Was it triggered by plant roots?
An plants, most people admire flowers, leaves, and maybe a strong trunk or twining tendrils. Underground roots rarely get a sigh of wonder. Plants without roots would languish at the level of algae. Sequoias could not stretch to gigantic heights and steppe grasses could not stoically brave the storms.
Two years ago in Scotland, researchers from Oxford and Münster found out exactly when roots developed. They were helped by fossils from a paleontological site near the village of Rhynie in Aberdeenshire. The rocks there are a window into the Devonian period around 400 million years ago. At that time, what is now Scotland was part of a landmass that later formed the northern part of the supercontinent Pangea. The area was criss-crossed by rivers and lakes, geothermal vents and geysers provided heat and minerals for life. The downright cozy terrain gave the plants an evolutionary boost: they put down roots.
are in Rhynie Fossils of Asteroxylon mackiei preserved, a vascular plant belonging to the clumping mosses (lycopsids), which had small roots in the ground as early as 407 million years ago. These early roots were much simpler than today’s, but still: the root principle was invented. And it had so many evolutionary advantages that it originated in several places and then developed enormously. The consequences of rooting the world were immense. Plants conquered the land, the soil was stabilized on the continents. Minerals were loosened from the rock and fed into the material cycle. The evaporation of water from the leaves created a humid atmosphere.
But recently scientists have hypothesized that the roots fueled species extinction at the end of the Devonian. Life flourished 360 million years ago, many different species emerged. By the end of the Devonian, more than half of all animal and plant species had become extinct. A severe lack of oxygen in the oceans would be a possible cause. The researchers use sediment analyzes from five lakes in Scotland and Greenland to justify the fact that roots may have played a role.
The team led by oceanographer Matthew Smart from the American Naval Academy showed that the phosphorus content in the soil fell with the emergence of roots. The plants extracted it from the rock and built it into their stems and leaves. When they died, they turned into phosphorus-rich soil, which in turn was washed into lakes and seas by heavy rains. Everyone knows the effect of over-fertilized fields and heavy summer rain: when nutrients are washed into the water, algae multiply and use up oxygen. The researchers were able to demonstrate the phosphorus and oxygen fluctuations in the sediments of the lakes studied. For many species, the sudden lack of oxygen meant death, they write in the Bulletin of the Geological Society of America.
But the new root hypothesis is facing headwind: The phosphorus and oxygen fluctuations could also have other causes, writes Maya Elrick from the University of New Mexico. The mass extinction in the Devonian should not be blamed lightly, maybe volcanic eruptions or a meteorite impact were to blame? If not by walkers, it’s nice that roots meet passion, at least in science.