Multicellular organism

A multicellular organism is an organism that consists of more than one cell, and more than one cell type, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of many identical individuals joining together to form a colony. However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". There are also macroscopic organisms that are multinucleate though technically unicellular, such as the Xenophyophorea that can reach 20 cm.

Evolutionary history

Multicellularity, where organisms are made of many cells, has evolved independently at least 25 times in eukaryotic life forms and also in some prokaryotic ones like cyanobacteria. Complex multicellular organisms developed only in six main eukaryotic groups: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants.

Some groups have later lost their multicellular nature and returned to being single-celled, such as certain fungi and algae. Multicellular organisms also face challenges like cancer, where cells grow out of control. In some groups, there is a clear separation between cells that make up the body and cells that are used for reproduction.

Origin hypotheses

There are several ideas about how multicellular organisms might have begun. One idea is that groups of cells came together to form a mass, like what some slime molds do. Another idea is that a single cell’s nucleus divided but the cells did not separate, leading to many connected cells. A third idea is that when a cell divided, the new cells stayed together instead of splitting apart, which happens in the early stages of animal and plant development.

Because early multicellular organisms were soft and had no hard parts, they rarely fossilized. However, some possible fossils from very old rocks show patterns that might be from early multicellular life. Scientists study both living organisms and fossils to understand how multicellularity might have developed, using tools like comparing DNA to see how different species are related.

Experimental evolution

We cannot know exactly how single cells became multicellular organisms hundreds of millions of years ago, but experiments show it is possible. Scientists have made changes to single-celled organisms that cause them to stick together, a key step toward becoming multicellular.

For example, yeast can be changed to form clusters called “snowflake” yeast, which grow into tiny groups visible to the naked eye. Green algae like Chlorella vulgaris and Chlamydomonas reinhardtii can also be encouraged to form larger groups when faced with predators. These experiments help us understand how life may have become more complex.

Advantages

Multicellular organisms can grow larger than single-celled organisms because they can share nutrients and resources more effectively. This allows them to live longer, even when some of their cells stop working. Multicellular organisms can also have different types of cells, which helps them perform more complex tasks.

However, not all scientists agree that being larger or living longer is always better. Most living things on Earth are still single-celled, and they are very successful in their own ways. Being multicellular comes with its own challenges and tradeoffs.

Main article: differentiation

Gene expression changes in the transition from uni- to multicellularity

When organisms evolved from single-celled to multicellular forms, the way genes worked changed a lot. In single-celled organisms, genes help the cell survive and reproduce better. But in multicellular organisms, these genes started working differently. Some cells became experts at reproduction, called germline cells, while others focused on keeping the body alive, called somatic cells. As this happened, cells began to work together and couldn’t survive or reproduce on their own anymore.