英语时差：DNA

You may already know that DNA is a long molecule found in almost every cell in your body that contains your genetic blueprints. Encoded in the particular sequence of your DNA is a description of what kind of organism you are, how you work, and how to make more of you. DNA can be extracted from living things–run your fingernail along the inside of your cheek to harvest a DNA sample right now–or from dead remains. Now, it turns out DNA can also be extracted from the soil, even without a body anywhere near. Researchers from the University of Copenhagen took a sediment core from a permafrost plain in Siberia. A “sediment core” is just a long, strait tube of earth. Imagine pushing a straw slowly into ice cream and then removing the straw: inside it would be a long thin ice cream sample. Since the surface of the earth is constantly covering itself over. With a sediment core, the lowest parts of the tube correspond to the oldest time period, while the highest parts are more recent.