Earth's geologic history is recorded in its layers of rock—each layer contains information about the climate, life, and conditions at the time of its formation. Sedimentary rock is formed by the deposition of organic or inorganic sediments or the precipitation of minerals from a solution. As sedimentary layers are deposited one over another, they keep a time-sequenced record of Earth's history; the oldest layers are at the bottom and the youngest layers at are the top. Within these layers of rock, fossils provide indirect evidence of ancient life. The original organic materials of these ancient living things have long since been destroyed, but fossilized remains—essentially rock formed into the shape of the organic material—help scientists piece together an understanding of Earth's history.

The Salado Formation in New Mexico (featured in the video segment) is composed primarily of the mineral halite, commonly known as rock salt, deposited about 250 million years ago during the Permian period. It is a large salt bed, about 200 feet underground, which was formed by the evaporation of salty water; it is evidence that there was once a large body of salty water covering the area. As the water evaporated and the salt crystallized, some pockets of liquid solution were trapped within the salt.

These liquid inclusions (an inclusion is material that becomes trapped inside a mineral during formation) are providing scientists with a surprising view into the past: in contrast to the indirect evidence provided by fossilized remains, they contain direct evidence of life. Due to the low permeability of the deposit, samples carefully collected from the salt are uncontaminated. These samples contain the same materials that were deposited 250 million years ago! Amazingly, scientists have found intact cellulose molecules hidden in the pockets of liquid, unaltered from their original form. Cellulose is a common organic compound found in plants and algae, and produced by some bacteria. The ancient cellulose fibers found within the salt deposit behave just like modern cellulose: they look the same and they react the same way to particular enzymes.

In addition, scientists have found some small fragments of deoxyribonucleic acid (DNA). It is extremely unusual for organic materials to survive over such a long time period, but apparently the DNA was preserved by the salty environment. Scientists hope to be able to further study this ancient DNA to learn more about where it came from and what the DNA of living organisms was like so many millions of years ago. These original organic materials provide a unique opportunity to perform biological tests to study ancient life.

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