How correct are our dates?
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A Question of Time: How We Date Human Evolution
Our aim?What we are aiming to achieve is a tightly-grouped (precise) collection of shots in the centre of the bullseye (accurate). Now this is all well and good when everybody can see our brightly-lit target right there in front of us. This is how we set up our dating methods. We date material that we already know the age of, to see how precise and accurate we are. The result is clear, we can see it and you can see it, right there on the brightly lit target. Now we move the target further down the range – right to the edge of the radius of light from our spotlight. We can just make out the overall shape of the target, but not the details. We take our bow, select our first arrow and begin the process again. We know that our equipment is good, because we set it up earlier for the short-range target. Our method also appeared to be working, as our precision and accuracy were good at short range. But now we need to make some adjustments to accommodate for the longer range. We understand the physics, but we need to factor in a few variables. We fire the shot, it looks pretty good, and we all hear the resounding thunk! as the arrow strikes the target. This is how we calibrate our dating methods, such as adjusting radiocarbon dates using calibration curves. How precise were we? Well, we fire off some more shots, watch the flight of the arrows and listen to the thunk! Precision is easy to measure from the spread of our arrows. How accurate were we? Well, here is the problem: the target no longer has a known bullseye. We assume that the bullseye is within the spread of our arrows. It certainly was when the location of the bullseye was known.
A little furtherNow we move the target right to the back of the range, way beyond the limits of the spotlight and into the impenetrable darkness. We take our bow and arrow and fire off our first shot… and… thunk! Ok, we have a result. We try again. The first thing we discover – our arrows scatter widely. The precision is not good. But is the bullseye within the spread of our arrows? If nothing has changed, it should be. But what if there is a sidewind further down the range that we don’t know about? A wind blowing from left to right. All our arrows would be pushed to the right. No matter how often we shoot at the target, none will be close to the bullseye. If we knew about the existence of the sidewind, we can adjust our shots. But what if the sidewind wasn’t constant, and changed in strength from day to day? How did we go? How accurate were we? We feel pretty good about our equipment. For our method, however, we are starting to make some significant assumptions. This is how we use models and theories to map environments and conditions of the past in order to arrive at a result. Is our result correct? Only if all of our assumptions have been correct. A colleague then arrives, steps up with a different bow and she takes a shot. Thunk! But her arrow strikes quite some distance from our shots. We ask her to take another shot and she makes a few adjustments and does. Thunk! Ah, this time her arrow seems to strike more closely to where our shots went.
Many methodsAny analogy can be stretched – and we may be guilty of that here. The point is that we don’t know exactly what the target date is. We can only report our results, based upon our equipment, our methods and the assumptions that we are making. Along with a margin of precision. (We’ll look at some of these assumptions and margins when we consider our three dating methods later this week.) We try to take as many measurements as we can with the same equipment and method. We try to use as many different dating methods as we can on the same target. Then we compare the results. If three completely different methods all return a date range very close to each other, then that would appear to be a confirmation of the date. If three completely different laboratories and scientists using the same method return a date range very close to each other, then that would also appear to confirm the date. All teams would include their margins for error in their reported results. However – reverting back to our analogy – all six results may be experiencing the same unrecognised side wind. We asked Mathieu why it was important to use different dating methods.
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Your taskCongratulations for working your way through that analogy. So, what do you think? Select the comments link below and let us know if this has surprised you or confirmed what you already knew. Perhaps it is a topic that you have not given much consideration to before now. Take a moment to let us know.
ReferencesGrün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48. Aitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.
A Question of Time: How We Date Human Evolution
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