Thomas Dupois, age 10, of Saugus, Massachusetts, for his question:
How long has life been on earth?
A century ago, many sensible people thought that the earth was about 6,000 years old. Now we know that our planet has had at least four billion birthdays. In all that time, a great many things have happened. The job of tracing back the history of the earth is far from finished.
When the dinosaurs departed, they left their bulky bones behind and many of them became fossilized. We can reassemble whole skeletons and figure out the shapes of the giant reptiles. We can use radioactive tests to prove when the creatures lived. Modern science can retell the tale of the dinosaurs with no trouble at all. But their reign is just a small chapter in the story of life on earth. What's more, even though they thrived 100 million years ago, the big reptiles were late comers.
Modern experts agree that life on earth began in a very simple way. The first living things were almost certainly soft, little one celled plants and animals. Countless trillions of these simple organisms still thrive on the earth. And every day, countless numbers of them perish, leaving hardly a trace. They have no bones and no teeth, no hard claws or tough scales to form durable fossils.
We cannot, then, expect to find fossilized specimens of the earth's first living things. The earliest known fossils belong to crusty shellfish that thrived in the fresh water oceans more than 500 million years ago. They were far, far more advanced than the original living things. And the first living things were here millions of years before them. We can be fairly sure of this, even though they left no fossils to prove it.
All living things, even single celled living things, must keep in touch with their surroundings. They give and take and exchange substances in their bodies with substances in their surroundings. These processes of life cause changes in the rocks, the minerals, the water and other chemicals in the earth. Soft bodied, single celled alga and marine animals turned non living chemicals into graphite and limestone. The rocks they formed endured, giving indirect evidence of the early living things that made them.
In tracing back the story of life to its beginning, scientists look for certain deposits of iron, graphite or limestone. Many of these samples are older than 600 million years. We can be fairly sure that life has been on earth for at least 600 million years and almost sure that this was not the very beginning.
We can trace back the story of life for more than half a billion years. But even then, the earth was very old. Perhaps we can grasp the vast span of the ages if we scale the age of the earth down to one year, beginning January 1. The first known life would arrive in early November and the dinosaurs would rule the world for one week in December. The human family would arrive in the afternoon of the last day of the year.