Found in rock samples recovered in Australia more than 60 years ago, microfossils dating to the Lower Ordovician Period fill a knowledge gap of approximately 25 million years By reconciling the molecular clock (or rate of evolution) with the fossil spore record, the physical evidence for early plant life collected by scientists over the years.
This reconciliation supports an evolutionary developmental model that connects the plant origins with freshwater green algae, or carophyte algae, said Boston College paleobotanist Paul Strother, co-author of the new study, which is published in Science. The “evo-devo” model posits a more nuanced understanding of plant evolution over time, from simple cell division to early embryonic stages, rather than large leaps from one species to another.
“We found a mixture of fossils linking older spore-like microfossils and more problematic with younger spores that are clearly derived from land plants, “Strother said.” This helps align the fossil spore record with molecular clock dates if we consider the origin of land plants as a long-term process that implies the evolution of embryonic development “.
The fossil record preserves direct evidence of the evolutionary assembly of the plant’s developmental and regulatory genome, Strother added. This process begins with the evolution of the plant spore and leads to the origin of macroscopic and complete tissues, organs, and eventually plants, perhaps something like mosses that live today.
“When we consider spores as an important component of land plant evolution, there is no longer a gap in the fossil record between molecular dating and fossil recovery,” Strother said. Without that gap “We have a much clearer picture of a completely new evolutionary step: from simple cellularity to complex multicellularity.”
Rethinking the origin of plants
As a result, researchers and the public may need to rethink how they view the origin of land plants, that fundamental advancement of life from water to land, Strother said.
“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 Cambrian and Devonian divisions within that era, “Strother said. “This requires a serious reinterpretation of problematic fossils that have previously been interpreted as fungi, not plants.”
Strother and co-author Clinton Foster of the Australian National University (ANU) set out to simply describe a set of spore-shaped microfossils from a deposit dating to the Early Ordovician era, roughly 480 million years ago. This material fills a gap of about 25 million years in the fossil spore record, linking well-accepted, younger plant spores with more troublesome older forms, Strother said.
Strother and Foster examined fossil spore populations extracted from a rock core drilled in 1958 in northern Western Australia. These microfossils are composed of highly resistant organic compounds in their cell walls that can structurally survive burial and lithification. They were studied at Boston College and the ANU School of Earth Sciences Research, with standard light microscopy.
“We use fossil spores extracted from rock drill cores to build an evolutionary history of plants that goes back in time to the very origin of the plants of their algae ancestors, “Strother said.” We have independent age control on these rock samples, so we study evolution by looking at changes in the types of spores that they happen over time. ”
Molecular biologists also analyze evolutionary history through time by 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 during which mutations accumulate in the genome of a species.
Hypothesis of a “lost fossil record”
However, there are huge discrepancies, up 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 the time when whole plants first appear.
These gaps gave rise to hypothesis about a “lost fossil record” of the first terrestrial plants “, Strother said.
“Our work seeks to solve some of these questions by integrating the fossil spore record into an evolutionary developmental model of the plant origins of algae ancestors,” said Strother.
Eddie is an Australian news reporter with over 9 years in the industry and has published on Forbes and tech crunch.