In many branches of science, a good clock can make all the difference. The better we can determine how old things are and when events happened, the better we can put the pieces of history back together. It’s important to know that the universe started 13.73 billion years old, for example, and that the Earth is 4.56 billion years old.
There is no one clock to rule them all, though. Each science requires a clock of its own, and some of them require a whole wall of timepieces. For the universe, we have to use old light to tell time. For the Earth, certain radioactive elements like strontium have ticked away accurately since the planet’s formation.
Without a good clock, scientists are left to struggle to determine which things came before other things, which are the causes and which the effects, which are the first of their kind and which are late arrivals. The discovery of a good clock can immediately cast some versions of history into the trash, and cause us to take others more seriously. Just such a rethink is now happening in the study of human origins.
Ancient paintings on cave walls are among the most important markers in our history. Whoever painted a lion tens of thousands of years ago–or a series of parallel lines or even the outline of his or her own hand–must have had a mind that could conceive that expressive act, that could generate the desire to travel deep inside a cave to do so.
For all its importance, though, cave art is particularly hard to date. If there’s enough carbon in the paint—from charcoal, for example—you can use radiocarbon dating to determine its age. But some paintings have no carbon to measure. And if that art is old–older than 50,000 years, roughly speaking–the radiocarbon clock becomes useless.
In recent years, scientists have created a different clock for cave art. Sometimes water flows over those paintings, and it gradually deposits lumps or sheets of rock on them—known as flowstones. Those flowstones are laced with tiny levels of uranium—parts per billion. But if you know how to measure those trace amounts, you have yourself a clock, one that will put a minimum age on the paintings underneath.
Some of the paintings in European caves turn out to be very old. So old, in fact, that modern humans could not have painted them. Who did? You can get the whole story in my column this week in the New York Times.
The Happiness of Readers
Thanks to Kelly Weinersmith (co-author of the delightful Soonish) for tweeting kindly about my book. Making readers happy is my mission! Just a reminder–subscribers to Friday’s Elk will be eligible for a book giveaway on March 15. If you know of friends who might be interested, please forward this email, or let them know they can sign up for Friday’s Elk here.
My latest book, She Has Her Mother’s Laugh: The Power, Perversions, and Potential of Heredity, comes out in May. You can pre-order it here. You can find information and ordering links for my other books here. You can also follow me on Twitter, Facebook, Goodreads. LinkedIn, and Google+. If someone forwarded this email to you, you can subscribe to it here.
Best wishes, Carl
Originally published February 25, 2018. Copyright 2018 Carl Zimmer.