How Did Multicellular Life Evolve?

Conservative religious culture is full of creationists -- people who believe that life was created, and did not evolve without guidance -- posing skeptical questions about the nature of evolution, often claiming that there are gaps in the theory of evolution that scientists can't explain or that are inexplicable without intelligent design. Almost every list of "Questions for Evolutionists," as they tend to be presented, contains one pressing (and interesting!) question: "How did multicellular life originate?"

One fairly detailed article by Shaun Doyle of Creation Ministries International goes so far as to claim that multicellular life is impossible under an evolutionary model, and intelligent design must have somehow been involved in the process. Unfortunately for Doyle and other proponents of creationism, this is far from true. Multicellular life has arisen at least 25 different times in the history of life on Earth, and has even been created in a laboratory by mimicking natural selection by predation. It's likely that it arose by several different paths at varying points in history, but here are the leading theories about the origins of multicellular life:

Colonial Evolution of Multicellular Life 
Some paths of evolution likely involved the colonial origin of multicellular life-- that is, large colonies of cells clumping together and forming huge networks, then eventually splitting off from each other after reaching a certain size.

Today, we can see something like this in certain strains of algae. Algae are single-celled organisms that don't necessarily separate after reproducing. Instead, they clump together in gigantic groups of thousands of individual cells. Within these algae colonies, only a few cells actually continue reproducing. The others maintain and regulate the colony without adding to it. Like multicellular organisms, these algae colonies are made up of cells with the exact same DNA but each have a different role to play in the regulation of the colony as one organism.

Eventually, these algae colonies mature and split apart, becoming separated from one another. Fascinatingly, some of these algae colonies reproduce sexually and can be male or female, while others are hermaphroditic. They provide insight not only into the evolution of some forms of multicellular life, but also into the origins of sexual reproduction.

Cellularization as the Origin of Multicellular Life 
Some multicellular plants, animals, and fungi may have come to be through a process called cellularization. In theory, this path for multicellular life began with one syncytial cell -- that is, a cell with more than one nucleus. This single cell may have developed new membranes around each nucleus, separating them into what eventually became completely different cells. Since they all originated in one unified cell, each of these new cells would share DNA with one another, but might play a different role in maintaining the organism.

We can see something like this in a few modern one-celled organisms, including certain slime molds that have multiple nucelii. In these single-celled organisms, one nucleus meets most of the needs of the organism while the smaller nucelii are used for reproduction. In theory, over the course of a long period of time and with a lot of external pressure, these one-celled organisms could eventually become multicellular.

Symbiotic Origins of Multicellular Life 
Multicellular life could have arisen as a result of symbiotic relationships between single-celled organisms. Symbiosis occurs when two living creatures form a connection that benefits them both. Some of the most well-known forms of symbiosis include lichens (an alga and a fungus living together) and the relationships between parasite-eating birds and large grazing animals. In some cases, like those between clownfish and anemones, the participants are both so dependent on one another that one species would likely become extinct with the other. Multi-celled life may have started in much the same way, on a smaller level.

In theory, multicellular life may have started when single-celled organisms entered close cooperative relationships that became essential for each other's survival. Unable to survive without each other, they eventually formed bonds and networks that incorporated all of their own ancestry, biology, and genetic material into one much larger organism: a complete ecosystem operating cooperatively as one creature. The only problem with this theory is that we don't yet know exactly how symbiosis could have led to the fact that the cells within a multicellular organism all share the same DNA and are not genetically separate from one another.

Although we don't know the specific path that evolution took in any of the dozens of times that multicellular life arose, we do know that multicellular life is possible without necessarily involving intelligent design. There are many current examples in nature of single-celled organisms unifying in ways that closely mimic multicellular life, and multicellular life can even be created by man in a laboratory setting. Although this fascinating field of natural history still needs more research, the question of how multicellular life evolved can be answered scientifically.