Great Moments in Earth History: Colonization of Land by PlantsPosted: February 28, 2013
Up until around 500 million years ago, the continents of Earth were practically lifeless, harboring – at most – slimy mats of bacteria on rocky, barren wastelands. Around this time plants began to creep out of the oceans, gradually developing adaptations that allowed them to expand further and further inland over millions and millions of years. But there is a dark side to this story: the increasing success of plants on land may have contributed to one of the largest set of extinctions known to the fossil record.
Plants colonized land over a period as long as tens to hundreds of millions of years. But there were a number of evolutionary advances that brought about swift change. Each advance allowed plants to either expand to new habitats or grow larger. And with each advance, the roots of these pioneering plants broke more and more earth apart. To Tom Algeo, a geologist at the University of Cincinnati, this process may have created a chain of events that removed massive quantities of oxygen from the ocean.
Although the first land plants evolved around 500 million years ago, they remained close to the waters edge and did not grow very large for around 100 million years. But these early plants paved the way for the future success of larger plants. This later success is largely due to lignin, tissue that gives plants structure and support.
Lignin, which developed around 390 million years ago, allowed plants to grow tall and produce deep roots. Eventually this resulted in the first forests, composed of massive, 100-foot tall ferns named Archeopteris. Because all plants at this time used spores, which can dry up and become useless without water, they could not expand into dry habitats. This changed around 370 million years ago, when plants began using seeds. This freed them from the burden of being near moist environments, and it allowed plants to colonize deep into the continental interiors.
The time period that contains these massive evolutionary leaps is known as the Devonian. For Algeo, it is no coincidence that these major milestones in plant evolution coincide with major extinction events in the ancient world’s oceans. In a widely cited paper published in GSA Today in 1995, Algeo presented the hypothesis that the evolution of the first lignified plants, then forested ecosystems, and finally seed bearing plants, each produced widespread biotic crises.
Algeo argues that each of these events lines up with two things: evidence for oxygen free marine waters and extinctions. Oxygen-free, or anoxic, water leaves a distinctive mark on the geologic record. Without oxygen, organic material is preserved, forming dark sludgy rocks called black shales. The middle and late Devonian, a time when nearly every marine group of organisms suffered some degree of extinction, is littered with black shales.
According to Algeo, the start of black shale deposition occurs at the same time that plants evolved lignin. The period of time when the most significant extinction occurred, known as the Frasnian-Famennian mass extinction, lines up in time with the peak of Archaeopteris forests. And finally, the last extinction of the Devonian period occurs at the same time that seeds evolved.
It all has to do with roots. When plants produce roots, they weather away rock using chemicals, and physically break them by expanding into the Earth. The process produces soil, but it also increases the amount of erosion on land. Both of these factors are important. As soil begins to develop, more and more organic matter is created, but as more erosion occurs as the result of these expanding root systems, much of this soil is transported away into the oceans. This organic matter can be converted into nutrients, flooding the ocean with the very chemicals organisms need to live.
Though this may seem like a good thing, it can actually have disastrous consequences.
With more nutrients in the water, the ocean can sustain more and more photosynthetic organisms, creating massive algal blooms. While these can be toxic in and of themselves, what is just as damaging is what happens after all of these organisms die. Their remains are decomposed by a variety of different bacterial organisms – a process that requires oxygen. Without oxygen, much of the ocean becomes uninhabitable. Those organisms that could not escape the anoxic waters would have ultimately perished.
The evolution of terrestrial plants can certainly be considered a great moment in Earth history. In addition to greatly expanding the area which life could live on this planet, it created the habitats necessary for animals, such as ourselves, to follow onto land and evolve. This one, however, may have come with a monumental cost.