Sunday, May 20, 2012

Archaeology: Date with history

By revamping radiocarbon dating, Tom Higham is painting a new picture of humans' arrival in Europe.

Beside a slab of trilobites, in a quiet corner of Britain's Oxford University Museum of Natural History, lies a collection of ochre-tinted human bones known as the Red Lady of Paviland. In 1823, palaeontologist William Buckland painstakingly removed the fossils from a cave in Wales, and discovered ivory rods, shell beads and other ornaments in the vicinity. He concluded that they belonged to a Roman-era witch or prostitute.

“He did a good job of excavating, but he interpreted it totally wrong,” says Tom Higham, a 46-year-old archaeological scientist at the University of Oxford's Radiocarbon Accelerator Unit. Buckland's immediate successors did a little better. They determined that the Red Lady was in fact a man, and that the ornaments resembled those found at much older sites in continental Europe. Then, in the twentieth century, carbon dating found the bones to be about 22,000 years old1 and, later, 30,000 years old2 — even though much of Britain was encased in ice and seemingly uninhabitable for part of that time. When Higham eventually got the bones, his team came up with a more likely scenario: they were closer to 33,000 years old and one of the earliest examples of ceremonial burial in Western Europe.

“It is another sobering example of cocked-up dates,” says Higham, whose laboratory is leading a revolution in radiocarbon dating. By developing techniques that strip ancient samples of impurities, he and his team have established more accurate ages for the remains from dozens of archaeological sites. In the process, Higham is rewriting European history for around 30,000–50,000 years ago — a time referred to as the Middle-to-Upper Palaeolithic transition — when the first modern-looking humans arrived from Africa and the last Neanderthals vanished. Higham thinks that better carbon dating will help to resolve debates about whether the two ever met, swapped ideas or even had sex. It might even explain why humans survived and Neanderthals did not.

“I admire him,” says Paul Mellars, an archaeologist from the University of Cambridge, UK, and an expert on this period in Europe, for “the sheer doggedness and sense of vision” he has for improving radiocarbon dating of the Palaeolithic. That vision sometimes clashes with other scientists' views, but Higham makes no apologies for his interpretations as long as the dates are solid. “I want to know the truth” is something he says a lot.

A woolly field

If you Google 'archaeologist' and 'Higham', the first hit is likely to be Charles Higham, a 72-year-old professor who has charted the origins of agriculture and government in southeast Asia. Tom was born in Cambridge, where his father was based until 1966. Charles then moved the family and nine-month-old Tom to New Zealand's rugged south island to start an archaeology department at the University of Otago in Dunedin. As a teenager, Tom spent summers at Ban Na Di, a study site in northeastern Thailand, where his duties included helping with human excavations and brewing tea for the crew.

Tom didn't originally plan to follow his father's path. As a child he was obsessed with the history of the American West. At university, he planned to study geography and glaciology, but switched to archaeology after excelling in an introductory course taught by his father that he had signed up for on a whim. But his enthusiasm soon waned. “I got less and less interested in archaeology because it was so subjective and woolly.”

The reasons for that woolliness were partly technical and partly historical, dating back to before the Highams' time. Archaeology before carbon dating relied on two principles: older things are buried beneath younger things, and people with cultural ties make similar-looking objects, such as stone tools. But dates were hard to come by. In the early nineteenth century, the Danish historian Rasmus Nyerup wrote that most of early human history was “wrapped in a thick fog”3. “We know that it is older than Christendom,” he wrote, “but whether by a couple of years or a couple of centuries or even by more than a millennium, we can do no more than guess.”

The fog began to lift in the middle of the twentieth century, when US chemist Willard Libby and his colleagues4 showed that all formerly living things bear a clock powered by radioactive carbon-14. Organisms incorporate tiny amounts of this isotope as they grow, and they maintain a constant ratio between it and other, non-radioactive, carbon isotopes throughout their lives. After death, the carbon-14 decays with a half-life of about 5,730 years, and the dwindling ratio serves as a time stamp. Libby's team proved the accuracy of this 'clock' on objects of known age, such as Egyptian mummy tombs, and bread from a house in Pompeii, Italy, that was burned during the eruption of Vesuvius. Libby earned the 1960 Nobel Prize in Chemistry for his work.

The clock gets less accurate as the samples age, however; cruelly, it begins to fail at one of the most interesting times of human history in Europe. Within 30,000 years, 98% of the already vanishingly small quantities of carbon-14 in bone is gone. And carbon-14 molecules from surrounding soil start to seep into the fossils. Collagen, the part of bone that contains the most carbon suitable for dating, sops up contaminants like a sponge, creating a false record. If just 2% of the carbon atoms are contemporary, then a 44,000-year-old bone will return a carbon date of 33,000 years old, Higham calculates.

Most of the thousands of carbon dates from archaeological sites from the Middle-to-Upper Palaeolithic era are wrong, say scientists, perhaps even as many as 90%. As a result, archaeologists can agree on the history of this era only in the broadest of brushstrokes.

Tom found himself drawn to the quantitative side of archaeology to help fill in those details. His father had counselled that if he wanted a future in the field, Tom ought to join the push to make it a more rigorous science, emphasizing testable theory, experiment and statistics. So, at his father's urging, Tom applied for and completed a PhD at the University of Waikato's Radiocarbon Dating Laboratory in Hamilton, then did a postdoc there. And when a faculty position became available at a better-funded lab at the University of Oxford in 2000, he moved back to his birth country.

Any idea that archaeology hasn't gone in the direction that Charles predicted is dispelled by a visit to his son's workplace. Its centrepiece is a giant £2.5-million (US$4-million) particle accelerator, which is used to tot up the number of radioactive carbon molecules in a sample.

Similar machines have been used for carbon dating since the 1970s and have allowed scientists to date smaller samples with more precision than before. But they have also produced their share of erroneous dates. “People used to take bones, grind them up and date them, and you got all kinds of dates because no one bothered to check if there was collagen or not,” says Ofer Bar-Yosef, an archaeologist at Harvard University in Cambridge, Massachusetts. And rather than damage valuable human bones or animal bones marked with cuts from stone tools, scientists tended to date fragments of unidentified animal bones found alongside human remains, assuming, not always correctly, that they coincided with human occupation. “It just breaks your heart to see what people have dated before. They've basically dated pieces of shit,” Higham says.

To continue reading this article, visit the site.

Callaway, Ewan. 2012. "Archaeology: Date with history". Nature. Posted: May 3, 2012. Available online:

No comments: