
14. For any element, beta decay happens at a unique, steady pace known as its “half-life.” Half-life is a probabilistic measure. It is the length of time it will probably take for half of the atoms in a sample of organic matter to decay. (Kaare Lund Rasmussen of the Institute of Physics, Chemistry and Pharmacy at the University of Southern Denmark, who has recently radiocarbon dated apostle relics at Santi XII Apostoli Church in Rome, says that in his work the sample has to contain at least 1 gram of pure carbon, and that requires about 20 grams of bones.) The half-life of carbon is 5,730 years.
15. To radiocarbon date an object, scientists decontaminate it and convert it into a gaseous, solid, or liquid form depending on which technology is to be used to count its carbon atoms. Once the scientists know how much radiocarbon and carbon-12 are in the sample, they compare the ratio of the two forms of carbon to the ratio of radiocarbon and carbon-12 in the atmosphere. If they find half of the radiocarbon that the 1-to-1-trillion ratio predicted, they know that the matter is about 5,730 years old. If they find one quarter of what the ratio predicted, they know the matter is about 11,460 years old. And so on.
16. But, oops. The radiocarbon to carbon-12 ratio in the atmosphere isn’t as unwavering as one might hope. That’s because tests and uses of nuclear weapons create additional radiocarbon. Weapon use in the 1940s and weapons testing in the 1950s and 1960s significantly increased the amount of radiocarbon in the atmosphere. By 1965, the concentration had nearly doubled. That’s called the “bomb effect,” and although the tests are now outlawed and the effect has waned considerably, it hasn’t disappeared. Magnetic field reversals (like the one that may have caused mass extinctions and prompted early humans and Neanderthals to live in caves) allow more solar radiation to enter the atmosphere. This results in more collisions with nitrogen and the creation of more radiocarbon.
17. Working against that increase, burning fossil fuels decreases the relative amount of radiocarbon in the atmosphere. Scientists must take both the bomb effect and fossil fuel effect into account when radiocarbon dating organic matter.
18. Because, eventually, all of the radiocarbon in a finite sample of organic matter beta decays, radiocarbon dating can’t help determine the age of anything older than 50,000 or 60,000 years. This means that some Neanderthal remains can be radiocarbon dated. Some cannot, for the earliest archeological evidence of Neanderthal communities may be as much as 400,000 years old. Dinosaur fossils definitely cannot be radiocarbon dated.
19. At last! Here’s something to worry about, if you’re given to that sort of thing. Radiocarbon is a part of every strand of DNA in the human body. Over the course of a human life, over 50 billion or so radiocarbon molecules emit radiation. This means that the radiocarbon that is the very stuff of life may be a source of dangerous genetic mutation.
20. Oh! And cosmic rays themselves can cause mutations in DNA. This is more of a problem for astronauts and airplane crews than for the average person. Still, there seems to be no escaping life — or the cosmos.
Kaare Lund Rasmussen of the University of Southern Denmark recently radiocarbon dated relics long venerated at the Santi Apostoli church in Rome. While his findings closed the book on at least one key fact about the relics, it brought to light a puzzle about an important dogma of the Catholic Church. See With a ‘Handshake,’ Science and Religion Pry Open Mysteries of the Catholic Church.