The Quiet Evolution of Trees

The Quiet Evolution of Trees

by Eric McLamb
with Special Contributions from Dr. Jack C. Hall

Long before trees appeared on Earth, animals and other land plants were scattered among the land masses. And before land plants came the land animals. How did it all happen, and where did they all come from?

Trees evolved from the oceans in a string of plant evolution, but they did not just start growing on the shorelines. They evolved quietly millions of years after the first plants emerged from their aquatic origins. (UK Photo, Hampshire County Council)

They came from the oceans, where just a few billion years earlier the first animal life evolved followed by plants. So, you see, the occurrence of trees was quite full of twists and turns and came about quietly long after land plants had struggled to rise from the oceans. Yet, their existence is the essence of all life on the planet.

There was a time about 550 million years ago when life "exploded" on the planet. Animal life. Nearly all of the animal groups in existence today -- as well as many that no longer exist -- first appeared on Earth during this time. It was the Cambrian Period, and this time of tumultuous and colossal animal diversity is called the Cambrian Explosion.

Land plants evolved a little more quietly about 90 million years later, with trees evolving some 100 million years after the first land plants began to emerge from their oceanic origins. But neither animals nor plants could have evolved were it not for the protection and nurturing of the ocean.

According to the fossil record -- about 3.5 billion years ago -- the first preserved life are found in the form of bacteria. They appeared in the oceans after the surface (crust) began to cool and stabilize, the land masses began to take shape, and clouds formed to produce massive volumes of rainwater that created the seas. The atmosphere was much different than today and the surface was unprotected from the Sun. This period is known as the Pre-Cambrian, the time that immediately followed the formative, molten and gaseous stage of Earth as it and the rest of the solar system started to come together -- or coalesce.

...some 470 million years ago, Earth would seem lifeless, inhospitable and very barren. For one thing, no trees -- no plants at all -- lived on the land! (NASA photo of Mars surface)

The first plants on Earth were a form of blue-green algae which appeared and lived in the oceans about 3.4 billion years ago according to the fossil record, protected from the harmful high energy radiation of the Sun. In the oceans, these plants were able to grow and photosynthesize as this high energy radiation was absorbed by the water. Now, to be perfectly clear about it, the first true algae (the kingdom protoctista) most likely made their first appearance about 2.4 billion years ago, but for sure by 1.8 billion years ago as the first acritarchs.

Although animals were the first life forms on Earth, it was plants that paved the way for land animals to evolve. Plants did this by simultaneously increasing the percentage of oxygen in the Earth's atmosphere and decreasing the percentage of carbon dioxide, a powerful greenhouse gas (Journal Science, August 2001).

Still, if you were to travel back about some 470 million years ago, Earth would seem lifeless, inhospitable and very barren. For one thing, no trees -- no plants at all -- lived on the land !

The first land plants made their appearance way before trees started driving their roots into the hard crust of Earth's surface, about 460 million years ago in the Ordovician period. Algae were the first land plants, moving from their aquatic origins to marshy and wet environments on land. It took consistent growth and diversification of land plants -- including the eventual evolution of trees -- to help break up the mostly iron-clad surface of the Earth.

http://ecology.com/features/quietevolutiontrees/quietevolutiontrees.html