The first complex cells

The story

Two kinds of cells

All living things are made of cells, but not all cells are the same. There are two big kinds. The simplest cells, those of bacteria and archaea, are small and their DNA floats loose inside. The complex cells, called eukaryotic, are much bigger, often ten times longer or more, and their DNA is kept in a separate compartment, the nucleus. Inside they also have many small structures with different jobs, called organelles. Plants, animals, fungi and all the larger single-celled organisms, such as amoebas, are made of eukaryotic cells. So are you.

Eukaryotic cells appeared after at least a billion and a half years of life made only of bacteria and archaea. The exact moment is not clear. The oldest certain fossils, small spheres with decorated surfaces called acritarchs, are about 1.8 billion years old, although they are rare and debated. Studies of the DNA of living things today suggest that eukaryotic cells arose between 2 and 1.6 billion years ago, in a time that scientists estimate with a wide margin of error.

A great partnership

How did such a complicated cell come about? The most widely accepted idea is called the endosymbiotic theory, that is, the theory of “partnership inside the cell”. It says that some parts of complex cells were originally independent bacteria. One day a larger cell swallowed a smaller bacterium; instead of digesting it, the two began to cooperate. The bacterium inside received protection and food, and the host cell received energy. Over many generations, the two became inseparable.

The bacterium that turned into the mitochondrion, the “power station” of the cell, was able to use oxygen to get energy from food. Later, in some cells, a second guest was added: a cyanobacterium that became the chloroplast, the organ of photosynthesis. The evidence is plentiful. Mitochondria and chloroplasts have a little DNA of their own, circular like that of bacteria; they divide by themselves inside the cell, as bacteria would; they have a double membrane, as if they had been swallowed. The mitochondrial DNA of a person, for example, is made of only 37 genes and is almost always inherited from the mother.

The idea was put forward in the early 20th century by some Russian scientists and was taken up strongly in 1967 by the American biologist Lynn Margulis. At first it was met with a lot of scepticism; today it is in school books all over the world.

Who was the host?

It remains to be understood who the host cell was. In recent years geneticists have found the most likely answer in a group of archaea, discovered from 2015 on the seabed near Greenland, close to an area of hot springs called Loki's Castle, which is why they are called “Asgard archaea”. In their DNA they have many genes that were once thought to belong only to eukaryotic cells. It is therefore likely (although not all scholars agree) that our cells were born from the union of an archaeon of this kind and a bacterium able to use oxygen. The order of the steps is still open: for example, whether the nucleus appeared before or after the mitochondrion.

Why it was such a big step

A bacterium has little room for keeping energy: it produces it along its surface. A eukaryotic cell, with many mitochondria inside, can produce far more energy and so can afford bigger DNA, many more proteins and complex structures. It is like going from a bicycle to a factory with lots of small power stations. This extra energy also made other innovations possible, such as the cell's internal skeleton, which allows it to change shape and to engulf food, and later sexual reproduction, in which the DNA of two parents is mixed and individuals are born that are always a little different.

Almost everything we call “visible life” starts here. For a long time, however, eukaryotic cells stayed microscopic and lived on their own: the next step, joining together in one body, is told in the next card.