The earliest plants to gain height were radically different from the modern trees—gymnosperms and angiosperms—that dominate the planet today. These ancient woody species represent a collection of separate evolutionary experiments that independently achieved a tree-like form, establishing the first forests millions of years before the appearance of true wood or seeds. Their existence marks a profound shift in the planet’s terrestrial ecosystems, fundamentally altering global atmospheric and soil conditions.
Defining the Earliest Tree-Like Forms
The concept of a tree in a paleobotanical context is defined by a plant’s ability to achieve significant height using lignified tissue, a feature known as secondary growth. The appearance of these upright forms is traced to the Middle to Late Devonian period, roughly 390 million years ago. This evolutionary development was a direct response to intense competition for sunlight among the emerging land flora. Plants that could lift their photosynthetic structures above their neighbors gained a significant survival advantage, which drove the rapid development of tall growth. The first organisms to achieve this form were not the ancestors of modern conifers or flowering trees, but rather members of extinct groups like the cladoxylopsids and progymnosperms.
The Strange Structure of Pre-Modern Trees
The internal and external structure of these first trees would appear bizarre to a modern observer, as they lacked the centralized, continuous cambium that forms modern wood. Instead of a single, solid trunk that expands radially with annual rings, the stem was often composed of numerous interconnected strands of woody tissue called xylem. In some cases, these strands were only located near the outer layer of the trunk, leaving the center hollow. Each of these hundreds of individual strands could produce its own set of growth rings, resulting in a far more complex internal anatomy than the smooth, uniform growth of a modern tree.
These early arborescent forms also lacked true leaves, instead bearing highly modified, three-dimensional branchlets that served the function of photosynthesis. The structural support system was comparatively weak; the vascular tissue’s primary function was water transport, with mechanical support being a secondary benefit. This contrasts with the dense, continuous wood of modern trees, which provides immense structural stability. Their root systems were generally shallow and unbranching, radiating outward near the surface rather than forming the deep, anchoring taproots common in many modern species.
Key Examples of the First Forests
The cladoxylopsid Wattieza first appeared approximately 385 million years ago. Fossil evidence, including the famous “Gilboa stumps” from New York, shows that Wattieza grew to a height of about 8 meters (26 feet). Its overall appearance likely resembled an oversized, primitive tree fern, with a flared base and a crown of frond-like branches. These branches were not true leaves but complex, three-dimensional structures that were likely shed as single units.
Archaeopteris, a progymnosperm, dominated the Late Devonian landscape, growing up to 30 to 40 meters tall. Archaeopteris is often considered the first modern tree due to its more advanced wood and ability to form extensive forests. It possessed an extensive root system and had branches that dovetailed into the trunk for added strength, though it still reproduced via spores like a fern. Its foliage consisted of flattened, fern-like fronds that were shed seasonally, providing the first major source of leaf litter and profoundly changing the nutrient cycling of the world’s soils.
The Evolutionary Shift to Modern Wood and Seeds
The reign of these spore-reproducing trees set the stage for two major evolutionary innovations that defined the modern forest. The first was the evolution of true secondary growth, characterized by a single, bifacial vascular cambium that produces a continuous cylinder of wood (secondary xylem) to the inside and bark (secondary phloem) to the outside. This continuous growth layer allowed for the development of the dense, structurally strong wood and the distinct annual rings seen in most modern trees. This more stable construction allowed trees to reach even greater heights and longevity.
The second transformative development was the evolution of the seed, which freed plant reproduction from the requirement of a moist environment. The earliest seed plants, appearing in the Late Devonian, offered their embryos protection and a food supply, enabling them to colonize drier, upland habitats. This innovation led to the rise of the gymnosperms, such as conifers, which ultimately replaced the spore-bearing lycopods and cladoxylopsids as the dominant tree forms.