Life Moves to Land
Why did plants move from the water to land?
Algae were diverse during the Proterozoic (2.5 billion–541 million years ago), and some groups must have evolved adaptations that allowed them to move from saltwater to freshwater like ponds and lakes
Freshwater green algae can live close to the shoreline of the ponds, where some could "root" in the wet soil, and where light is plentiful.
Sometimes during droughts, the water level drops in a pond/lake, killing the shoreline algae that need constant moisture or submersion.
Researchers think that over millions of years some algae groups adapted to survive drought conditions for short periods of time.
Over time, these plant-like algae evolved to survive longer and longer periods of drought
The true land plants evolved from these tough freshwater algae around 550 million years ago (the Cambrian Period).
Why didn't plants invade the land earlier?
Land was available for life early in the earth’s history, but ultraviolet radiation was lethal to anything attempting to live on land
As we all know, UV radiation causes cancer... and, "cancer" is just a defect in DNA and how cells divide (called mitosis). This is true of plants and algae, as well as animals
Water acts as a good filter of ultraviolet rays, which allowed algae and animals to survive and evolve
As a result, atmospheric oxygen (O2) levels on Earth increased over time
This increased oxygen in the atmosphere allowed for the formation of the Earth’s ozone (O3) layer
Ozone impedes ultraviolet rays which can harm DNA in cells.
The ozone layer made it possible for organisms to evolve onto the land
Life may have existed on land in the Cambrian Period, with soil bacteria and fungi, and there is evidence of spores that hint at this early colonization.
What did these early land plants look like?
The earliest land plants were probably small and flat, looking like small leaves that gripped the ground. They were very similar to modern day liverworts, cousins of mosses.
In order to reproduce, liverworts produce spore cases about 1 inch above the ground. Spores are released into the air, and they fly away from the parent plant.
The earliest evidence for true land plants comes from spores found in rocks 475 million years ago (Ordovician Period).
Above: Modern day liverwort, which looks similar to early plants that invaded the land
What animals were present, land or water?
Ocean animal groups, like sponges, worms, jellyfish (i.e. invertebrates) flourish, as well as complex animals evolve and thrive during the Cambrian
Trilobites, which look like roly-polys but are cousins to the horseshoe crab, are dominant in the ocean
During the Ordovician, animals with backbones such as fish, appear and diversify in the oceans.
During this time, there were no large animals on the land, only small invertebrates such as mites, springtails, and relatives of spiders.
There is some evidence that sea scorpions were able to come on land for short periods of time, but probably not permanently.
What other organisms were present?
These were organisms were not plants, but had algae- and fungi-like features. Some were tree-sized.
Nematophytes went extinct and left no known descendants. These may have been "experiments" in evolution that never succeeded for different reasons.
As mentioned before, fungi may have been present on the land during the Cambrian, and by the Ordovician, there is evidence of fungi living throughout the land.
Above: Fossil trilobites
Above: Reconstruction of tall nematophytes, called Prototaxites, during the Silurian Period
What was the Earth like during this period?
As these early plants and fungi grew, they weathered rocks while they absorbed minerals, such as calcium and nitrogen.
Carbon dioxide in the atmosphere reacted with these minerals, trapping the carbon as it travels into streams. This process caused chemical changes in rivers and oceans.
This reduction in CO2 caused global cooling and ice ages, leading to a massive extinction for marine life around 445 million years ago.
The Earth quickly warms during the middle Silurian, which allows for the rapid diversification of land plants.
Plants become both taller in size, and more complex in form and anatomy.