Mitochondria lie at the center of all multicellular life.

Some 4 billion years ago, when life on Earth was just beginning, all that existed were algae and single-cell bacteria.

Six hundred million years ago – a mere sixth of the time that life has existed on Earth – more complex forms of life started to develop. These complex lifeforms are called multicellular organisms – they are a combination of numerous cells that have a variety of functions.

Not only did these new multicellular organisms have more cells, they also had different types of cells that fulfilled various functions. These multicellular organisms are eukaryotes, and each of their cells contains a nucleus. These eukaryotic cells are what humans and animals are made up of. More simple organisms, such as bacteria, are known as prokaryotes. Prokaryotic cells contain no nucleus.

For a long time, biologists thought that prokaryotes evolved into eukaryotes, which then transformed into complex entities such as humans. However, this isn’t exactly the case. These two types of cells are distinct from each other; for instance, eukaryotes are ten to 100 times larger than prokaryotes.

The biggest difference, though, is that complex multicellular organisms are made up of eukaryotic cells, which all have – or once had – mitochondria. Mitochondria live inside cells and produce energy.

So, if all complex life forms consist of eukaryotes, and eukaryotic cells only exist when they’ve come in contact with mitochondria, then it follows that mitochondria are at the center of all multicellular life.

Since all that existed in the beginning were prokaryotes such as algae and bacteria, it’s likely that eukaryotes came into existence via a merger between two prokaryotes: one being a mitochondria and the other a host cell. We’ll take a more detailed look at this later on.