Multicellular organism

A multicellular organism is an organism that has more than one cell, and more than one type of cell. This is different from unicellular organisms. All species of animals, land plants and most fungi are multicellular. Many algae are also multicellular. Some organisms, like slime molds and social amoebae such as the genus Dictyostelium, are partly unicellular and partly multicellular.

Multicellular organisms come about in different ways. They can form through cell division or by many single cells coming together. Colonial organisms are made when many identical individuals join to form a colony. Sometimes it is hard to tell the difference between colonial protists and true multicellular organisms. This is because the two ideas are not completely separate. Colonial protists are sometimes called "pluricellular" instead of "multicellular". There are also large organisms that are multinucleate but are technically unicellular, such as the Xenophyophorea, which can grow up to 20 cm.

Evolutionary history

Multicellularity means that an organism is made of many cells. It has evolved independently at least 25 times in different life forms. Some examples include cyanobacteria.

Complex multicellular organisms developed in six main groups: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants.

Some groups have later returned to being single-celled, such as certain fungi and algae. Multicellular organisms can sometimes face problems, like when cells grow out of control. In some groups, there is a clear separation between cells that make up the body and cells 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. Another idea is that a single cell’s nucleus divided but the cells did not separate. A third idea is that when a cell divided, the new cells stayed together instead of splitting apart.

Because early multicellular organisms were soft, they rarely fossilized. However, some possible fossils from 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 do not know exactly how single cells became multicellular organisms, but experiments show it is possible. Scientists have made changes to single-celled organisms so they stick together, an important step toward becoming multicellular.

For example, yeast can be changed to form clusters called “snowflake” yeast, which grow into tiny groups we can see. Green algae like Chlorella vulgaris and Chlamydomonas reinhardtii can also be encouraged to form larger groups when they face threats. These experiments help us understand how life may have become more complex.

Advantages

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

However, not all scientists think 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 has its own challenges and tradeoffs.

Main article: differentiation

Gene expression changes in the transition from uni- to multicellularity

When organisms changed from single-celled to multicellular forms, the way genes worked changed. In single-celled organisms, genes help the cell survive and reproduce. In multicellular organisms, genes work in new ways. Some cells became experts at reproduction, called germline cells. Other cells focused on keeping the body alive, called somatic cells. Cells began to work together and could not survive or reproduce alone anymore.