The oldest fossilized spores of land plants are making scientists rethink how plants evolved

When plants made from freshwater algae ventured into Earth more than 500 million years ago, they changed the planet. By removing carbon dioxide from the air, they cooled the Earth, and by eroding rocky surfaces, they helped build the soil that now covers a large area of ​​land.

These changes in the planet’s atmosphere and surface paved the way for the development of the biosphere as we know it. Terrestrial plants make up about 80 percent of the Earth’s biomass.

The pioneering algae-like plants had to overcome two major challenges to survive on Earth: avoiding drought and surviving the sun’s harsh ultraviolet rays.

In rock samples from the Canning Basin in northwest Australia, we discovered 480 million-year-old fossilized spores from ancient land plants along with spores from the ancestors of aquatic algae.

These are the oldest plant spores ever found, and they give us new clues about when and where plants jumped to the ground and how they managed to survive. Request chest in one Sciences.

When plants inhabit the earth

Estimates of the initial time of plant colonization of Earth are based on large fossilized plant remains and calculations of how long different species took to evolve (calledmolecular clock“Data), a record of plant spores.

Molecular clock data indicate that the Earth was colonized about 515 million years ago (in the Cambrian), while the oldest fossils of plant stems occurred about 430 million years ago (in the middle Silurian period).

These small, early plants lack root systems or hard woody tissues, which may explain why their fossil remains are so rare.

Alternatively, we can look at the spores of plants. Spores are simple reproductive units that carry genetic material (much simpler than seeds that evolved much later).

In order to reproduce successfully, the spore walls of terrestrial plants must be strong enough to withstand drought and UV damage.

These flexible spore walls also allow spores to be preserved in ancient sediments for hundreds of millions of years and extracted from these sediments with strong acids, as used in this study. Then we examined the shapes of the spores under the microscope.

shape of germs

The spores of early land plants appear as somewhat geometrically arranged groups of two or four cells. These spores have been found in sediments that are 465 million years old (in the Ordovician), at least 35 million years before larger plant fossils were known.

However, older spores (about 505 million years ago) have been found in the United States. Paul Strother (of Boston College, co-author of the new Canning Basin Paper) and his colleagues suggest that these ancient spores likely came from freshwater algae called Charophytes.

These ancient spores appear as “packets” of irregularly shaped cells. The same “packages” of spores were found in the fossils we found in the Canning Basin, which date back about 25 million years.

Charophyte algae are semi-aquatic. To survive in this state, they developed genes to resist dehydration and the harmful effects of ultraviolet rays.

The oldest terrestrial plants either captured parts of the genome of the algae’s ancestors, possibly through “horizontal gene transfer,” in which bacteria transfer genes from one organism to another, or they evolved similar genes themselves.

Given the time frame of millions of years, this suggests that the emergence of land plants did not occur as a unique event.

We found wild plant spores containing two or four cells and irregularly packed algal spores in the canning pond basin, indicative of the coexistence of wild plants and their algal ancestors in the same area.

It also narrows the time gap between estimates of land colonization from molecular clock data (515 million years ago) and fossil evidence.

The Canning Basin record is about 480 million years old and is the oldest in the world.

Where did the earth’s plants begin?

Our finding comes from previous studies of terrestrial plant spores in a canning pond. 1991 dated Keime About 440-445 million years ago found and more It dates back to 460 million years ago Found in 2016.

These two records were only found after examining extracts from about 100 core samples to determine the age of the rock sequences, showing that spores are rare.

The sediments deposited in the canning basin during this period come mainly from the marine environment, as we see in mussels and microfossils such as conodontin.

Early land plants, like their ancestors of granular algae, grew in freshwater areas on the edges of the sea.

In these areas, spores and sediments were washed away. The fossil discoveries that have come down to us are based on the geography of antiquity.

In 2020, Geoscience Australia, in partnership with the Western Australian Geological Survey, drilled a hole in the southern part of the Canning Basin to understand the geology of the deep rock.

After acid extraction of rock samples from a geological formation called the Nambet Formation, which dates back to the early Ordovician period (485 million to 470 million years ago), we identified spores from terrestrial plants with typical regular arrangements of two or four cells.

As part of this work, we examined preparations of plant spores that were actually superimposed on microscopic slides from the original section of the nambit formation excavated in 1958.

Here we found the first evidence of plant spores on Earth associated with the spores of their algal ancestors. Our discovery would not have been possible without access to this prior material provided by the Western Australian Government.

Further studies are needed to locate additional algal spores and terrestrial plants in Australian sediments from the Late Cambrian and Ordovician.

The new data could also reveal where the land plants originated from: were they present on this continent as others have suggested?

The present work has highlighted the importance of access to previous data and materials and acknowledged the critical role of scientific sampling infrastructure and geological data by the Western Australian Government.

Clinton FosterProfessor Emeritus Australian National University.

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