The Molecule Hunt: Archaeology and the Search for Ancient DNA

  • Martin Jones
Allen Lane: 2001. 280 pp. £18.99

Traditional archaeological research describing skulls and pottery fragments has expanded in recent years to the use of genetic and biochemical methods in pursuit of the origins of humans, domestic animals and agriculture. The rewards have been great, and structural determinations of DNA and other macromolecules in ancient materials have transformed the field almost as much as DNA analysis has enhanced forensic medicine. For example, an unknown genius in southeastern Turkey, about 10,000 years ago, must have dramatically altered human life patterns by domesticating wild cereals, thereby initiating agriculture. The later exploitation of wild maize in America may have been a more gradual process.

Credit: DAVID NEWTON

DNA sequencing has enabled us to trace the origins of horses, dogs and cattle as human domestic animals in a detail that was hard to imagine a few years ago. The huge and fierce aurochs, depicted in cave paintings and at ancient grave sites, now emerges as a kind of bovine Neanderthal, extinct and not a direct precursor of modern cattle.

Martin Jones tells the intriguing story of the new field of bioarchaeology in this timely book. From his position as the George Pitt-Rivers Professor of Archaeological Science at Cambridge University, and chairman of the UK Ancient Biomolecules Initiative, he has had a broad and unique overview of the rapid evolution of this new area of research, and his lucid and authoritative writing conveys the excitement of the field. Human migration patterns in ancient times, the causes of terrible epidemics in the distant past, and many similar questions can now be defined by DNA analysis.

Because DNA decomposes by chemical decay with time, it has not been possible to extend molecular analysis into the very distant past, beyond about 100,000 years. However, the development of ultrasensitive sequencing methods for proteins, which are more stable, might push the time barrier a little further. The oldest DNA fragments yet recovered are short stretches of mitochondrial DNA from woolly mammoths and mastodons. Not surprisingly, their DNA sequences show great similarities to modern elephants. But the interesting fact has emerged that there were many quite distinct types of mammoths, and so elephants represent merely a small remnant of a once-great pattern of diversity.

The retrieval of DNA sequences from Neanderthal bones, slightly less old than the mammoths, has been a particular triumph in this field of research, and has greatly elucidated the distinct differences between early humans and Neanderthals. A decade ago, before it was realized that DNA cannot be preserved for millions of years and contamination with modern DNA is a major technical problem, there were several overly optimistic reports on the apparent recovery of DNA from insects entombed in amber and other potential sources more than 100 million years old.

The recovery of DNA from a dinosaur bone made the headlines in both the scientific and daily press. It was followed by the dampening discovery that this particular DNA sequence was identical with a piece of human junk DNA, a fragment of mitochondrial DNA that had found its way to the cell nucleus and been integrated as a pseudogene during evolution. It is not often that molecular biology turns into farce, but in the published discussion that attempted to explain this amazing finding, the possibilities were raised that a roving dinosaur had entered the laboratory and contaminated a piece of equipment, or that in the distant past intercourse had taken place between a dinosaur and a human individual. Jones recounts the absurd story totally straight.

The contamination problem remains the greatest difficulty in this dynamic new area of research. In a recent conversation with one of the leaders of research into ancient DNA, I was told how he had handed a famous palaeontologist a brownish, ancient human skull from a museum collection with a query as to whether the skull had been coated with varnish. The palaeontologist gripped the skull with both hands, stretched out his tongue, licked the top of the skull and decided from the taste that the skull indeed had been varnished. In the process, large amounts of modern human DNA would have been transferred to the skull, hampering any future search for traces of remaining ancient DNA fragments. Perhaps archaeologists should change their field techniques and use sterilized instruments, protective clothing, plastic gloves and face masks during their digs.

The book's final chapters describe how the molecule hunt continues. Jones is more circumspect and less authoritative here dealing with microbiological problems, in particular with regard to recent bizarre claims that very ancient bacterial spores might be revived. It requires considerable wishful thinking to believe that bacteria recovered from an excavated fossil or salt crystal are as old as the fossil itself, especially if the bacteria are common extant ones. The only credible way to retrieve very ancient inherited information would be by identifying a slowly propagating organism, halted in its evolution and possessing unique properties clearly not shared by other existing species.

Perhaps some cold and hostile ecological niche, with a reducing, anoxic environment similar to that of Earth a couple of billion years ago, continues to shelter a lurking primitive microorganism that has retained an inefficient early version of the genetic code, or even a chemical precursor form of modern DNA. The isolation and propagation of such a molecular coelacanth would be received with astonishment even by the scientific world, and would teach us much about the origins of life.

The chances of finding something may not be great, but very little work has been done on anaerobic systems from this point of view. In fact, the odds seem rather better than those on the futile and costly search for life on Mars.