Human DNA has been recovered from a Neanderthal fossil 70,000 years old. That's a record, but there may be plenty of DNA recoverable from a human body 10, 50 or even 150 years after death.
The bodies of the Romanov royal family, executed during the Russian revolution, were identified from DNA comparison in the last decade. US scientists recently confirmed the death of the outlaw Jesse James after they exhumed a disputed corpse. Time, says Mark Thomas, a forensic anthropologist at University College London, is not really the problem, even in the tragic circumstances of the Indian Ocean tsunami.
The technique of DNA sampling is less than 20 years old, but has become standard practice the world over. Standards, however, are not quite the same the world over, which may be why a Japanese forensic delegation has announced that it will re-examine all the bodies originally handled by Thai experts, why China has offered to collate the data, why a US team has announced that it will handle the samples from all "foreign-looking" victims, and why the Israeli government sent a 19-member team to help search for Israeli victims.
Both volunteers and resident experts have a host of challenges - stricken relatives, rapidly decaying bodies and damaged infrastructure for a start - but at least time is on their side. DNA is vulnerable. It breaks down in sunlight and water, and there are enzymes that naturally destroy it. But long after death, samples would survive in teeth and bones. Police forensic scientists - often working with only the tiniest samples from a fingerprint or a spatter of saliva - have to worry about the possibility of contamination. But the teams of forensic experts working in Thailand to identify the thousands of victims have no such worries.
"It would be very straightforward to take a biopsy which would be a very reliable source of DNA. That would be stored either in a chemical buffer that prevents the breakdown of DNA, or frozen," Thomas says. There might be greater administrative difficulties about where the samples are kept, how they were recorded and how easily people could have access to them, but these are administrative challenges rather than scientific. Altogether, the tsunami swept the shores of 10 countries and killed an estimated 150,000 people, citizens of around 50 nations. The administrative challenges are likely to be huge and the work of identification slow. Ten days after the disaster, Interpol officers met in Lyon and called for a standardised method of recording DNA profiles, fingerprints, tattoos and birthmarks, all accessible from one centralised source.