A SCIENTIST’S GUIDE TO DATING
How do we know how old things really are? As Rachel Williamson reports, it can be more controversial than it seems.
How to measure the age of the Earth? The lifespan of a whale? RACHEL
WILLIAMSON explores the technologies measuring time – and the controversies that suggest that not all great minds think alike.
In 2018, a Dutch pensioner tried to legally change his age. Having to admit to a chronological age of 69 was torpedoing Emile Ratelband’s swipe-right rate on the dating app Tinder, and besides, his doctors claimed his “true” biological age was a sprightly 45. Ratelband decided a more accurate official age should knock at least two decades off his current count.
The perky pensioner’s argument for the law to recognise biological over chronological age didn’t wash with the Netherlands court, however. It declined to let one man’s dating prospects alter the legal definition of age.
But although global media chuckled at Ratelband’s chutzpah, what our lusty pensioner needed to show the court was Horvath’s Clock, a molecular diary of DNA changes published in 2013 by US geneticist Dr Steve Horvath that measures biological, or epigenetic, age. One of the foundations of the current ageing and disease predictor research, Horvath’s Clock tracks the patterns of chemical tags on DNA made by methyl molecules over time. These correlate with a person’s chronological age; life in the fast lane can speed it up – smoking will do that – while exercise and diet can slow it down. Youth-seekers need to be careful which body part they’re basing their preferred biological age on, however: breast tissue ages faster than other cells, but the heart has been found to be up to 10 years “younger” than blood and 12 years younger than a person’s chronological age.
Ring cycles
The science of chronological dating relies on a museum-cabinet’s worth of research that seeks to find the age of everything from the Earth itself, to sharks and crustaceans, to Indigenous culture in Australia. It uses methods that range from cutting-edge technology to simple, careful counting.
Ring counting is a common and longstanding age marker for many types of flora and fauna. In sharks, you can count vertebra rings; in dinosaurs, it’s bonegrowth rings; in whales, the colour of earwax layers show different seasons.
While these are generally accepted measures, one of the limitations rests in not always knowing the age of a significant data set of animals from which to validate these methods. The hundreds of whale earplugs that have been studied, for example, have all been taken from dead animals whose actual age was never known. Researchers like Baylor University biologist Stephen J. Trumble have compensated for the lack of earplugs from whales with a known age by using re-sightings of individuals, hormone data to find how many times a cow has been pregnant, and even versions of Horvath’s Clock as earwax age validation.
“While not perfect, ageing using the earplug, especially when we have additional data (re-sights, ovaries, etc) makes it one of the best indices thus far,” Trumble says.
Sometimes, it’s the science itself that’s called into question. In 2012 researchers thought they’d cracked the mystery of how to find the age of crabs, shrimps and rock lobsters, a problem for fisheries’ managers charged with sustainably managing stocks of the popular crustaceans. A rock lobster study suggested tiny calcified rings on the eyestalks and on secondary teeth in their stomachs were markers of annual moulting, but research since has cast doubt on this finding.
Perhaps the best-known counting method is dendrochronology – counting the annual growth rings in tree trunks. Species like the kauri (Agathis robusta) in New Zealand and the bristlecone pines (the oldest species of which is Pinus longaeva) in North America reveal rings that correspond to every year going back for millennia. Dendrochronology doesn’t work on every tree species worldwide – the critical factor is consistent seasonal variability; so species growing in, say, a year-round damp tropical climate won’t be so “readable”. The oldest known sequence, from consecutive rings matched to multiple generations of trees, is from oaks in Europe, which go back 12,460 years. But even dendrochronology is utilising new frontiers of technology.
“Molecular dating uses the genetics of the species to see how different it is from its closest relatives,” says University of Tasmania plant biologist Dr Greg Jordan. “If you assume the amount of change is somehow related to how long they’ve lived, and the difference in genetics is related to how long it’s been since those species split, then you can work out when that split happened – and you can make an estimate.”
The dating of the nothofagus, or Antarctic beech, is a case in point, utilising both high-tech applications and our knowledge of scientific eruptions. It’s considered to be a “Gondwana thing”, Jordan says, but molecular dating put the species split some 50 million years after the supercontinent broke up, despite species being spread over several of Gondwana’s new continents. “That kind of research has been going on for about 20 years. People have been fighting over that,” Jordan says.
In recent years, it’s become apparent that some plants are actually part of the same individual – every King’s holly (Lomatia tasmanica), for example, is a genetic clone of its parent. Scientists are looking at how a still-theoretical method of genetic analysis could be used to date plants of this kind by creating a map of genetic mistakes over time.
Counting carbon
One of the best-known methods of establishing age is by radiocarbon dating, first proposed in 1946. It measures the ratio of volatile carbon-14 to carbon-12.
Carbon-14 has a half-life – the amount of time it takes to reduce or decay into a new chemical element, in this case carbon-12 – of 5730 years. Living tissues, like bones, stop accumulating carbon-14 when they die, so theoretically all a scientist needs to know is how much carbon-14 and carbon-12, both of which fluctuate over time, were in the atmosphere around the time of death. Then, by measuring the variation in these numbers now, they come up with an age.
Of course, it’s not nearly that simple, according to ANU archaeologist and bone-dating specialist Dr Rachel Wood. “Any sample material for radiocarbon beyond about 30,000 years gets difficult, because it’s incredibly sensitive to young contamination,” she says. “If you add 1% modern carbon to a sample that’s 50,000 years, we’re going to measure 37,000 years.”
To find out how much carbon-14 and carbon-12 were in the atmosphere at any given time, radiocarbon daters need to go back to those tree rings, which also lay down an atmospheric carbon record in each ring.
“Without calibration, radiocarbon wouldn’t work,” says Wood. “The tree-ring method, dendrochronology, is the reason why we can do radiocarbon. We’re still building calibration curves. It’s an absolutely enormous effort to build these…particularly when you go further back in time.”
Despite its limitations, radiocarbon dating is still a handy tool for those times when you need to figure out the age of Greenland sharks (simple: just date the carbon-14 in their eye lenses – a research breakthrough in 2016 that revealed one shark had lived for 392 years). And just this year, University of Melbourne PHD candidate Damian Finch pioneered the use of wasps nests’ radiocarbon to date a painting of a kangaroo on the ceiling of a rock shelter on the Unghango clan estate, in Balanggarra country in the north-eastern Kimberley region of WA. The painting was found to be between 17,100 and 17,500 years old – Australia’s oldest known cave painting.
“Constraining the age of rock art older than approximately 6000 years (ka) has remained a largely intractable scientific problem, particularly for rock engravings and for paintings where the paint no longer contains any original organic material,” Finch wrote in Science Advances last year.
So he painstakingly sorted through the particles of wasps nests underneath and on top of the paintings to find particles of charcoal from bushfires. Finch dated the charcoal, which wasps picked up in the mud they used to build their nests, to provide upper and lower age limits for the paintings.
But if scientists are right, we may need to imagine a time when radiocarbon dating is impossible, says Dr Chris Turney, the head of the Chronos 14Carbon-cycle facility at UNSW Sydney.
“We’ve gone from a situation in the 1960s where we flooded the atmosphere with radioactive carbon from bonkers thermonuclear bomb testing, and suddenly now we’ve gone into reverse and we’re flooding the
atmosphere with lots of what we call dead carbon – it’s got no radioactive carbon,” he says. “The practical upshot is that effectively radiocarbon won’t be able to be used, because you’ll have a massive spike in radioactive carbon from the 1960s when we did the bomb testing, and then suddenly the atmosphere appears to age really dramatically because of our emissions.”
Bones of contention
With new methods come different age determinations, and bare-knuckle fights in Australian age dating aren’t uncommon. A war of words erupted in 1989 when Bert Roberts, Rhys Jones and Mike Smith claimed the Arnhem Land site of Malakunanja II wasn’t 18,000–23,000 years old, but more likely to be 52,000–61,000 years old. Their team had used the then-experimental method of thermoluminescence, which measures the electrons trapped within quartz crystals of long-buried grains of sand. The method is “relative” (the same as Finch using the wasp nests), dating sand around the artefacts and fossils rather than the object itself.
It set off a battle by followers of absolute dating methods such as archaeologist Sandra Bowdler. Bowdler didn’t believe that Roberts et al. had “convincingly demonstrated a clear association of dated sediments and the artefacts intended to be dated” as she was “sceptical” that the rock layers in the luminescence sample site were strongly enough associated with the artefacts.
“That caused quite a controversy, because up to that point it was believed Indigenous occupation of Australia was probably something like 40,000 or 45,000 years,” says Dr Zenobia Jacobs, a South African archaeologist at the University of Wollongong who helped Roberts redate the site between 2012 and 2015. “But it became quite clear that we were bumping up against what we call the radiocarbon barrier. It was very difficult to get past about 45,000 years back then, and even now it’s only 50,000 years with radiocarbon dating because there’s just too little left to measure. And even a very tiny amount of contamination can make your sample quite a bit younger.”
