The planet was transformed when plants first walked on the land. They evolved from freshwater-dwelling algae more than 500 millions years ago. They cooled Earth by drawing carbon dioxide out of the atmosphere and eroding rock surfaces to create the soil that covers most of the land.These changes in the atmosphere and land surface of the planet paved the way to the evolution of the biosphere that we now know. Around 80 percent of Earth’s biomass is made up by land plants.These pioneering plants were tiny and mossy-like and had to overcome two major challenges in order to survive on the ground: not to dry out and to withstand the harsh ultraviolet light of sunlight.We found fossilized spores of ancestral water-dwelling alga alongside early land plants in rock samples taken from Canning Basin, north Western Australia.These are the oldest land plantspores ever discovered. They provide new clues as to when and where plants first made the leap to land. Science published the research.The first time plants colonized landLarge fossilized plant remains and calculations of how long it took for different species to evolve (known as "molecular clock") are used to estimate the timing of colonization of land.Data from the molecular clock suggest that land colonization took place around 515 million years ago (in Cambrian time), while the earliest fossilized plant stems are around 430 million years old (in the middle-Silurian time).This may be why fossil remains of these early small plants are so rare. They did not have roots or hard woody tissue.Alternately, we could look at the spores that plants produce. Spores, which are simpler reproductive units than seeds and carry genetic material, are much simpler than seeds.The spore walls of land plants must be strong enough to resist drying and ultraviolet radiation damage in order to reproduce successfully.These resilient spore walls allow the spores can be preserved in ancient sediments for hundreds of million of years. They can then be extracted using strong acids such as those used in this study. The spore shapes were then examined under the microscope.The shape of sporesThe spores from the earliest land plants are organized in regular, geometrically-arranged groups of two to four cells. These spores have been discovered in sediments dating back to 465 million years (in Ordovician), which puts them 35 million years ahead of any larger known plant fossils.But, the United States has also found older spores that date back to 505 million years ago. Paul Strother, a Boston College professor and my co-author in the Canning Basin research, has shown that these older spores likely come from freshwater algae called Charophytes.These older spores are irregularly shaped "packets” of cells. These "packets of spores" are also found in fossils from the Canning Basin that were dated to approximately 25 million years later.Semi-aquatic living is possible for Charophyte Algae. They developed genes that can resist the harmful effects of UV and desiccation to survive in this environment.The first land plants captured parts of the ancestral algal genome or created similar genes themselves.It is clear that the origin of land plants was not a single event, given the time span of millions of years.Both land plant spores with two or four cells were found, as well as irregularly packaged algalspores. This shows that land plants and algal ancestors lived in the same area.It also reduces the time difference between land colonization estimates based on molecular clock data (515,000,000 years ago) or fossil evidence.The Canning Basin record, which is approximately 480 million years old and was discovered in 2005, is one of the oldest ever found anywhere on the planet.From where did the land plant get its start?This discovery is based on earlier studies in Canning Basin of land plant spores. The spores were discovered in 1991, and they were dated between 440-45 million years ago. More spores dating to 460 million year ago were also found in 2016.These two records were found only after examining extracts from around 100 core samples to determine the rock sequences' age. This shows that spores are very rare.This period's sediments were primarily from marine environments. We can see this from shelly fossils as well as microfossils like conodonts.Like their charophyte alga ancestors earlier land plants, they grew in freshwater at the edges of the ocean.These areas were contaminated with spores and sediments. The geography of the ancient world is what has shaped the fossil records we have.Geoscience Australia and the Geological Survey of Western Australia jointly drilled a well at the Canning Basin's southern end in 2020 to study the geology of subsurface rocks.Acid extraction of rock samples from the Nambeet Formation geological formation, which dates back to the Early Ordovician Period (485 million to 477 million years ago), allowed us to identify land plant spores that had the regular arrangement of two to four cells.We also examined plant spores already mounted on glass slides from the Nambeet Formation, which was drilled in 1958.Here we discovered the first record of land plantspores associating with algal spores. Without the WA government's earlier materials, our discovery would have been impossible.Additional studies are required to determine if additional land and algal spores exist in Australian sediments dating from the Late Cambrian or Ordovician periods.New data could also shed light on the origins of land plants: Was it on this continent?This work has highlighted the importance of having access to historical data and materials. We also acknowledge the crucial science infrastructure role played by the WA government in curating geological data and samples.Clinton Foster, Honorary Professor, Australian National UniversityThis article was republished by The Conversation under Creative Commons. You can read the original article.
