When did life start on earth?

A.S. is making a logical mistake in his comment, assuming that we cannot apply a measurement scale before the scale existed! However, I can coherently say that the distance from A to B was x miles long before the scale of miles existed. The world existed before A.S. was born, just as time existed before we came up with our measurement scale. Similarly we can use our current convention of measuring a year as roughly 365 days and apply it before the arbitrary calendar was created! Using scientific techniques such as measuring the distance of stars from us, their expansion relative to us and each other, and looking at their redshift, we can calculate their speed and distance from us and hence approximate the age of the universe and, closer to home, by analysing our planet and the age of rocks work out when the earth came into existence. Taking all this into account and using our current scale for the measurement of a 'year' we can work out with reasonable accuracy a date for the universe.
By: Dan

Date of comment: Wed, Mar 22nd 2006

How could anyone possibly know that the earth has been around for 4.5 billion years? Creation is undated. There was no measurement of time as we know it until the Greek and Roman civilizations existed, and that was not millions of years ago. The Romans began a calendar at the time of Julius Caesar, at the time and death of Jesus Christ. Hence, we are in the year of our Lord (AD) 2006.
By: AS

Date of comment: Sun, Mar 19th 2006