Life Moves 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

In particular, the green algae were very successful in freshwater. An environment that they are still very successful today (think "pond scum" like Spirogyra).

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). 

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

Around 2.7 billion years ago, the process called photosynthesis evolved with bacteria. 

• 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.

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).

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.

Around 460 million years ago (Middle Ordovician) until the Early Devonian, we see strange organisms, called nematophytes, present in the fossil record

• These were organisms were not plants, but had algae- and fungi-like features. Some were tree-sized.

• One example is Prototaxites, which is sometimes called the humongous fungus. It was a tree-sized lichen-like organism existing early in the middle Paleozoic.

• 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.

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.