The question of Earth’s first tree requires an exploration into deep time and the evolutionary history of plant life. Trees, as we recognize them today, evolved over millions of years through various plant lineages. The fossil record reveals the answer is not a single species, but a progression of plant forms that developed tree-like characteristics. The earliest forms appeared roughly 400 million years ago, marking a transformative period for the planet.
Defining a Tree
Defining what constitutes a “tree” from a scientific perspective, especially when examining the ancient fossil record, presents challenges. There is no single, universally accepted botanical definition. Generally, a tree is a perennial plant with an elongated stem or trunk supporting branches and leaves. Key characteristics include a woody trunk, secondary growth for annual thickening, a branching structure, and significant height. However, applying these criteria to primitive plants can be ambiguous. Some early tree-like forms lacked the true wood or the ability to increase in girth characteristic of modern trees. The tree growth form has evolved independently across diverse plant groups, indicating it is not a unified taxonomic category.
Early Tree-like Plants: The Contenders
The Devonian Period (approximately 419-359 million years ago) witnessed the appearance of the earliest tree-like plants, including lycophytes, ferns, and horsetails. Among the leading candidates for Earth’s first tree are Wattieza and Archaeopteris.
Wattieza
Wattieza, discovered in New York State fossils, is currently recognized as the earliest known tree, dating to the Middle Devonian, around 385 million years ago. These plants grew to heights of 8 meters (26 feet) or more, resembling modern tree ferns. Wattieza belonged to cladoxylopsids, distant relatives of contemporary ferns and horsetails, and reproduced using spores. The 2007 discovery linking Wattieza’s crown to fossilized trunks (previously Eospermatopteris) provided a more complete understanding of its structure.
Archaeopteris
Before Wattieza’s 2007 discovery, Archaeopteris was widely considered the earliest known tree. This extinct progymnosperm genus existed from the Late Devonian to Early Carboniferous periods, with its oldest fossils dating back 385 million years. Archaeopteris had a woody trunk and branching patterns similar to modern conifers, but its foliage resembled ferns, and it reproduced via spores. These trees reached heights of up to 24 meters (80 feet), with trunks sometimes exceeding 1.5 meters (5 feet) in diameter. Archaeopteris also featured reinforced branch joints and extensive root systems, making it a dominant component of forests during the Late Devonian.
The Evolutionary Leap
The emergence of large, woody plants transformed Earth’s ecosystems and atmosphere. These early trees, including cladoxylopsids like Wattieza, formed the planet’s first widespread forests during the Devonian Period. The proliferation of these forests significantly altered global carbon cycles. Before the Devonian, atmospheric carbon dioxide levels were considerably higher, but widespread photosynthesis by these new forests absorbed substantial CO2. This absorption contributed to a notable drop in global temperatures, changing the planet’s climate.
The development of extensive root systems by these early trees also significantly impacted Earth’s geology. These roots helped break down rocks and facilitated the formation of richer, more stable soils, which reduced erosion. Decaying organic matter from these forests provided new food sources for early terrestrial organisms and fueled the evolution of freshwater fish. The establishment of these vast forest ecosystems created new habitats, fostering the diversification of terrestrial life, including arthropods and, eventually, vertebrates.