When vascular plants invaded the land

The colonization of primeval land masses by the first plants was a significant milestone in Earth’s history. But until now it is not clear when the first higher factories were able to take this step. Now, isotopic analyzes of mercury in ancient sediments in southern China provide new evidence. They show that the isotope signature, closely related to terrestrial vascular plants, is clearly recognizable from the beginning of the Silurian, about 444 million years ago. According to this, the first vascular plants could already have spread widely on Earth at that time – about 14 to 25 million years earlier than was previously assumed on the basis of fossil finds. This means that far-reaching changes caused by terrestrial plants could have begun earlier than expected.

The first life probably originated in the sea, or at least in a watery habitat. Only after hundreds of millions of years of evolution did the first creatures begin to colonize the land regions of the primeval earth. The first terrestrial organisms were dense, mat-forming microorganisms, followed by fungi, and then the first plants in the form of algae-like growths. Genome analyzes and family tree reconstruction using the molecular clock indicate that this step occurred in algae about 515 million years ago. However, the fossils of this fragile early vegetation have not survived, and the first fossil evidence comes from the Silurian, about 426 million years ago. After algae appeared the first vascular plants – tall plants with specialized vascular bundles and roots that gave them greater growth and drought tolerance.

Mercury isotopes as evidence

But when the first vascular plants spread on land is still a matter of dispute. While some researchers date the appearance of the first vascular land plants to around 470 million years ago based on DNA comparisons, others hypothesize that extensive vascular plants on Earth did not appear until the late Silurian period, and perhaps even the late Devonian period. The oldest clearly recognizable terrestrial vascular plant fossils are about 420 million years old. To clarify this discrepancy, Wei Yuan of the Chinese Academy of Sciences and colleagues have now looked for indirect evidence of higher plants on Earth. They explained that “such indirect imprints of terrestrial vegetation in the form of organic and inorganic geochemical markers can be found in the sediments.”

One of these signs are isotopes of mercury – atomic variants of this metal, which is gaseous at room temperature, carried over long distances by air currents. During photosynthesis, plants always absorb mercury atoms with air. Although isotopes of mercury are also contained in rainwater and groundwater, studies show that plants prefer to incorporate atmospheric mercury into their tissues. As a result of this selective incorporation, they contain less Hg-199 and Hg-200 than, for example, aquatic plants or geological sources. When terrestrial plants die, their typical isotope ratios find their way into the soil and with sediments into bodies of water and the sea. “Hg inputs from terrestrial plant biomass can overprint positive signals for these mercury isotopes normally found in marine sediments,” Yuan and colleagues explain. For their study, they examined mercury isotopes in former marine sediments in southern China that were deposited between 550 and 250 million years ago.

The green presence is already in the transition to the Silurian

Analyzes showed that sediments from the Early Cambrian to the Middle Ordovician, about 460 million years ago, contain high natural ratios of mercury isotopes 199 and 200 – as expected. “The evolution of marine invertebrates and other microorganisms during this period did not cause changes in the effects of mercury in marine sediments,” the researchers stated. The first algae evolved on Earth, but due to their slow growth and slow biomass increase, left no apparent traces in isotope values. But that changed about 450 million years ago: “The interval between the late Ordovician to the Silurian marked the beginning of a significantly negative mercury isotope signature,” according to Yuan and colleagues. “We see this signature in the context of the rapid colonization and subsequent spread of land plants. Because vascular plants grow faster, produce more biomass, and die faster, they transfer more atmospheric Hg-199 and -200 to terrestrial reservoirs.”

According to the scientists, their results indicate that the vegetation of vascular plants has been present on Earth for about 444 million years. “Our data show that land plants profoundly affected terrestrial ecosystems as early as the Ordovician to Silurian transition — although no large plant fossils have been identified from this period,” Yuan and his team say. If confirmed, vascular plants invaded Earth’s territory about 14 to 25 million years earlier than is assumed on the basis of fossil finds. In addition, it apparently did not last until the Devonian period before vascular plants developed dense vegetation. “To leave such a long-lasting trace in the mercury imprints of marine sediments, these terrestrial pioneers must have been widespread before the advent of the Devonian period—at least in low-latitude coastal and coastal areas such as southern China,” the researchers write.

Source: Wei Yuan (Chinese Academy of Sciences, Guiyang) et al., Science Advances, Available here. doi: 10.1126/sciadv.ade9510

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