Evolution

Multicellular life occurred 25 times

When the single-celled green algae Chlamydomonas was exposed to the tiny predator Paramecium, the alga quickly added cells. The new, multi-cellular form had about 2.5 times more chance of surviving the predator's attack than the old, single-celled form.

© Boyd et al

A few algae cells cling to each other in a compact ball - until it falls apart. The parts float away and each starts to form new, larger balls.

The originally single-celled algae have thus developed a primitive form of multi-cellular life. But that major evolutionary leap did not last millions of years. It was done in 350 days - in an American laboratory.

The researchers behind this 2017 experiment managed to get a single-celled green alga to become multi-celled by exposing it to a greedy predator: a single-celled ciliaat that lives on algae or algae.

Thanks to the presence of the ciliaat, the algae that clump together easily had twice as many chances of survival as their lonely relatives.

When the single-celled green algae Chlamydomonas was exposed to the tiny predator Paramecium, the alga quickly added cells. The new, multi-cellular form had about 2.5 times more chance of surviving the predator's attack than the old, single-celled form.

© Boyd et al

The transition to multicellularity is one of the most important events in the history of life, and that transition now appears to be surprisingly easy.

Fossils and DNA show that life on earth took that step at least 25 times, and possibly much more often. And it is now dawning on us why single-celled organisms can work together so well.

Multiple cellity is still not there

Just about all current life forms are multicellular. This applies to you, the flowers in the vase, the blackbird in front of the window and the mushrooms in the forest. Some multicellular organisms are very simple - they consist of cells that form a permanent colony.

Others, such as yourself, are much more advanced. A characteristic is that the germ cells can produce a completed offspring. Individual cells are also highly specialized, although each cell contains exactly the same DNA.

Specialization means that cells evolve to perform specific tasks, such as the nerve cells transporting electrical signals through the organism, and as immune cells protect you against diseases. Plants on land and multi-cell fungi often have 10 to 20 cell types, while animals, including humans, can contain up to 200.

The researchers have studied the enormous diversity of multicellular organisms of today and discovered that they each developed multicellularity.

Multi-cellularity offers organisms some clear advantages. They can move efficiently and find a good environment to hunt, and they can flee. However, multicellularity has not always existed and the researchers do not know exactly when the first multicellular organism emerged.

Some finds indicate that there were simple versions 2 to 3 billion years ago. These probably consisted of loosely cooperating cells, which are more reminiscent of algae globules than of animals, plants and fungi.

The first more developed multi-cell life probably saw the light of day 750 to 660 million years ago. At that time, the single-cell life forms had ruled the earth for over 3 billion years.

The long wait suggests that the switch was difficult, but that conclusion is not consistent with other evidence.

The earliest multicellular organisms were very simple. This is the 560 million year old Dickinsonia tenuis.

© Masahiro Miyasaka

The researchers have thoroughly studied the enormous diversity of multicellular organisms on our earth and discovered that they have developed their multicellularity independently of each other. Plants, animals and fungi have evolved from three types of single-cell ancestors.

And with red, green, brown and pebble algae, multicellularity has arisen at least five times. Today's multicellular life can be traced back to at least 25 single-cell ancestors - and probably even more than 50.

Now scientists are wondering why this huge explosion of multi-cell life only started after 3 billion years.

Tree of life

The genes of multicellular organisms show that many have developed their own multicellularity, so independently of each other.

Algae are separate from each other

Plants and algae - multicellular algae - originated from a number of different single-cell algae, and many of these algae were not even closely related.

Attempts to multicellularity: ≥ 9.

Mold is a receptacle

Slime molds, oomycota, real fungi and other organisms were all once considered fungi. Now we know that they come from various single-cell precursors.

Attempts to multicellularity: ≥ 6.

Animals risk the jump

Animals - from the simplest jellyfish to the largest whale - most likely stem from one organism that developed multi-cellularity around 800 million years ago.

Attempts to multicellularity: ≥1.

Cells form a simple colony

Protists - an old word for all organisms that are not plants, animals, bacteria or fungi - have repeatedly developed a simple multicellularity, such as feather-shaped colonies.

Attempts to multicellularity: ≥ 5.

Bacteria make flowers

Bacteria and primordial bacteria (archaea) are often single cells, but some, such as myxobacteria, can form flower structures that can be seen with the naked eye.

