Inside our cells, each of us has a second set of genes completely separate from the 23 pairs of chromosomes we inherit from our parents.
And this isn't just the case for humans — it's true of every animal, plant, and fungus, and nearly every multicellular organism on Earth.
This second genome belongs to our mitochondria, an organelle inside our cells.
They're not fully a part of us, but they're not separate either — so why are they so different from anything else in our bodies?
Approximately 1.5 billion years ago, scientists think a single-celled organism engulfed the mitochondria's ancestor, creating the predecessor of all multicellular organisms.
Mitochondria play an essential role: they convert energy from the food we eat and oxygen we breathe into a form of energy our cells can use, which is a molecule called ATP.
Without this energy, our cells start to die.
Humans have over 200 types of cells, and all except mature red blood cells have mitochondria.
That's because a red blood cell's job is to transport oxygen, which mitochondria would use up before it could reach its destination.
So all mitochondria use oxygen and metabolites to create energy and have their own DNA, but mitochondrial DNA varies more across species than other DNA.
