Walking through Bwindi Impenetrable National Park on a misty morning, it is easy to feel that you are moving through a world much older than any human civilisation. That feeling is not entirely imagination: some of the plant families represented in Bwindi’s understorey—the ferns, the cycad relatives, the mosses, and the club mosses—are genuinely ancient, representing lineages that predate the evolution of flowering plants, the dinosaurs, and in some cases the appearance of true forests on earth. Understanding the evolutionary antiquity of these plant groups transforms the forest from a scenic backdrop into a living record of deep biological time.
Tree ferns: survivors of the Carboniferous
Tree ferns—members of the families Cyatheaceae and Dicksoniaceae—are the most visually dramatic ancient plants in Bwindi. They grow slowly, reaching heights of 3 to 8 metres on their woody, unbranched trunks, crowned with a rosette of large, feathery fronds. The trunks are not true wood but compressed masses of old frond bases and adventitious roots. In Bwindi’s cooler, wetter zones—particularly around Ruhija at higher altitude—tree ferns are locally abundant in clearings and along streams, creating a landscape that resembles illustrations of Carboniferous coal swamp forests from 300 million years ago.
The Carboniferous period (approximately 358 to 299 million years ago) was characterised by vast swampy forests of giant club mosses, horsetails, and tree ferns that formed the coal deposits now extracted across Europe, North America, and Asia. The tree ferns of that era were different species from today’s—the evolutionary lineage has persisted while individual species have changed—but the structural form of a tall, unbranched stem topped with a crown of fronds represents one of the most successful architectural solutions in plant evolution, persistent across hundreds of millions of years and multiple mass extinction events.
Club mosses: relatives of the Carboniferous giants
In the Carboniferous, club mosses (lycopsids) were among the dominant trees: Lepidodendron and Sigillaria reached heights of 30 metres or more, with trunks a metre in diameter, forming the canopy of the great coal swamp forests. Their living relatives in Bwindi—small, creeping plants in the genera Lycopodium and Selaginella—are a humbling contrast in scale: plants a few centimetres tall, spreading across mossy rocks and forest floor soil, representing a lineage that once dominated terrestrial ecosystems before being displaced by seed plants. The ancient body plan is recognisable: scale-like leaves densely arranged on dichotomously branching stems, spore-bearing cones at the shoot tips, no flowers and no seeds.
Selaginella species in Bwindi are particularly diverse and often iridescent: their chloroplasts are arranged to reflect blue-green light when the forest is dim, creating a shimmering effect that catches the eye of observant trekkers. This iridescence is thought to be an adaptation for capturing the rare diffuse light that penetrates the forest canopy at very low angles, allowing photosynthesis in conditions that would challenge conventional plant arrangements.
The stability of ancient plant forms
The persistence of ancient plant architectures in Bwindi reflects a principle called “morphological conservatism”—the tendency of successful biological forms to be retained across geological time even as species within those forms change through evolution. Fern fronds, club moss creeping stems, and moss mats appear in the fossil record almost unchanged for hundreds of millions of years. This is not evidence that evolution has stalled but that these forms function so well in their ecological contexts—capturing light in shade, retaining moisture in humid environments, reproducing without the energy investment of flowers and fruit—that radical change provides no selective advantage.
Standing in the presence of mountain gorillas—evolutionary newcomers in geological terms—while surrounded by ferns and club mosses whose lineages predate them by hundreds of millions of years inverts the usual sense of where temporal depth resides in a forest. The gorillas are among the youngest elements of the ecosystem; the ferns at their feet are among the oldest. Both are present and alive in Bwindi at this moment, sustained by the same ancient forest that has persisted through climate shifts, volcanic activity, glaciations, and everything else that geological history has offered. That persistence is its own kind of wonder.
