Life in the old fossil yet

WHEN most animals die, nature likes to tidy up by making their bodies disappear. The remains get eaten by scavengers, bones are scattered, tissues rot away and anything left over tends to get destroyed by the elements. Very occasionally, though, these destructive processes get disrupted. This usually happens when the corpse is quickly buried by sediment deposited by a river or blown in by the wind. Then begins a slow process in which minerals precipitate from groundwater into the encased organic material, eventually replacing it with a stony replica: a fossil.

Such was palaeontological doctrine for decades. But in recent years traces of soft tissue, such as blood vessels and bone cells, have been found in some dinosaur fossils. Now researchers have come up with an explanation for how these tissues were preserved for millions of years, which just might make it possible to extract some elements of prehistoric DNA.

That there was more in a fossil than meets the eye emerged in 2005 when Mary Schweitzer, a palaeobiologist at North Carolina State University, found something unusual after her team used acid to dissolve minerals from a fossilised piece of Tyrannosaurus rex bone. Left behind were some fibrous tissue, transparent blood vessels and cells. Many argued that this material must have come from modern bacteria and not a T. rex, since nothing organic could possibly survive the 68m years since the creature had walked the Earth.

In 2012, however, Dr Schweitzer and her colleagues went a step further, revealing the presence of proteins in a dinosaur fossil that had been freshly dug up and carefully protected from any potential contamination. Moreover, one of the proteins the researchers identified could be found only in birds. Since dinosaurs were the ancestors of modern birds, the discovery made it hard to argue that soft-tissue material in the fossil could have come from bacterial contamination. Still, many scientists wondered how such a thing was possible.

Cast-iron evidence

In a new report in Proceedings of the Royal Society Dr Schweitzer and her colleagues collaborated with a team led by Mark Goodwin, a palaeontologist at the University of California, Berkeley, to seek an explanation. Organic material from dinosaur bones was studied using micro x-ray absorption spectroscopy, which allows scientists to examine the structure of matter using intense light beams. This led Dr Goodwin to notice something remarkable: the organic material in the samples was thickly laced with iron nanoparticles. In animals, iron is most commonly found in blood and this led the researchers to wonder if the iron had come from blood cells that had once flowed through their dinosaur’s veins. Could it have played a part in the preservation of the tissues?

To test this idea, the researchers designed an experiment using freshly slaughtered ostriches which, being large and flightless birds, seemed to be a reasonable modern equivalent to dinosaurs. They extracted blood vessels from the bones of the birds and soaked them in a haemoglobin solution obtained from ruptured ostrich blood cells for 24 hours. The samples were then placed in both a saline solution and sterile distilled water. As a control, some of the blood vessels were put straight into saline solution or water without being pre-soaked in blood.

As expected, the ostrich tissues that went directly into the water and the saline solution fell apart rapidly and were entirely consumed by bacteria or heavily degraded in just three days. The same thing happened to the tissue soaked in haemoglobin and placed in water. But the treated sample in the saline solution remained intact and has stayed that way for two years now, with no signs of bacterial growth.

Dr Schweitzer and Dr Goodwin believe that highly reactive ions known as free radicals, which are produced by iron as it is released from the haemoglobin, interact with the organic tissue causing abnormal chemical bonds to form. These bonds effectively tie proteins in knots at the molecular level, much as the preservative formaldehyde does. This knot-tying makes the proteins unrecognisable to the sorts of bacteria that would normally consume them. This, they theorise, is how the soft tissues manage to survive for millions of years without rotting away.

The iron nanoparticles, however, may be doing more than just preserving tissues. Despite what happens in the science fiction world of “Jurassic Park”, no dinosaur DNA has yet been found. The reason for this is that DNA is thought to have a half-life of 521 years, which means that, after that much time, half of the bonds between the proteins that make up DNA have broken apart; after another 521 years, another half have gone, and so on. This leaves very little behind after hundreds of thousands of years yet alone the 65m years or so that stand between humanity and dinosaurs. Even so, Dr Schweitzer and Dr Goodwin still wondered if the iron-based preservation process might allow DNA to bypass its typical half-life and last a lot longer.

To find that out, the team used an iron-removal compound known as pyridoxal isonicotinic hydrazide to delicately pull iron away from the dinosaur tissues without damaging them. They then added four different stains that react only with either DNA itself, or with proteins closely associated with it in organisms other than microbes. Remarkably, in all cases, these specific stains lit up inside the ancient cells in the tissue samples. This hints that something chemically very similar to DNA can remain in a fossil and might yet be hidden precisely where it had resided during life.

As to whether any DNA strands can ever be read remains to be determined because of the complex knots that they have been tied into by the iron. But Dr Schweitzer and Dr Goodwin plan to try. Many things other than iron tie proteins into knots. One is sugar which, when exposed to tissues in high enough concentrations, can also cause abnormal bonds to form. This process damages tissues in diabetics. Some doctors suspect the process might be reversible by using drugs, like N-phenacylthiazolium bromide, that selectively cleave abnormal bonds while leaving normal ones alone. With this in mind, the researchers are considering trying some of these drugs on their dinosaur proteins to see if they can untangle them.

In the end, such tactics will not be quite as poetic as Hollywood’s notion of collecting dinosaur DNA from bloodsucking mosquitoes preserved in tree sap. But if it results in sequencing even part of the genes of a T. rex to determine more about these and other prehistoric animals, then no one is going to mind.