If 500 million years ago we had been standing on a beach, in front of the immense ocean that covered the Earth, the Pantalasic, we would be unable to foresee that that world would become what we see today. First, because we should be wearing an astronaut suit : the oxygen level in the atmosphere was below 6% and the lethal ultraviolet radiation sterilized the surface of the continents due to the absence of the necessary ozone layer. Looking inland we would see high volcanic cones as a result of tectonic changes that were taking place due to the breakup of a supercontinent that occurred during the Snowball event that covered the entire Earth with a white blanket. It is even possible that we felt the occasional earthquake. This rupture leaves beaches with shallow waters where life finds a niche to flourish. If we look carefully we can see the very young Cambrian fauna swarming under the crystal clear waters, something that contrasts with the desolation that we find on the mainland : we are standing on a gray world with mounds of black lava and where the surface is covered with rubble and rocks with sharp edges product of the intense glaciation from which the planet has just emerged. We see no trace of life on the mainland and we only find green in the few stromatolites that still survive near the beach, descendants of a very ancient empire born when the Earth was young and that dominated for two billion years.
But something was changing. Little by little life took positions. The first living things to venture beyond the calm shallow waters near the coast were the ubiquitous cyanobacteria, followed by mosses, fungi and lichens: they not only began to change the color of the land but also began to prepare the soil for those who were to come. Without nutrients to absorb, plants cannot survive.
The first proof we have that plants began to colonize the mainland are microscopic spores found in Ordovician rocks : they had harder and more resistant walls -to survive the scarcity of water- and could spread through the air. Apparently the first plants that reached the mainland were small non-vascular plants (which do not have a system of vessels to transport water and nutrients) similar to liverworts (named for their peculiar liver shape): they are small – they do not exceed 10 cm-, without deep roots and we can find them in humid environments and in the shade ; only a trained eye is capable of differentiating them from moss.
Although lately this scenario is being questioned: it is possible that the liverworts were not truly the first land plants . Philip Donoghue, a paleobiologist at the University of Bristol in the UK, put together plaeontological data with genetic data collected from more than 100 species of plants and algae and has concluded that land plants first appeared earlier than which was believed, during the Cambrian explosion. In addition, Donoghue’s team questions whether the most primitive plant was physiologically a liverwort. One of the reasons given for this is that they lack roots and pores for the exchange of gas and water, something normal in terrestrial plants but useless in aquatic plants. According to their analysis, the liverworts had roots and pores but ended up losing them. Donoghue suggests that the yet-to-be-discovered ancestor of all land plants probably had rudimentary pores and roots, and thus could grow better, process more soil and more carbon dioxide, and thus have had more influence on Earth’s biogeochemistry than previously thought.
If Donoghue’s results are confirmed, “this would change the entire timeline for the origin of terrestrial life and the rate of evolutionary change in plants and associated groups of animals (and fungi),” said Pamela Soltis, an evolutionary biologist at Plants from the University of Florida. By delaying the passage to dry land, it makes the evolutionary changes that happened later not as fast as we might think. This would please Darwin, who advocated slow and gradual evolution.
Whatever the first land plant was, charophyte green algae are thought to be its ancestors . In fact, from the study of the genetic material of the current charophytes it is deduced that they have more in common with the first land plants than with any current plant. Of all of them, the Trochiliscus podolicus found in the Ukraine is the oldest charophyte known with certainty as such. Now, and against all odds, it seems that the jump to the mainland was not made from the oceans but from fresh water. Paleobotanists suggest that some type of these green algae adapted to live in rivers during that warm season could have done it (there were places where temperatures reached 60 degrees).
Thefirst plantsof which we have a recorddate from 440 million years ago, in the Silurian. Unlike the animals, which came to land in groups starting with the arthropods, theColonization by plants was done all at once.The Silurian was a time of recovery and consolidation of the species that survived the first global extinction, that of the Ordovician-Silurian, which was much worse than the one that wiped out the dinosaurs: 85% of animal species disappeared. But life is robust;During the Silurian, large coral reefs appearedsimilar to the Australian Great Barrier Reef, different types of fish evolved andthe first vascular plants appeared, in what is the most important adaptation in the evolutionary history of plants.Not only do xylem and phloem play a similar role to blood vessels in animals, but they also serve as a support, a “skeleton”, for the plant to remain vertical. The first vascular plants appear for the first time in the fossil record 410 million years ago and of these, the earliest known isCooksonia: it had a V-shaped stem, it did not rise more than 10 cm from the ground and was anchored to the ground thanks to a horizontal stem from which new shoots emerged, a rhizome. It lived in the mud of estuaries and in other wet and low habitats, forming dense mantles. Like all the first plants, it multiplied by spores that formed at the ends of each stem, although it is not well known how.This was the starting signal for the diversity of plants that we see around us today.
