LAND PLANT ORIGINS REVEALED

Most of what we know about the deep history of life on Earth comes from fossils, and according to that record, plants migrated from the sea onto land about 420 million years ago. But the story told by those dusty old rocks is patchy at best. New research led by the University of Bristol has wound back the evolutionary clock to determine that plants likely invaded dry land much earlier, about 500 million years ago.

The first complex plants were algae, living in ponds as far back as 1.6 billion years. The earliest fossils of land-based plants are dated to about 450 to 440 million years ago, during the late Ordovician Period. But just because fossils haven't been found, doesn't mean that plants weren't already well established by that time.

The researchers study ground-covering plants, resembling the earliest land-dwelling plants, on a lava field in Iceland

There are other techniques that can be used to fill in the gaps in the fossil record. Since life tends to evolve at a particular rate, clues can be found in the DNA of modern organisms. Using evolution models, researchers can wind back time and predict when certain traits likely appeared, and when different species first branched off from one another. This is called the "molecular clock", and it's often used in conjunction with fossil studies to check if the bones match up to what the models say certain species should have looked like at certain points in time.

"The fossil record is too sparse and incomplete to be a reliable guide to date the origin of land plants," says Mark Puttick, co-lead author of the new study. "Instead of relying on the fossil record alone, we used a 'molecular clock' approach to compare differences in the make-up of genes of living species – these relative genetic differences were then converted into ages by using the fossil ages as a loose framework."

Using this technique, the researchers found that plants were likely to have migrated onto land about 500 million years ago. That's between 50 and 80 million years earlier than previously thought, placing the first terrestrial plants to the middle Cambrian Period – close to when the first land animals arose.

A 400-million-year old plant stem fossil from Scotland, which is one of the earliest known plant fossils

The evolution of terrestrial plants seems to line up with a huge climate change event. Carbon dioxide concentrations were much higher than now, and with plants absorbing increasing amounts of the gas, atmospheric levels dropped and global cooling kicked off.

"The global spread of plants and their adaptations to life on land, led to an increase in continental weathering rates that ultimately resulted in a dramatic decrease of the levels of the 'greenhouse gas' carbon dioxide in the atmosphere and global cooling," says Jennifer Morris, co-lead author on the study. "Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value – our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised."

Since it's hard to tell how different species might affect each other, the researchers modeled several possible relationships to check whether that could change the results on when plant life emerged on land. No matter what they tweaked, the scientists say, the date came back the same.