Plants May Date Back Earlier Than Thought
Plants and fungi may have appeared on land hundreds of millions of years earlier than previously thought, according to new research that employed so-called genetic clocks to reconstruct the evolution of land plants.
The earliest records of land plants are fossils, the oldest of which date back 480 million years. But evolutionary biologists at Penn State University have used the genetic code to determine that plant life spread from the oceans onto land as early as a billion years ago, and fungi 300 million years before that.
The early plants may have lowered the Earth’s carbon dioxide levels--reducing the temperature at the same time--and increased the supply of oxygen, allowing hundreds of new species of animals to emerge.
The first fossil records of animals appear all at once, dating from about 530 million years ago. The period is called the Cambrian Explosion because so many species seem to have evolved suddenly and simultaneously. The phenomenon continues to perplex scientists, but lowered temperature and carbon dioxide levels on Earth may have made the environment hospitable enough for animals to evolve. The authors of the new research, published in Thursday’s edition of the journal Science, speculate that the early land plants may have contributed to those conditions.
During so-far-unexplained events that occurred 580 million to 750 million years ago, the Earth’s temperature lowered so dramatically that the oceans may have frozen solid.
Scientists have theorized that the sudden decrease in temperature, referred to as the Snowball Earth era, and the Cambrian Explosion were both heavily influenced by geological changes. But the new work provides the first inkling that we may now “actually have a biological explanation for these events,” said Blair Hedges, lead author of the study.
Other scientists were more cautious. Paleobotanist Andrew Knoll of Harvard University, for example, cautioned that the results of the study do not confirm that early land plants affected the two events.
There are no fossil records of land plants older than 480 million years, presumably because they were too soft to be preserved as fossils. But history is preserved another way: in the genetic code.
Genetic mutations, random changes in the genetic code, occur continuously over time. Hedges counted the mutations that occurred over time in animal genes and used the animal fossil record, which is better than that of plants and fungi, to determine the rate at which mutations occurred. The number of mutations was thus used as a clock, providing a timeline for when the earliest plant and fungi species emerged onto land.
Because mutations are random, “molecular clocks are not like Rolex watches,” said Hedges. The researchers studied changes in 120 different genes to make the best estimate of how much time had elapsed since the first plants.
The data suggest that plants appeared on land 700 million to 1 billion years ago and fungi appeared 1.3 billion years ago.
Plants consume carbon dioxide. Because carbon dioxide helps trap heat in the atmosphere--the greenhouse effect--reducing levels of the gas could lead to lowering the Earth’s temperature. As a byproduct of photosynthesis, the process by which green plants produce food, plants also release oxygen.
