A fresh look at spore-like microfossils has challenged conventional understanding of how plants evolved.
In a study, published in Science, researchers analysed a 480-million-year-old microfossil, retrieved from Western Australia, that connects plant origins to freshwater green algae. This suggested that plant evolution was gradual and nuanced as it evolved from using simple cell-division to complex reproduction, instead of experiencing large evolutionary leaps from one species to another.
Simple plants, like moss and algae, use spores to reproduce instead of pollen and ova (eggs). This was thought to indicate that the simple plants were evolutionarily older, and only distantly related to flowering plants, because they were less complex.
“We found a mix of fossils linking older, more problematic spore-like microfossils with younger spores that are clearly derived from land plants,” says lead author Paul Strother of Boston College, US.
This new discovery suggests that plant evolution wasn’t quite so linear, filling a 25-million-year gap on a molecular clock – a type of technique used to estimate the rate of evolution and age of species.
“This helps to bring the fossil spore record into alignment with molecular clock dates,” says Strother.
Previously, spore development and embryo development were seen as two distinct types of reproduction that occurred after plant tissue evolved, but this discovery suggests that some plant tissue evolved from spore-like cells, too.
Read more: The evolution of flowering plants
“When we consider spores as an important component of the evolution of land plants, there is no longer a gap in the fossil record between molecular dating and fossil recovery,” says Strother.
“[Now] we have a much clearer picture of a whole new evolutionary step: from simple cellularity to complex multicellularity.”
This suggests that plant development – and how they went from water to land – might be quite a different process, because some spore-like fossils may have been mistaken by previous researchers for fungi, which all use spores to reproduce, instead of plant ancestors.
“We need to move away from thinking of the origin of land plants as a singularity in time, and instead integrate the fossil record into an evolutionary-developmental model of genome assembly across millions of years during the Paleozoic Era – specifically between the Cambrian and Devonian divisions within that era,” says Strother.
“This requires serious re-interpretation of problematic fossils that have previously been interpreted as fungi, not plants.”
“Our work seeks to resolve some of these questions by integrating the fossil spore record into an evolutionary developmental model of plant origins from algal ancestors.”