A new examination of microfossils resembling spores from 480 million years ago indicates that the first land plants evolved from freshwater algae.
Found in rock samples mined in Australia more than 60 years ago, microfossils dating back to the Lower Ordovician fill a knowledge gap of about 25 million years by reconciling the molecular clock (or rate of evolution) with the spore record. Fossils, physical evidence of early plant life that scientists have collected over the years.
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This alignment supports an evolutionary phylogenetic model linking plant origins to green, freshwater algae, or carophyte algae, according to Boston College paleobotanist Paul Strother, co-author of the new study, which is published in the journal Science. The “evo-devo” model assumes a more accurate understanding of plant development over time, from simple cell division to early embryonic stages, rather than large jumps from one species to another.
“We found a mixture of fossils linking older, more disturbing spore-like microfossils with younger spores clearly derived from wild plants,” Strother said. “This helps align the fossil spore record with molecular clock dates if we consider the origin of terrestrial plants as a long-term process involving the development of embryogenesis.”
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Strother added that the fossil record preserves direct evidence for the evolutionary assembly of the plant’s developmental and regulatory genomes. This process begins with plant spore development and leads to the origin of macroscopic and complete tissues and organs, and eventually plants, and possibly something like algae living today.
“When we consider microbiota an important component of the evolution of Earth’s plants, there is no longer a gap in the fossil record between molecular dating and fossil recovery,” Strother said. Without this gap, “we have a much clearer picture of an entirely new evolutionary step: from simple cellularity to complex multicellularity.”
As a result, researchers and the public may need to rethink how they see the origin of Earth’s plants, and the fundamental progression of life from water to land, Strother said.
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“We need to stop thinking of the origin of land plants as a singularity in time and instead integrate the fossil record into an evo-devo model of genome assembly over millions of years during the Paleozoic era, specifically between the Cambrian and Devonian divisions of that era,” Strother said. This requires a serious reinterpretation of the problematic fossils previously interpreted as fungi, not plants.”
Strother and co-author Clinton Foster of the Australian National University (ANU) proceeded to describe a group of spore-shaped microfossils from a deposit dating to the early Ordovician period, approximately 480 million years ago. This material fills a gap of about 25 million years in the fossil spore record, Struther said, linking spores from younger, well-accepted plants to more disturbing ancient forms.
Strother and Foster examined assemblies of fossil spores extracted from a rocky core excavated in 1958 in northwest Australia. These microfossils consist of highly resistant organic compounds in their cell walls that can survive skeletally from burial and rock. They were studied at Boston College and the ANU Research School of Earth Sciences, using a standard optical microscope.
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“We use fossil spores extracted from the core of rock pits to build an evolutionary history of plants back in time to the origins of their algal ancestors,” Strother said. “We have independent control over the age of these rock samples, so we study evolution by looking at changes in spore types that occur over time.”
Molecular biologists also analyze evolutionary history through time using genes from living plants to estimate the time of plant origins using “molecular clocks,” a measure of evolutionary divergence based on the average rate at which mutations accumulate in a species’ genome.
However, there are huge discrepancies, amounting to tens of millions of years, between direct fossil data and molecular clock dates, Strother said. Also, there are similar time gaps between the oldest spores and when full plants first appeared.
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These gaps have led to hypotheses about a “fossil record loss” of early land plants, Strother said.
“Our work seeks to resolve some of these questions by integrating the fossil spore record into an evolutionary phylogenetic model of the plant origins of the algal ancestors,” Strother said.
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