Thermoluminescence has been surpassed by an even more powerful version, called optically stimulated luminescence (OSL).
Both luminescence methods date crystalline materials to the last time they were heated – whether by human-made fires or sunlight. The mineral grains in sediments absorb ionising radiation over time, which charges the grains in “electron traps”. Exposure to sunlight or heat releases these, removing the charges from the sample.
The material is stimulated using heat (thermoluminescence) or light (optically stimulated luminescence), which causes a signal to be released, the intensity of which can provide a measure of how much radiation was absorbed after the burial of the material – provided you know the amount of background radiation at the site.
One of the most profound dating fights surrounds Mungo Man, aka Lake Mungo 3 or LM3, found in 1974 in south-western NSW. LM3’S are the oldest human remains found in Australia. The body had been interred with careful positioning and ceremony and sprinkled with red ochre; it’s among the world’s earliest known examples of sophisticated, ceremonial burial practices. (See also African burial in Digest, page 8).
Initial dating relied on stratigraphic comparison with the sediments in which the cremated remains of Mungo Lady (LM1) were found in 1969 – not direct examination of samples from LM3. The result was an estimate that LM3 was 28,000 to 32,000 years old.
In 1987 a new method called electron spin resonance (ESR) dating was used on bone fragments from LM3, and determined an age of 31,000 ± 7000 years.
“ESR measures changes in the electron structure of the tooth enamel, which are caused by radiation, for example from small amounts of naturally occurring uranium, thorium and potassium in the sediment,” Wood says.
In 1999, thermoluminescence dating of sediment from the burial site returned results indicating that the burial was older than 24,000 ± 2400 years and younger
than 43,300 ± 3800 years. The same year, a team led by anthropologist Alan Thorne used information from uranium-thorium (which measures the decay of uranium-234 into thorium-230), ESR and OSL dating of both remains and surrounding sediment to propose a new age of 62,000 ± 6000 years.
“We don’t have any uranium in our skeletons, but when you put them in the ground, the groundwater normally has just a little bit of uranium in it and that infuses into your bones,” says Wood. “It’s often a bit younger than the age of the bones themselves, but it gives you a minimum age, and that can be combined with another method, of dating teeth, called electron spin resonance, which is a bit like OSL.”
For a variety of reasons, there was widespread disagreement about Thorne’s estimate.
Finally, in 2003, LM3’S discoverer – geomorphologist Jim Bowler – led a multi-disciplinary team that reviewed all the available information from different dating methods to reach a determination that LM3 is about 40,000 years old. This study also determined that LM1 was the same age, not younger as was originally believed. LM3 is among the oldestknown modern human fossils, and LM1 is thought to be the oldest known example of human cremation. The lesson to take from the changing ages of LM1 and LM3 is that new techniques and new ways of preparing materials for age dating allow researchers to hone dates and push past the old limits.
Who listens to the radiometry?
It is through the study of uranium and other radiometric “clocks” that age-dating science enables us to look beyond the history we can see in fossils and move into geological time.
Uranium-238, for example, has a half-life of 4.468 billion years, meaning it takes that long to turn into lead. Potassium-40 takes 1.248 billion years to become argon-40, and rubidium-87 takes 47 billion years to turn into strontium-87. Measuring the ratio of the “parent” elements to their “daughter” elements helped scientists arrive at the age for the Earth: 4.54 billion years, give or take 50 million years.
That number was determined by dating the oldest rock found on Earth, a zircon from Jack Hills, in WA, in 2014, which set the Earth’s minimum age at 4.3 billion years. The oldest samples of Moon rocks and asteroids have been dated between 4.4 billion and 4.5 billion years, which leads scientists to say the Solar System is about 4.57 billion years old.
Age-dating methods are a moveable feast of nuclear forensics, biological investigation, pollution monitoring and, in the case of tree rings, being good at counting.
The history of trying to figure out how long Aboriginal peoples have been in Australia proves how tricky it can be to put a solid date on anything, but also how much things can change when new techniques appear to shake things up.
Be they an aged but living lothario in the Netherlands or a fabulously preserved skeleton in the Australian desert, almost everyone wants to know how old they really are.
Jacobs says that working with Aboriginal people on sites like Madjebebe has set her thinking about the meaning of time.
“The way we’re trained and the way we create our narrative is all about arrival,” she says. But, she points out, “from their perspective, they’ve always been here.”
The study of uranium and other radiometric “clocks” helped scientists arrive at the age of Earth.