Plants’ evolutionary history began with algae over a billion years ago, but land plants emerged 700 million years ago. The first land plants were non-vascular bryophytes, followed by vascular plants and seed-bearing plants. The “Devonian explosion” saw huge forests of seed-bearing plants, and angiosperms emerged during the Cretaceous period, with grasses evolving just 35 million years ago.
The evolutionary history of plants begins a long time ago. Algae have probably existed on moist soils for over a billion years, but plants, such as in the kingdom Plantae, didn’t emerge until 700 million years ago. This number comes from a molecular genetics analysis that suggests land plants split from green algae around this time, although this figure is not corroborated by fossil evidence. The earliest evolutionary history of plants on earth appears in the Early Ordovician Period, about 475 million years ago, although many paleobotanists suspect there were plants during the Cambrian Period, 500 million years ago.
The first land plants were non-vascular bryophytes, represented today by mosses, hornworts and liverworts. These plants, lacking circulatory tissue, were quite short, between 1 and 100 mm (4 in) thick. These bryophytes represented the basal group in the evolutionary history of plants. They could only survive in very humid areas, where all cells can easily absorb water directly and their spores could be dispersed easily. Lacking a protective coating, the spores are relatively fragile and prone to desiccation (desiccation). Scientists believe that the first land plants may have set the stage for animals to colonize the earth by sequestering carbon dioxide from the atmosphere in the biopolymer lignin. This increased the portion of the atmosphere containing oxygen, making it more available to oxygen-breathing animals such as early arthropods and terrestrial molluscs.
About 425 million years ago, the first vascular plants appeared, such as the simple Cooksonia forked, sporangia (spore-producing structure) and the unusually advanced Baragwanathia, found in Australia. Slowly, the plants grew in height, from a couple of centimeters to about 20 centimeters (8 inches). At this point, the plants spread mainly through vegetative growth, as the spores cannot be dispersed very far from the parent plant. Scientists who study the evolutionary history of plants are hard at work trying to determine which land plant actually came first and what its ecosystem looked like.
Throughout the Devonian (416 – 360 million years ago), plants progressively grew in height until they became as large as today’s massive ferns. At the beginning of the Devonian, plants were mostly non-vascular and consequently reduced, but by the end of the period seed-bearing plants had evolved, forming huge forests. The explosion of botanical diversity during this period has been called the “Devonian explosion”. Meanwhile, fish ruled the seas.
The next major innovation in the evolutionary history of plants was much later, during the Cretaceous period, when flowering plants (angiosperms) first emerged. By using flowers to attract bees, which would then pollinate other plants, angiosperms were genetically diverse and a major evolutionary success. One of the latest plant varieties are grasses, which evolved from angiosperms just 35 million years ago.
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