Recent news that scientists had recovered actual marrow from a dinosaur fossil aroused much amazement. But the achievement is not a surprise. In fact, it doesn't even set a new record for the oldest pieces of life.
Most of us think of fossils as dead things that have been petrified, turned to stone. But that doesn't have to be. The actual bodies of once-living things can escape being petrified for a very long time under the right conditions.
A fossil is defined as any evidence of life from the prehistoric or geologic past that is preserved in the Earth's crust. A prejudice against preservation may have kept scientists from looking for meat in the ancient bones, but now we know better, and a race is on to find ever-older tissues.
Creatures in Ice
Ötzi, the 5,000-year-old "ice man" found in an Alpine glacier in 1991, is the best-known example of a frozen fossil. Mammoths and other extinct polar animals are also known from permafrost. These fossils are not as pretty as the food in your freezer, as they undergo a kind of slow mummification in the frozen condition. It's a geologic version of freezer burn in which ice migrates out of the tissues into the surroundings.
Frozen bison bones nearly 60,000 years old were analyzed in 2002, yielding DNA fragments and bone proteins that could be compared to existing species. Mammoth hair turns out to be even better than bones for preserving DNA.
But Antarctica holds the record in this field, with microbes in deep ice that are 8 million years old.
The desert preserves dead matter by desiccation. Ancient humans have been naturally mummified this way, such as the 9,000-year-old Nevadan known as Spirit Cave Man. Older material is preserved by various desert packrats, which have the habit of making piles of plant matter cemented into rock-hard bricks by their viscous urine. When preserved in dry caves, these packrat middens can last tens of thousands of years.
The beauty of packrat middens is that they can yield deep environmental data about the American West during the late Pleistocene: vegetation, climate, even the cosmic radiation of the times. Similar middens are being studied in other part of the world.
Even the remains of extinct creatures still exist in dried form. Mammoths are most famous for their permafrost carcasses, but mammoth dung is known from desiccated specimens.
Of course "Jurassic Park" put amber in the public consciousness with its plot based on the idea of retrieving dinosaur DNA from blood-sucking insects trapped in amber. But progress toward that movie's scenario is slow and possibly stopped. Lots of different creatures are documented from amber, from frogs and insects to bits of plants. But the published DNA retrievals have not yet been duplicated.
In a few places plant matter has been preserved in sediment for many millions of years. The Clarkia beds of northern Idaho are between 15 and 20 million years old, putting their origin in the Miocene Epoch. Tree leaves can be split from these rocks still displaying their seasonal colors, green or red. Biochemicals including lignins, flavonoids and aliphatic polymers can be extracted from these fossils, and DNA fragments are known from fossil liquidambar, magnolias and tulip trees (Liriodendron).
The current champions in this field are the Eocene dawn-redwood forests of Axel Heiberg Island, in the Canadian Arctic. For about 50 million years the stumps, logs and foliage of these trees have been preserved almost totally unmineralized, thanks to swift burial in conditions that kept oxygen out. Today this fossil wood lies on the ground, ready to pick up and burn. Tourists and coal miners alike threaten this scientific treasure.
Mary Schweitzer, the North Carolina State University professor who documented soft tissues in Tyrannosaurus rex leg bones, has been exploring biomolecules in ancient fossils for several years. The presence of those in the 68-million-year-old bones was not the oldest of her finds, but actual tissues of this age are unprecedented. The discovery challenges our notions of how fossils form. Surely more examples will be found, perhaps in existing museum specimens.
Naturally the claim brought criticism: the laboratory or the salt bed was contaminated, and in any case the DNA of the microbes (the genus Virgibacillus) was too close a match to more recent species. But the discoverers have defended their technique and raised other scenarios for the DNA evidence. And in the April 2005 Geology they published evidence from the salt itself, showing that it (1) matches what we know of Permian seawater and (2) appears to date from the time of the salt's formation, not a later event. For now this bacillus holds the title of Earth's oldest living fossil.