Attempts to multicellularity: ≥ 4.

Unity is strength

The oxygen content of the earth was much lower in the first 3 to 4 billion years than it is today. It only rose 850 million years ago - shortly before multicellular life started.

Because of this confluence of circumstances, some scientists think that the low oxygen content interfered with multicellular life.

The British paleontologist Nicholas Butterfield, however, thinks the multicellular organisms could have bypassed that lack of oxygen by pumping water over a surface that absorbs oxygen - just like fish do with their gills.

Butterfield suggests that the advanced multi-cell life was delayed because the cells did not yet have their coordination capacity in order.

In organisms like us, such a capacity ensures that every cell produces the right proteins at the right time.

This allows our cells to perform different tasks, even though they have the same DNA, they no longer divide when they no longer need to grow, and some commit suicide to secure the survival of the entire group.

Video: This is how your cells divide the work

If it was difficult to achieve better coordination, we understand that multicellularity did not take place - but not why it suddenly arose in the most diverse organisms.

So that switch may not be that difficult at all, as long as the right evolutionary motivation is present. The big advantage of multicellularity is that it offers protection against predators.

But the first 3 billion years of life were predators small and primitive - so life didn't necessarily have to become multi-cellular. 800 million years ago, however, a more advanced predator emerged, so many other organisms clung to each other as self-defense.

Early predators, such as the 530 million-year-old Anomalocaris, started the development of multicellular organisms well.

© Jose Manual Canete

The American biologist Nicole King may have become obsolete, so many groups have become multicellular so quickly.

By investigating so-called single-cell collar flagellates, Kings team discovered that single-cell organisms can have variants of many genes that are essential to our own multicellularity: genes that keep our cells together, communicate or kill themselves for diseases like cancer that threaten the group.

Only single-cell organisms use those genes for other tasks, such as catching food or observing the environment; however, not much is needed to change the function of these genes.

Cells are made to work together

Thanks to a number of proteins our body is a whole and our cells work together. But the tiny collar flagellates show that the proteins can also be found in single-celled organisms. And here they serve a very different purpose.

1. Bacteria trap keeps cells together

Collar flagellates contain cadherins (light spots), which help them catch bacteria. In our bodies, however, the cadherins hold the cells together so that we do not fall apart.

2. Sensitive cells receive signals

To detect molecules in the surrounding water, collar flagellates use so-called tyrosine kinase receptors (light green). In humans, these receptors ensure that our cells can receive signals - such as hormones - from other cells.

3. Repair tool inhibits cancer

The protein p53 (turquoise) repairs damaged DNA (red) in people and collar flagellates. And with us, this protein causes the cells to commit suicide if the DNA cannot be repaired. This prevents damaged cells from developing into a tumor.

Thus, single-celled organisms have everything to become multi-celled, and therefore multi-celledness has always been able to arise.

Since our own ancestors took the plunge, our genes have been modified over millions of years, which means that our bodies are now unlikely to coordinate any of its 37 trillion cells. However, it has also been discovered that we sometimes fall back to the single-cell stage.

Relapse explains cancer

Australian scientists led by biologist David Goode investigated in 2017 how the activity of a number of genes in cancer cells deviates from what is normal.

Goode discovered that the genes that have changed the least since we became multi-cell are very active in cancer cells. Conversely, the more developed genes are inactive in the cancer cells.

Cancer therefore arises as soon as mutations break down the mechanisms that allow cell coordination and stimulate the selfish mechanisms that a single-celled organism needs.

Cancer cells divide uncontrollably and destroy the healthy tissue in their environment.

© Shutterstock

Cancer is the decline from multicellularity to single-celledness, and proliferating cells present themselves as single-cell parasites that attack our bodies.

Rare forms of cancer, such as Sticker sarcoma, which occurs in dogs, can even spread through sexual contact, just like gonorrhea and other bacteria.

In recent years, researchers have learned a lot about the transition of our ancestors from single cell to multi-celledness.

And their discoveries can not only tell us something about the past, but also clarify how our cells can fall back to single-celledness. That paves the way for new cancer therapies.

Video: Pastor Chui: Could Life Have Evolved Multicellular Systems? (February 2020).

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