Earth's magnetic field is not only useful for sea captains to determine where the North is. It is a life-protecting mechanism. Space surrounding our planet has large amounts of radiation, like the radiation coming from our sun. This radiation is so strong, no living tissue could survive it. Fortunately for us, our magnetic field deflects much of this radiation, making life on our planet possible. However, if this magnetic field was too strong, life would not be possible either. Fortunately for us, the strength of our magnetic field is just right.
Earth's magnetic field looses strength with time. Every century, it decays by about 5%. The decay of the Earth's magnetic field has been recorded for over a century by modern scientists, and proved to be consistently decaying. If the magnetic field was 5% stronger each century, as we go back in time, very quickly, it would become so strong, life on Earth would become impossible.
Study of magnetization of bricks, pottery, campfire stones, have shown that around 1000 A.D., earth's magnetic field has been about 40% stronger than today.
How do we know that the Earth's magnetic field did not fluctuate over time, though? Good question. And the fact of the matter is that it probably did. The study of magnetization locked into rocks at the time of their formation, has shown that the Earth's magnetic field has reversed its direction hundreds of times as the geologic layers were laid down. But during these reversals, the magnetic field has been decaying even more rapidly. Taking that into account, life on Earth would become impossible, as late as 8700 years ago. Even if this energy decay is due not only to free decay, but also due to residual motions in the core fluid, the maximum age safe for life would only be about 100,000 years.
Humphreys, D.R. 1993. The Earth's magnetic field is young, Acts & Facts. 22(8), icr.org/article/371