How Did Multicellular Life Evolve?

The first evidence of multicellular organization, in which unicellular organisms link behavior and may be the precursor to evolution in true multicellularity, comes from organisms such as cyanobacteria that lived 3-3.5 billion years ago.

Scientists are discovering ways in which a single cell may have developed entities that are central to group behavior, opening the way for multicellular life. These discoveries may shed light on how complex extraterrestrial life might evolve in alien worlds.

Dividing Cell

The first known single-celled organisms appeared on earth about 3.5 billion years ago, about a billion years after the formation of the Earth. The most complex species of humans took a long time to emerge, and the first multicellular animals did not appear until about 600 million years ago.

The emergence of multicellular organisms from simple single-celled organisms has been an important moment in the biological history of the Earth and has greatly reorganized the planet's ecosystem. However, another mystery regarding the biology of many cells is why cells have not returned to the same single cell.

"Unicellularity is clearly successful - On our Earth Unicellular organisms are much more abundant than multicellular organisms and have existed for at least 2 billion years," said lead research author Eric Libby, a mathematician at the Santa Fe Institute in New Mexico.  "So what's the benefit of being multicellular and staying that way?"

The answer to that question is often a cooperative one since the cells benefited more from working together than when they were working alone. However, in co-operative cases, there is always a tempting opportunity for "cells to escape their functions - that is, to cheat," Libby said.

"For example, consider the ant colony where only the queen lays eggs and the workers, who are unable to reproduce, have to make sacrifices for the colony," Libby said. “What prevents an ant worker from leaving the colony and establishing a new colony? However, it is clear that the ant worker is not able to reproduce, so it cannot start its own colony. But if it gets a mutation that makes it able to do that, then this could be a real problem for the colony. This kind of struggle is rampant in the evolution of multicellularity.

Studies have shown that a group of microbes that produce useful molecules that all members of the group can benefit from can grow faster than non-beneficial groups. But in that group, freeloaders do not waste resources or energy to release these fast-growing molecules. 

To solve the mystery of the survival of multicellular life, scientists have proposed what they call “ratcheting mechanisms.”  Ratchets are devices that allow movement in one direction. By analogy, ratcheting methods are factors that provide benefits to groups but are harmful to individuals, ultimately preventing them from returning to a single-cell state, said Libby and research co-author William Ratcliff at the Georgia Institute of Technology in Atlanta.

In general, when a feature makes group cells more dependent, it acts as a ratchet. For example, groups of cells may divide a function so that some cells grow into one important molecule while other cells grow a different essential component, so these cells perform better collectively than lonely, a theory supported by recent experiments with bacteria.

Ratcheting can also explain the symbiosis among the ancient bacteria that cause symbionts to live inside cells, such as mitochondria and chloroplasts that help their hosts to utilize oxygen and sunlight. Single-celled organisms known as Paramecia do not do well when they are detected by examining photosynthetic symbionts, and symbionts often lose the genes needed for life without their host.

These methods of ratcheting can lead to seemingly insignificant results. For example, apoptosis, or planned cell death, is the process by which a cell commits suicide. However, research shows that high rates of apoptosis can have benefits. In large clusters of yeast cells, apoptotic cells act as weak links whose death allows small lumps of yeast cells to relax and continue to spread elsewhere where they may have more space and nutrients.

This advantage does not apply to single cells, which means that any cell that discards a group will be at risk, ”said Libby. “This work demonstrates that a cell that resides in a group can receive a very different environment than a living cell alone. The environment may be so diverse that harmful elements in living organisms alone, such as increased mortality rates, may be helpful to cells within the group. ”

Speaking about what these findings mean in the quest for an unknown life, Libby said the study suggests that foreign behavior may seem strange until one has a better understanding of how a living thing can belong to a group.

"Living organisms in communities can have behaviors that may seem strange or contradictory without proper consideration of their social contexts," Libby said. "It's a reminder that a piece of the puzzle is confusing until you know how it fits into the larger context."