When the vascular plants conquered the land

When the vascular plants conquered the land

Germinating vascular plants. © amenic181/ iStock

The colonization of the primeval land masses by the first plants was a milestone in the history of the earth. But until now it was unclear when the first higher plants managed to take this step. Now, analyzes of mercury isotopes in ancient sediments in southern China provide new clues. They show that the isotopic signature, which is closely linked to terrestrial vascular plants, is clearly recognizable as early as the beginning of the Silurian, around 444 million years ago. According to this, the first vascular plants could already have spread widely on land at that time – around 14 to 25 million years earlier than previously assumed on the basis of fossil finds. This means that the far-reaching changes caused by land vegetation could have started earlier than expected.

The first life probably arose in the sea or at least in a habitat characterized by water. Only after hundreds of millions of years of evolution did the first creatures begin to colonize the land areas of the primeval earth. The first terrestrial organisms were dense mat-forming microorganisms, followed by fungi, then the first plants in the form of moss-like growths. Genome analyzes and reconstructions of the family tree using the molecular clock suggest that this step happened in mosses around 515 million years ago. However, fossils of this fragile first land vegetation have not survived, the earliest fossil evidence comes from the Silurian around 426 million years ago. After the mosses came 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 disputed. While some researchers date the emergence of the first vascular land plants to around 470 million years ago based on DNA comparisons, others assume that extensive vascular plant vegetation on land did not appear until the late Silurian, possibly even as late as the Devonian . The oldest clearly identifiable fossils of terrestrial vascular plants are around 420 million years old. To clarify this discrepancy, Wei Yuan of the Chinese Academy of Sciences and his colleagues have now looked for indirect evidence of the presence of higher plants on land. “Such indirect fingerprints of land plants in the form of inorganic and organic geochemical markers can be found in sediments,” they explain.

One such marker is mercury isotopes — atomic variants of this metal, which is gaseous at room temperature, are carried over long distances by air currents. During the course of their photosynthesis, plants always absorb mercury atoms with the air. Although mercury isotopes 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 mercury-199 and mercury-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 the sediment, into bodies of water and the sea. “The inputs of mercury from terrestrial plant biomass can therefore overprint the positive signatures of these mercury isotopes normally found in marine sediments,” explain Yuan and his colleagues. For their study, they therefore examined the mercury isotopes in former marine sediments in southern China that were deposited between 550 and 250 million years ago.

Green presence already at the transition to the Silurian

The analyzes showed that the sediments from the early Cambrian to the middle Ordovician about 460 million years ago contained the normal, high proportions of the mercury isotopes 199 and 200 – as expected. “The evolution of marine invertebrates and other microorganisms during this period did not cause changes in the mercury signatures of marine sediments,” the researchers report. The first mosses developed on land, but due to their slow growth and slower increasing biomass, they did not yet leave any clear traces in the isotope values. But that changed around 450 million years ago: “The interval from the late Ordovician to the Silurian is marked by the beginning of a significantly negative mercury isotope signature,” according to Yuan and his 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 transport more atmospheric mercury-199 and -200 into terrestrial reservoirs.”

According to the scientists, their results suggest that vascular plant vegetation existed on land around 444 million years ago. “Our data demonstrate that land plants profoundly influenced terrestrial ecosystems as early as the transition from the Ordovician to the Silurian – even though hardly any plant macrofossils are known from this period,” Yuan and his team state. If this is confirmed, then the vascular plants conquered the earth’s land areas around 14 to 25 million years earlier than assumed on the basis of fossil finds. In addition, it apparently did not last until the Devonian before the vascular plants developed a dense vegetation cover. “To leave such a long-lasting trace in the mercury signatures of marine sediments, these terrestrial pioneers must have been widespread before the onset of the Devonian — at least in coastal and riparian areas of low latitudes such as southern China,” the researchers write.

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

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