Million years ago

A million years ago is a way to talk about a very, very long time in the past. We call this time period "Mya," which is short for "million years ago," or sometimes "Myr" (which means megayear) and "Ma" (which means megaannum). All of these words mean the same thing — 1,000,000 years.

When scientists study Earth or life on Earth from a million years ago, they are looking at a time when many things were very different. Animals, plants, and even the shape of the land might have looked quite different. This unit of time helps us understand how old things are and how they have changed over very long periods.

A million years is also a huge amount of time when we think about it in seconds. It is about 31.6 teraseconds, which is a very big number! This way of measuring time helps scientists compare events that happened long, long ago.

For example, if you hear someone say that a fossil is from two million years ago, they mean it is twice as old as one million years. This way of talking about time helps us understand the history of our planet and the creatures that lived on it.

Usage

Myr is often used in areas like Earth science and cosmology. It can be used together with Mya or Ma. They help us talk about time in the past.

Myr is not recommended in geology, but it is commonly used in astronomy. In geology, you might see "Myr," which means a million years. In astronomy, "Myr" is the standard way to talk about million years.

Debate

In geology, people sometimes talk about whether to use the term Myr along with Mya or just use Ma. Both ways follow special rules for writing science.

Some writers like to say things like:

The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr.

Here, "Ma" means "million years ago," and "Myr" means a length of time.

Others think it's better to say:

The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma.

In this way, "Ma" only means the number of years, not "years ago."

Conflicts with the International System of Units and possible solutions

The word "mya" does not follow the rules of the International System of Units (SI) in a few ways. First, the letter "m" in "mya" stands for "milli," which means one thousandth in the SI, but it is not the right prefix for measuring years. Second, the letter "y" for "year" is not an SI unit. Instead, the letter "a" (which means year) can be used, and it is based on the SI unit of seconds. Third, "mya" talks about a specific point in time long ago, not a period of time.

Because of this, many people use "Ma" instead of "mya." While this changes the meaning a little, it works well if everyone agrees on how to use it. To make it very clear that we are talking about a point in time in the past, some people add "bp" (which means "before present") to "Ma." This helps avoid any confusion, but it is not exactly the same as "mya."