For scientists, this is not an easy question to answer.

Some of the longest living organisms are trees. You might have counted the rings of a tree stump to determine its age, where each ring is one year of growth. To measure the age of many kinds of living trees, scientists do something similar. They drill a small hole into the trunk of a tree and take out a long piece called a “core.” This does not fatally damage a tree, and the rings of the core can be counted to determine a tree’s age.

The oldest measured tree, a bristlecone pine from the White Mountains in California, is often considered to be the oldest living organism. A core of this tree revealed it was about 5,070 years old. That's roughly the lifetime of 68 people combined!

However, the bristlecone pine isn’t necessarily the oldest living thing on earth.

Some organisms grow in “colonies.” A colony is a group of organisms living close together that are identical. They start as a single organism that reproduces by making a copy of itself. Bacteria often grow in colonies, but some larger organisms grow this way too. In Fishlake National Park in Utah, there is a clonal quaking aspen tree named Pando. Pando is so large, it looks like a forest! All of the trees share the same root system and came from one seed, so they could be considered one living thing.

It’s difficult to measure the age of organisms like this accurately. Often, the size of the colony is used to estimate age. Pando is estimated to be anywhere from a few thousand to 80,000 years old. There is also a colony of seagrass near Spain that is thought to be anywhere from 12,000 to 200,000 years old.

However, these clonal organisms aren’t necessarily the oldest living thing on earth, either!

Some organisms have a dormant stage of life. Certain bacteria can exist as spores, where they pause their life processes for a long period of time – sometimes a very, very long period of time. Though debated, spores may be considered living because they can resume their life processes normally when conditions are favorable. Scientists found spores preserved in amber that were suggested to be 25-40 million years old and spores in salt crystals that were suggested to be 250 million years old, though these findings have been somewhat controversial.

However, even if the findings are correct, spores may not be the oldest thing on earth!

Collectively, scientists have discovered approximately 1.7 million species, but it’s thought that there is anywhere from 5 to 10 million species living on earth. The oldest living thing could easily be an organism we have yet to find.

Currently, the shortest and most accepted answer this question is that the bristlecone pine is the oldest living thing on earth. But because “living thing” can have a few different meanings, age can be difficult to measure, and there’s still so many organisms we don’t know about, this question may not yet have a definite answer!