The first many-celled living things

The story

From one cell to many

For billions of years life was made of single cells, each one independent. Then, at some point, some cells stopped separating after dividing and stayed stuck together, cooperating. Something new had been born: the many-celled organism, a being made of many cells working together. We are many-celled organisms, like a tree, a mushroom or a frog.

Scientists do not know when it happened for the first time, because the first organisms with several cells were tiny and soft, and fossils of that kind survive very badly. Some researchers have proposed even older fossils, such as those found in Gabon, in Africa, about 2.1 billion years old, but they are disputed. One of the most convincing pieces of evidence comes from a red alga called Bangiomorpha.

Bangiomorpha, an alga a billion years old

Bangiomorpha pubescens was discovered in the rocks of the Canadian Arctic, on Somerset Island, by the palaeontologist Nicholas Butterfield (now at the University of Cambridge), who described it in 2000. It is made of thin filaments, formed of a row of cells arranged like little discs, and it attached itself to the seabed. A dating in 2018, by a team from McGill University in Montreal, gave an age of about 1.047 billion years.

What makes this fossil special is that it already had cells that were different from one another. Some served to fix the alga to the bottom, others to produce cells for reproduction. What is more, its remains show that it reproduced by a sexual process, that is, by mixing the genetic material of two individuals. For this reason it has been considered one of the oldest organisms that can be recognised as eukaryotes with sexual reproduction. It is regarded as one of the oldest certain complex organisms, although new discoveries could move the date.

How many times did it happen?

A surprising fact is that many-celled life has been “invented” many times, independently. Red algae, green algae, brown algae, plants, fungi and animals each have their own story. Scientists estimate that it arose at least twenty or thirty times in different groups. This means that once complex cells existed, staying together paid off in many different ways.

You can still watch the process today. Volvox is a green alga that lives in fresh water as a hollow sphere made of thousands of cells, some devoted to movement and others to reproduction. Some single-celled organisms called choanoflagellates sometimes live in colonies and are the closest living relatives of animals, which suggests that the first animals came from similar forms.

In 2012 a team of researchers at the University of Minnesota (where he then worked) led by William Ratcliff carried out a famous experiment: they took brewer's yeast, a single-celled organism, and grew it, selecting every day the clumps of cells that sank fastest. In about two months, in the laboratory, the yeast cells formed snowflake-shaped clusters. It was a small re-run, in the lab, of the birth of a many-celled body.

Why it pays to stay together

A group of cells has advantages. It is harder for a smaller predator to eat, it can grow bigger, it can share out the jobs: some cells take care of getting food, others of moving, others of reproducing. But there are problems too. The cells have to cooperate and stop competing with one another. If a cell reproduces without control, it harms the body: that, after all, is how a tumour works. Many-celled organisms therefore developed rules: chemical signals, checks on cell division and even a programmed “suicide” of damaged cells.

A human body has tens of trillions of cells, of more than two hundred different types, all born from a single cell, the fertilised egg, and all with the same DNA. Their cooperation is the result of a story that begins, in an ancient sea, with an alga a billion years ago.