Our solar system is 4.6 billion years old. Within our vast universe there are millions of older solar systems, many of them probably with planets and life. It is entirely possible that more intelligent life forms may exist elsewhere. If so, what may have they seen as they passed Mars and approached Earth?

If our visitors had passed by Mars early enough, they may have seen a planet with a rocky landscape with some active volcanoes and a considerable quantity of surface water. Over the land surface, barren rocky soil totally devoid of life.

Let's say our visitors came by recently, oh.. ...some 480 million years ago (90.6% of the time from Earth formation to today). Our visitors would first be struck by the vast watery expanse of our oceans, similar in area and volume as today. Different would have been the shape of the oceans and distribution of continents.

As our visitors moved closer, for a better view of the continents, they would have observed the same variety of topography as today, vast flat plains to high snow covered mountain ranges. Yet to these explorers, Earth's land surfaces would have been more like Mars, where they had just visited. WHY? Because there was no visible life, no green, no plants, except for perhaps some very limited cyanobacteria.

So when did plant life first inhabit the continents? That is the subject we now explore. First, plants had to solve the problems of desiccation (drying), support, and the effects of gravity. Land plants probably evolved from marine plants, moved into freshwater and finally onto land. This probable transition is from marine green algae to simple bryophyte type plants to vascular plants:

Marine Algae ---------> Bryophyta -----------> Tracheophyta

(green) "amphibious vascular

plants" plants

No fossil evidence of transition from marine algae to non-vascular plants

Non-vascular Land Plant Characteristics:

Lack specialized cells for the movement of nutrients and water. They are small and limited mostly to low, moist areas.

Vascular Land Plant Characteristics (not all evolved at the same time):

roots: to gather water and nutrients

leaves: photosynthesis

vascular system: for fluid transport

cuticle (cutin in outer wall layers): prevent drying

support: lignin and cellulose

Phylum: Bryophyta: "amphibious" plants

lack: 1. fluid transport system - small

2. means of preventing gamete desiccation - need moist environment

oldest fossils reveal evolution by the middle to late Ordovician Period (~450-440 m.y.a.)

These first non-vascular, "amphibious" plants, were limited primarily to lowland, wet areas, of temperate to tropical latitudes. Still most land areas were barren.

By some 420 m.y.a., seedless vascular plants evolved which were bigger and more diverse than their ancestors. These seedless vascular plants required water for sperm to reach the eggs and were still limited in their distribution to wet climates. These seedless plants grew to tree size, reaching 10 or more meters by the late Devonian (~370 m.y.a.). Lowland swampy environments had abundant seedless plants.

After the 1st land plants evolved (approximately 450 m.y.a.) they underwent rapid expansion and radiation with great forests by Middle Devonian Period. (385 m.y.a.). Although these forests were limited in their extent, vast quantities of organic material were deposited and preserved in swampy environments. Through time and burial, this organic material was converted to coal, forming the first significant coal deposits of Earth history.

At a similar time (late Devonian, ~375-360 m.y.a.), seed bearing gymnosperm plants (non-flowering) evolved. The development of the seed allowed plants to proliferate and spread to drier areas of continents. Gymnosperms became the dominant plant type between ~290 to 145 m.y.a. and are still common today.

During the final days of the dinosaurs, the final chapter of plant evolution began. This was the evolution of flowering plants (angiosperms). During the Cretaceous Period, angiosperms began overcoming non-flowering plants as the dominant land plants. Since this time they have become the dominant land plants.

Food for thought:

For most of the history of planet Earth, landmasses have been barren of life. Even with the advent of vegetation, plants first clung to lowland wet regions. Much later, with the arrival of gymnosperms, they spread to drier regions. Only within the last 100 m.y. did the planet's plant world begin to resemble the Earth we know.

1. Prior to plant colonization of land, how would geologic processes (such as erosion) have differed from today?

2. Why are land plants important as eventual providers of a fossil fuel source?

3. In what type of environment are plants preserved as fossils?

For most of Earth history (90%), the land has been void of all plant life!

What did the barren landscape look like prior to life?