I’m currently reading a Christmas present book, Ancient bones: unearthing the astonishing new story of how we became human , by the German palaeontologist and palaeoclimatologist Madelaine Böhme, co-written with Rudiger Braun and Florian Breier. I’ve become very taken with it, as it’s seriously challenging a hypothesis/theory I thought was now well established, namely the ‘Out of Africa’ hypothesis. But what would I know?
I hope to write about all that elsewhere, but here, in keeping with this blog’s raison d’être, I want to focus on a couple of dating techniques for ancient fossils that have been developed in recent decades, as solutions to the problem of providing the fullest picture of the planet’s living past and its evolution. Böhme’s book gives a brief overview of some of these technologies, and I want to see if I can fully understand them.
I note that the term ‘quaternary geochronology’, which I’ve just discovered, covers dating techniques for the quaternary period – the last 2.6 million years – though Ancient Bones is dealing with proto-humans going back as far as our links with bonobos and chimps, some 7 million years ago. That’s into the late neogene period, geologically speaking.
First, the most well-known, carbon-14 or radiocarbon dating. Carbon-14 is a rare, weakly radioactive isotope of carbon, first discovered in 1940, containing 8 neutrons instead of the usual 6 (carbon-13, a stable isotope, is much more common, making up 1% of all carbon). Carbon-14 is unstable and decays at a steady rate, hence its value as a tool in archaeology and geology. Böhme et al give a nice account of its use in dating:
Living plants regularly absorb C-14 out of the carbon dioxide in the air when they photosynthesise, and they incorporate it, along with C-12, into their tissues. The relationship between C-14 and C-12 in plants and in the animals that eat them therefore remains constant. After an organism dies, however, a kind of atomic clock begins to run. Because only the amount of C-14 present in the tissues at the time of death remains and no new C-14 is added, the portion of C-14 in the tissues declines at a constant rate through radioactive decay. This process is completely uniform, and it is therefore easy to translate it into time and use it to calculate the age of things.Ancient Bones, p154
I quoted this at length, as it’s a great explanation for novices. The limitation to this dating technique, though, is the relative rapidity of C-14 decay. It has a half-life of about 5,730 years, meaning that the amount of the C-14 isotope in samples reduces by half in that time. So it is only useful for dating items less than about 50,000 years old.
Radiometric dating, using the decay rates of uranium and thorium, can date fossils and rocks back to half a million years, and other radiometric measurements (uranium-lead and potassium-argon) can take us back further still, but this depends on whether rocks contain the required elements.
This is a fascinating technique which examines the orientation of magnetic particles in rocks. The Earth’s magnetic field (or geomagnetic field) – which interacts with and protects us from solar wind particles – is always shifting its orientation, and every so often, over millions of years, completely reverses itself (reversal of polarity). So different strata in rock formations can be dated in terms of these magnetic reversals. These are called magnetozones. Currently, our magnetic north pole is oriented roughly with our geographic North Pole, which is described as normal polarity as opposed to reverse polarity.
However, different rock types are more or less metallic in composition, and their structure varies. The best types of sediments for this kind of dating have fine-grained metallic elements that are more likely to orient with the ambient magnetic field at deposition.
The technique is particularly useful for gathering information about rates of sediment accumulation. Changes in magnetic orientations have been plotted on a Global Metallic Polarity Time Scale (GMPTS) and this can be plotted against the depth of sediment for a particular period of orientation, known as a chron (intervals of less than 200,000 years are called sub-chrons).
There are of course other dating methods, including relative dating based on reliably dated fossil samples, and new ‘absolute dating’ techniques are being developed all the time, including, for example, thermoluminescence, electron spin resonance and electron paramagnetic resonance for dating fossil teeth. But I’ll need to learn a lot more about them.
Madelaine Böhme, Rudiger Braun & Florian Breier, Ancient bones: unearthing the astonishing new story of how we became human, 2020