Before trees existed, Earth was home to a giant life form that seemed otherworldly.

Imagine walking through a landscape with almost no tall plants, where the horizon is broken by gigantic columns-solitary, silent and unexplained.

These colossal structures really did exist, long before forests. They left fossils that have puzzled scientists for more than 150 years, and even today no one can say with confidence what they were.

The forgotten colossi of an almost empty planet

Around 400 million years ago, during the Devonian period, Earth was a very different place. There were no trees and no dense forests. Vegetation was limited to low-growing plants, mosses and small lifeforms that barely reached ankle height.

In the middle of this minimalist scene, structures up to 7.5 metres tall appeared. They looked like enormous trunks, standing alone, rising from ground that was almost bare. These prehistoric organisms became known as Prototaxites.

The first fossils were described in 1843. In 1859 they were given this name, which in Latin means something like “primitive yew” (a yew is a tree). But as research progressed, one certainty emerged quickly: they were not trees.

Prototaxites was a solitary giant on a planet that was still only beginning to develop a diversity of life on land.

From that point on, a long scientific stalemate began: what exactly was this thing?

Not a plant, not a fungus… or something entirely new?

For decades, two hypotheses competed. Some researchers argued that Prototaxites was a type of giant fungus. Others suggested it belonged to a group of organisms unlike anything alive today.

A recent study, published in Science Advances, strengthened the second line of thinking. The team compared the structure of Prototaxites with fungal fossils found in the same rock layers. The results pointed to several incompatibilities.

Inside, Prototaxites was made up of a network of tubes. That resembles the filaments of fungi, known as hyphae. But on closer inspection, the similarity largely ends there.

• The tubes branched in an apparently chaotic way.
• There was no sign of the organised pattern typical of known fungi.
• A key component of fungal cell walls was missing: chitin.

The absence of chitin attracted particular attention. This compound is often found in fossilised fungi, including in other specimens from the same site. If Prototaxites were a typical fungus, you would expect to find at least some trace of it.

The structure of Prototaxites looks familiar at first glance, but dissolves into strangeness under the microscope.

Taken together, this evidence pushes scientists towards an uncomfortable conclusion: Prototaxites may represent a form of life with no modern equivalent-a lineage that vanished without leaving any obvious descendants.

A lifeform lost to history

Part of the scientific community is already considering treating Prototaxites as belonging to an extinct branch that does not fit neatly into the kingdoms we use today: animals, plants, fungi, algae, bacteria, and so on.

Other researchers prefer more caution. For them, it is still possible that it was an extremely unusual fungus-part of a now-extinct lineage, but still within the broad fungal umbrella.

This uncertainty even shows up in artistic reconstructions. In images of ancient ecosystems, such as the famous Rhynie chert site in Scotland, Prototaxites appears as a massive, isolated pillar surrounded by low vegetation. But its texture, colour and the precise form of its surface are largely educated guesses.

Characteristic	Known fungi	Prototaxites
Typical height	Millimetres to metres	Up to 7.5 metres or more
Internal organisation	Well-organised filaments	Irregularly branching tubes
Chitin in cell walls	Present	Not detected in the fossils analysed
Ecosystems it lived in	Varied, well-established environments	Low, sparsely diverse vegetation

The energy puzzle of these giants

One of the biggest questions is a practical one: how did an organism so large feed itself in an environment with so few tall plants and so little accumulated organic matter?

Earlier studies suggested Prototaxites fed on decaying organic material, playing a role similar to modern fungi: recycling nutrients, breaking down remains of other organisms, and keeping chemical cycles running.

But the setting was different. With little large vegetation, the supply of material to decompose was probably limited. That contrast troubles researchers: a giant organism in a world of small plants, needing energy to grow and remain standing.

The size of Prototaxites challenges the logic of a planet still poor in resources for large terrestrial organisms.

Some scientists propose that these organisms may have grown extremely slowly, over hundreds of years. Others suggest they might have been far more efficient at extracting nutrients than anything we know today.

What these columns reveal about early life on Earth

Even without agreement on its exact classification, Prototaxites has already taught some important lessons. The main one is that life on land became complex very early. There was no need to wait for the rise of trees to see large multicellular organisms occupying space in ecosystems.

The existence of these giants also changes how we imagine Devonian landscapes. Instead of a flat green carpet, we can picture scattered columns creating patches of shade, microclimates and perhaps even influencing local water cycling and near-ground winds.

For palaeontology, Prototaxites is a reminder that the great kingdoms of biology are human constructions, shaped by the life we see today. When we look deep into the past, we encounter forms that do not fit those boxes.

When a fossil doesn’t fit our categories

Terms such as “kingdom”, “fungus”, “plant” and “animal” can suggest rigid boundaries, but evolution does not respect these divisions so neatly. In very ancient periods, lineages were still in the midst of biological experimentation.

Prototaxites may represent one of those evolutionary experiments that succeeded for a time, dominated certain landscapes, and then disappeared-perhaps replaced by more efficient organisms, or by ecosystems with different dynamics.

To handle cases like this, scientists combine several approaches:

• Microscopic study of internal fossil structure.
• Comparisons with living organisms across all kingdoms.
• Chemical analysis of minerals and possible organic residues.
• Modelling of ancient ecosystems to test survival scenarios.

Possible scenarios-and what may still be revealed

Some models attempt to simulate what an environment dominated by Prototaxites might have been like. In one, these columns would act as large nutrient-recycling “towers”, concentrating organic matter around their bases and supporting other, smaller lifeforms.

Another scenario considers relationships with microorganisms such as bacteria and algae. Symbiotic partnerships may have helped these giants obtain additional energy, either from light or by breaking down chemical compounds present in wet soil.

There are also conceptual risks: forcing these fossils into familiar categories can lead to mistaken interpretations. A giant fungus? A bizarre plant? An organism entirely independent of any modern group? Each possible answer would change how we reconstruct the history of life on Earth.

As new techniques emerge-such as more precise isotope analysis and high-resolution imaging-the chances of finding a decisive clue increase. Until then, Prototaxites remains one of the most intriguing figures from a past in which Earth was experimenting, on a gigantic scale, with what life could be.