The Ancient Merger That Powers Almost Every Living Thing on Earth

Imagine you're at lunch, and instead of eating your sandwich, the sandwich somehow moves in with you permanently and starts doing your homework. That sounds ridiculous, but something a little like that actually happened about two billion years ago — and the result is one of the most important events in the entire history of life on Earth.

Here's what scientists believe occurred: one microscopic, single-celled organism engulfed, or swallowed, another smaller cell. Most of the time when that happens, the smaller cell gets digested. But this time, it didn't. Instead, the two cells settled into a partnership that turned out to be incredibly useful for both of them. Over hundreds of millions of years, that smaller cell slowly transformed into something called a mitochondrion — a tiny structure that now lives inside nearly every cell of every plant, animal, fungus, and many other living things on Earth. Your own body contains trillions of them. A single heart muscle cell can hold around 5,000 mitochondria. A liver cell holds between 1,000 and 2,000.

Mitochondria are often described as the "powerhouses" of the cell, and that label is actually pretty accurate. Their main job is to take the energy locked inside the food you eat and convert it into a form your cells can actually use. This process, called oxidative phosphorylation, is far more efficient than the simpler chemical processes that most single-celled bacteria use to get energy. Because of this upgrade, cells that contained mitochondria could grow more complex, store more information in their DNA, and eventually organize into the multicellular structures — like organs, muscles, and brains — that make up large living things. Some scientists argue that without mitochondria, complex life as we know it may simply never have evolved at all.

The scientific idea that explains all of this is called the endosymbiotic theory — "endo" meaning inside, and "symbiotic" meaning a relationship where two different organisms live together and benefit each other. The theory was first seriously developed by scientist Lynn Margulis in the 1960s, and today it is one of the most thoroughly supported ideas in all of biology. The evidence is surprisingly direct. Mitochondria carry their own DNA, separate from the main DNA found in a cell's nucleus, or control center. That mitochondrial DNA is organized as a small, circular molecule — the same shape as bacterial DNA — and contains about 37 genes. Mitochondria also reproduce by splitting in two, exactly the way bacteria do. Their internal structure looks more like a bacterium than like anything else inside a typical cell. In fact, certain antibiotics that work by targeting bacteria can accidentally interfere with mitochondria for exactly this reason — because at a biological level, the two are surprisingly similar.

Scientists have traced the original bacterial ancestor of mitochondria to a group called alphaproteobacteria. These are a large and varied group of bacteria, some of which still live freely in the environment today, while others survive as parasites inside other cells. Exactly which branch of this group is the closest modern relative to mitochondria is still being actively debated among researchers. Some studies suggest the answer might be a bacterium we haven't even discovered yet. The host cell that did the swallowing also remains somewhat mysterious. Research published in 2017 identified a group of microbes called the Asgard archaea — named after figures from Norse mythology — whose genes contain an unusual number of features once thought to be unique to complex cells. Many scientists now think the host cell was related to this group, though the exact details are still being worked out.

One of the most fascinating details about mitochondria is how they get passed down through generations. In humans and most other animals, mitochondria are inherited only from the mother. Egg cells carry mitochondria into the next generation, while the mitochondria in sperm cells are typically destroyed after fertilization. This means that every mitochondrion in your body right now is part of an unbroken chain stretching back — through your mother, her mother, and every maternal ancestor before that — all the way to that original accidental merger more than 1.5 billion years ago. The little bacterium that was almost someone's lunch never quite got eaten, and because of that, you exist.

Source: Space Daily