The Evolution of Multicellular Life

In a world filled with unicellular organisms, revolutionary changes led to the emergence of complex multicellular life in five major groups independently: animals, land plants, fungi, red algae, and brown algae. This article explores how these groups emerged and evolved, and how we use fossil records and molecular clocks to understand their history.

The Evolution of Multicellular Life

Multicellular organisms are far more complex than their unicellular ancestors, with cells specializing in specific functions and forming distinct tissues and organs. This revolutionary shift required new and advanced tools, such as sophisticated mechanisms for cell adhesion and communication throughout the organism. This evolution occurred independently in the five major groups.

The challenge in studying this evolution lies in understanding when and how these complex organisms first arose. The fossil record provides us with time markers, but it is not always sufficient, especially with fungi.

Challenges in Dating Evolutionary Branches

The fossil record is a primary tool for understanding when complex groups appeared. Red algae date back to about 1.6 billion years, animals to 600 million years, and land plants to 470 million years, while brown algae appeared millions of years later. However, fungi remain a mystery due to their soft, fibrous nature, which does not allow for clear fossil formation.

To overcome this gap in the fungal fossil record, scientists have relied on the concept of the molecular clock, which is based on the relatively constant rate of genetic mutations, to estimate when species diverged from a common ancestor.

Reading the Genetic Clock

The molecular clock is used to provide relative time estimates, but it requires calibration points from the fossil record to offer absolute dates. Due to the scarcity of fungal fossils, using this method traditionally has been challenging.

A team of scientists led by OIST developed an innovative method using horizontal gene transfer between different fungal lineages as a tool to provide new time points. These patterns of genetic exchange offer strong evidence of the evolutionary timeline.

A New History for an Ancient Kingdom

Analyses suggest that the common ancestor of modern fungi dates back to between 1.4 and 0.9 billion years, long before land plants. This supports the idea that fungi interacted with algae in early stages, contributing to the preparation of the terrestrial environment for colonization by land life.

Fungi play a vital role in ecosystems by recycling nutrients and partnering with other organisms. Establishing an accurate timeline for fungal evolution shows they were diversifying before plants, reinforcing the idea that these early partnerships may have helped pave the way for terrestrial ecosystems.

Conclusion

This study reshapes our understanding of the history of life colonizing the land, showing that fungi existed hundreds of millions of years before plants. This long period of preparation may have been necessary to make continents habitable. Through the analysis of rocks and nutrient cycles, ancient fungi could be the early engineers of ecosystems, preparing the first primitive soils and radically altering the terrestrial environment. In this new context, plants were not invading barren land but were following a world prepared by the diligent activity of the ancient and ongoing fungal kingdom.