Trees are perennial plants characterized by an elongated stem, or trunk, typically supporting branches and leaves. From an evolutionary perspective, “tree” describes a growth form that has evolved independently multiple times across various plant groups, allowing them to grow tall and compete for sunlight. These forms emerged through a series of adaptations over vast geological time.
Early Plant Ancestors
Plant life began in aquatic environments, with green algae, charophytes, considered the closest ancestors to land plants. Around 480 to 470 million years ago, during the Ordovician period, these early plant forms started to colonize terrestrial habitats. This transition presented significant challenges, as plants had to overcome desiccation, gravity, and the need to reproduce without being fully submerged in water.
Early plants developed crucial adaptations to survive on land. A waxy outer layer, a cuticle, emerged to prevent water loss, while rudimentary root-like structures, rhizoids, provided anchorage. Initial terrestrial plants, such as mosses and liverworts (bryophytes), lacked true vascular tissue and remained small, growing close to moist environments. Later, seedless vascular plants like ferns, horsetails, and lycophytes appeared around 400 million years ago, developing basic vascular systems to transport water and nutrients.
The Emergence of Tree Forms
True tree forms required several evolutionary innovations. A robust vascular system, comprising xylem and phloem, became essential for transporting water and nutrients from the ground to leaves, and sugars throughout the plant. The evolution of lignin, a complex polymer, provided structural rigidity to plant cell walls, allowing stems to grow vertically and withstand gravity. This enabled plants to achieve height, outcompeting smaller flora for sunlight.
Secondary growth, which increases stem girth and forms woody tissue, was another transformative adaptation. This continuous growth provided long-term structural support, enabling trees to become large and long-lived. One of the earliest true tree forms was Archaeopteris, emerging approximately 385 to 370 million years ago during the Late Devonian period. Archaeopteris had a woody trunk and branching patterns similar to modern conifers, but reproduced via spores. Its wood provided both mechanical support and efficient conduction, and its widespread presence led to Earth’s first true forests, profoundly altering global ecosystems.
The Age of Diverse Trees
Following the development of woody plants, trees continued to diversify, leading to two major groups: gymnosperms and angiosperms. Gymnosperms, characterized by their “naked seeds” not enclosed within an ovary, first appeared around 360 million years ago during the Carboniferous period. This group includes familiar trees such as conifers, ginkgoes, and cycads.
The next major evolutionary leap occurred approximately 125 million years ago with the emergence of angiosperms, or flowering plants. Angiosperms rapidly diversified and now constitute over 90 percent of all plant species. Their success is attributed to advantages like flowers, which attract pollinators, and fruits, which protect and disperse seeds. The evolution of these diverse tree types profoundly shaped terrestrial environments, creating the vast forests seen today.