Hundreds of millions of years ago, a towering life form known as Prototaxites dominated the prehistoric landscape. Reaching heights of up to 26 feet (8 meters), some species resembled tree trunks composed of intricate interconnected tubes. These giant organisms, believed to be among the first to thrive on land, have puzzled scientists for over a century and a half, sparking debate about their place in the evolutionary tree of life. New research suggests Prototaxites may have belonged to a previously unknown branch altogether, challenging our understanding of early life on Earth.
A recent study, currently available on the preprint server bioRxiv, examines the fossil remains of Prototaxites taiti. Researchers from the United Kingdom meticulously analyzed the fossils and concluded that Prototaxites likely belonged to a now-extinct lineage of multicellular terrestrial eukaryotes. Eukaryotes are organisms whose cells contain a nucleus, encompassing all animals, plants, and fungi. This research challenges the previously held belief, proposed in a 2007 study, that Prototaxites were a type of ancient fungi.
Challenging the Fungi Hypothesis
The researchers, including Corentin Loron from The University of Edinburgh’s School of Physics and Astronomy, meticulously compared the anatomy and molecular composition of Prototaxites taiti with contemporary fungi from the 407-million-year-old Rhynie chert, a sedimentary deposit in Scotland renowned for preserving Early Devonian period fossils (around 408 to 360 million years ago).
This comparison revealed significant differences. Prototaxites taiti stood as the largest organism in the Rhynie ecosystem, possessing a unique anatomy distinct from any known fungi, living or extinct. Crucially, the research team found no evidence of chitin or chitosan molecules, key components of fungal cell walls, within the Prototaxites samples.
Defining Characteristics of Prototaxites
The study highlights three defining characteristics of Prototaxites:
- Large structures composed of various tube-like components.
- Compounds similar to lignin, the molecules that provide rigidity to plants.
- A heterotrophic lifestyle, meaning they obtained nutrients by consuming other organisms.
These findings reinforce the unique nature of Prototaxites, setting them apart from known organisms. The researchers suggest classifying Prototaxites as members of a previously undescribed, entirely extinct group of eukaryotes, unable to place them within any existing lineage.
Rethinking the Kingdoms of Life
All life on Earth falls into three domains: Bacteria, Archaea, and Eukaryotes. Bacteria and Archaea are single-celled organisms lacking a nucleus, while multicellular organisms belong to the Eukaryote domain. Eukaryotes are typically divided into four kingdoms: animals, plants, fungi, and protists (a diverse group encompassing all eukaryotes that aren’t animals, plants, or fungi). The research on Prototaxites hints at the possible existence of a new, previously unknown kingdom, although the researchers don’t explicitly propose this classification.
A Glimpse into a Lost World
The study of Prototaxites offers a fascinating glimpse into the diversity of ancient life, suggesting that lost kingdoms may be more than just fantasy. The discovery underscores the ongoing quest to understand the complexities of evolution and the myriad forms life has taken throughout Earth’s history.
