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A plant fossil that gathered dust in a museum drawer for a century is the oldest fossil of large plants ever found.

The find suggests we need to rethink the plant family tree. It has been estimated that land plants first emerged but actual fossils are rare and not quite so old. Many botanists assume that the first land plants grew like mosses, and more complex plants like shrubs and trees evolved later.

However, the new find adds to growing evidence that this picture may be back-to-front. Mosses and their relatives might be more 鈥渆volved鈥� than we thought.

It all comes down to reproduction.

Plant sex

Like most animals, land plants can reproduce sexually, but the details can seem strange to our eyes. A plant embryo first grows into an organism called a 鈥渟porophyte鈥�, which has a full complement of chromosomes. The sporophyte can then give rise to a distinct organism called a 鈥済ametophyte鈥�, with just half the chromosome set. It鈥檚 this organism that produces the egg and sperm cells that fuse to form a new embryo.

This life cycle plays out differently in different land plants.

Most familiar plants like trees and shrubs are sporophytes that generate their own energy. The gametophyte is a small, short-lived structure that grows within the tree or shrub.

However, among mosses and their relatives it is generally the gametophyte that is the dominant organism. It grows for a long time, gathering enough energy to briefly support a small sporophyte, which relies on the gametophyte for its energy.

Conquest of land

Many botanists suspect that the moss condition evolved first, partly because mosses and their ilk seem to be the simplest living land plants. But a team led by at the National Museum Prague, Czech Republic and at the Charles University, Prague says that this idea might be wrong 鈥� and they have fossil evidence.

They have reanalysed a 432-million-year-old plant fossil that was discovered more than a century ago near Prague. The fossil belongs to a new species called Cooksonia barrandei. It is 4-5 centimetres high and up to 3 millimetres across.

鈥淸罢丑别闭 Cooksonia barrandei we describe is the oldest unambiguous plant macrofossil known to date,鈥� says 沤谩rsk媒. The older fossils are all tiny fragments, mostly spores.

鈥淣atural history collections hold a lot of undiscovered treasures,鈥� says at the Xishuangbanna Tropical Botanical Garden in Yunnan, China.

Beyond its age, the fossil is significant because it is a sporophyte. Since it is big, it鈥檚 likely that C. barrandei鈥檚 sporophytes could sustain themselves using photosynthesis. That makes this ancient plant more like a modern tree or shrub than a moss.

What鈥檚 more, since C. barrandei is one of the earliest well-preserved land plants we鈥檝e found, the researchers say it鈥檚 possible that the common ancestor of land plants also had these self-sustaining sporophytes.

鈥淯ntil now it was assumed that the 鈥榟igher plants鈥� evolved from [mosses and their relatives],鈥� says 沤谩rsk媒. But mosses, with their reduced sporophytes, might actually have evolved later.

Earliest history

鈥淚 was genuinely surprised to see this fossil 鈥� it鈥檚 very cool,鈥� says at Stanford University in California. He agrees C. barrandei may well have made sporophytes that could nourish themselves.

However, Boyce says it鈥檚 unclear what the first land plants were like. 鈥淚t鈥檚 now basically up in the air,鈥� he says, partly because the fossil record of plants is so patchy. He is reluctant to draw conclusions from C. barrandei because some of its more recent relatives are believed to have had a moss-like life cycle.

However, at the Natural History Museum in London, UK is open to the idea that the first land plants were not moss-like.

Kenrich co-authored a study published in March that used molecular data to infer the lifestyle of the first land plants. The research suggested descended from a more complex ancestor.

鈥淭his implies that [moss-like plants] and their life cycle may not be ancestral, but rather derived,鈥� he says. 鈥淟ife cycle evolution in the earliest emergent plants needs a rethink.鈥