Origin of Leaves

Eophyllophyton bellum, from the Lower Devonian (Pragian) of China, is the oldest known plant with megaphylls, or true leaves

• Megaphylls have multiple, branching veins compared to the leaves in lycophytes

The leaves appear to be flattened shoot systems, and may represent a different origin for megaphyllous leaves

This plant is superficially similar to the progymnosperms

Archaeopteris, the first true tree (lignophyte), had frond-like leaves 

Earlier members of progymnosperms, such as the aneurophytes, did not have laminate leaves

It is assumed that the leaves of the Archaeopteridales were megaphylls, and share developmental ancestry with the fern and seed plants

Some have called these leaves lignophylls, for the leaves in the lignophyte clade, which may have separate origin from true ferns.

Laceya hibernica (Famennian) has the ealiest known leaf from a seed-bearing plant

This hypothesis by F.O. Bower (1908) proposes the steps for the origin of microphyllous (lycopod) leaves from a leafless ancestor. Some early land plants, such as zosterophylls, had small, leaf-like outgrowths that lacked vascular cells; such outgrowths are called enations.

An interesting early lycopod fossil from the Rhynie chert, Asteroxylon, has enations each with a vascular trace that come up to, but not into, the vascular trace. These may represent a transitional stage between zosterophyll enations, and microphylls in derived lycopods.

This concept pertains mainly to clubmoss or lycophyte evolution. Current thinking proposes that clubmosses evolved leaves separately from other plants, such as ferns and seed plants. Botanists call lycophyte leaves "microphylls", due to their small size. A more definitive feature is that they exhibit a single vascular trace (or vein) that does not branch. 

The megaphyllous leaf was hypothesized to be created through some of the "telomic" processes, hypothesized by Walter Zimmermann (1952)

• Over-topping: Plants evolve from an isotomous branching habit to pseudomonopodial growth, creating lateral axes that are used for photosynthesis and reproduction.

• Planation: The three-dimensional branching systems evolve to grow in a two-dimensional or flattened plane. The lateral bracnhes, mentioned above, would growth in an orientation to intercept as much light as possible.

• Webbing: Tissues form in-between the axes of these lateral branches to create a laminate leaf

There is also explanation given to the origin of microphyllous leaves, through a different process

• Reduction: Multi-branched shoots become shorter and less branched over time, possibily becoming a single vascularized outgrowth like a microphyll 

Proposed by Kenrick and Crane (1997), which proposes that the first leaves that evolve (i.e. lycophytes) came from sporangia that were sterilized and repurposed for photosynthesis.

Research by Vasco et al. (2016) may provide evidence for this idea of sterilization as the origin of leaves

• They show that there is a common leaf developmental mechanism conserved between ferns and seed plants. 

• The C3HDZ genes express in lycophyte and fern sporangia, and these genes have conserved expression patterns during initiation of lateral primordia, such as leaves or sporangia.

• These researchers hypothesize that there is a deep homology of all leaves and that a sporangium-specific developmental program was coopted independently for the development of lycophyte and euphyllophyte leaves.