[Example] Freeze-dried chromosomes can survive for thousands of years

For palaeontologists, DNA is infuriatingly fragile. Its long chains begin to break apart shortly after death, destroying valuable information about the deceased parent organism. Unlike bones, footprints and even faecal matter, which can comfortably survive—in fossilised form—for millions of years, DNA rarely lasts much more than a hundred. In recent decades scientists have discovered that some exceptionally well-preserved bodies do still have readable fragments of genetic code hundreds of thousands of years after death. But these have been tiny scraps. They lack much of the valuable information that an intact genome provides.

A major advance may be at hand. A new paper in Cell, a scientific journal, describes the discovery of fossil chromosomes—coiled-up strands of DNA millions of base pairs long—inside the cells of mammoths that died tens of thousands of years ago. The research “really opens a door for a new kind of exploration of ancient life”, says Erez Aiden of the Baylor College of Medicine (BCM) in Houston, Texas, who is one of the paper’s lead authors.

DNA is a resolutely three-dimensional molecule. In animals and plants, long strands are coiled and folded into chromosomes, which store genetic information inside the nucleus of a body’s cells. The architecture of those chromosomes is as important as their content. It enables interactions between specific strands of DNA and regulatory proteins in the nucleus, thus determining which genes are expressed in different cells and tissues. Snippets of DNA recovered from ancient specimens lack this valuable information. Given the structural complexity of a chromosome, the chances of ever finding one were deemed to be extremely slim.

That did not stop Dr Aiden and his colleagues from looking. As water and the microbes that feed off it play a central role in the degradation of DNA, the researchers quickly realised that a dehydrated sample offered the best chance of preservation. More intriguing, they discovered that when a specimen dries into the equivalent of jerky, the complex structures of the chromosome also get preserved as a sturdy solid. Because the resulting object lacks the structural regularity of a crystal, it is technically known as a glass. Over nine years, Cynthia Perez Estrada, also at bcm, has searched for this “chromoglass” in all sorts of places: crickets, roadkill and mice—even in, she says, “my dinner from Thanksgiving”. Her research also suggested it would be theoretically possible to find such chromoglass in ancient specimens of extinct species, provided they were kept sufficiently cool and dry.

This hypothesis was borne out when the team was sent samples from two mammoths found in caves in northern Siberia. Both were females. One had been frozen for 39,000 years, the other for 52,000. They were so complete that hair, skin follicles, and even individual cells could be seen upon magnification. And, deep inside those skin cells, chromoglass had naturally formed.

The team in Houston collaborated with colleagues across Europe to dig out the information preserved in the glass. It revealed that mammoths had 28 pairs of chromosomes, the same number as in their living elephant relatives. The analysis also provided unprecedented information on the mammoth’s epigenome, the set of modifications made to DNA that determines which genes are actively influencing its parent organism and which are simply along for the ride. By comparing it with the epigenome of present-day elephants, the researchers found that one of these active genes was responsible for the long hair that helped the mammoth handle cold temperatures. Future analysis could shed light on the mammoths’ immune systems, revealing the adaptations they developed to cope with pathogens native to the cold steppe environment.

Dr Aiden expects chromoglass to be found wherever dehydration happens quickly, including in deserts and on mountain peaks. Excitingly, its durability suggests samples even older than the mammoths may be found. Under ideal conditions, the researchers say, chromoglass may survive for as long as 530m years. That would give palaeontologists a window on the past stretching as far back as the Cambrian era, a time when animal life had yet to make the transition onto land. Even if this estimate turns out to be optimistic, DNA may be more resilient than anyone imagined. ■