Around 500 million years ago–when the Earth was already a ripe 4 billion years old–the first green plants appeared on dry land. Precisely how this occurred is still one of the big mysteries of evolution. Before then, terrestrial land was home only to microbial life. The first green plants to find their way out of the water were not the soaring trees or even the little shrubs of our present world. They were most likely soft and mossy, with shallow roots and few of the adaptations they would later evolve to survive and thrive on dry land. And though scientists agree that these plants evolved from some kinds of seaweed, we know comparatively little about those green algal ancestors.
But a few recent papers–two based on molecular biology, and one on rare, precious fossils from 1 billion years ago–are helping to fill in the gaps in our understanding of those ancient algae and what allowed them to eventually make the transition to land.
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While fossils of land plants are abundant, ancient seaweed fossils are rare. To survive out of water, plants developed sturdy vascular systems and strong cell walls. Those same characteristics make for excellent preservation in fossils. “Most algae are little squishy things; they don’t form a skeleton. The first land plants had to have some sort of mechanical support,” said Shuhai Xiao, a professor of geobiology at the Virginia Polytechnic Institute and State University. “Overall, their material is much stronger than seaweed.”
The recently unearthed tiny fossil, smaller than a single grain of rice, appears to be the world’s oldest known specimen of green algae: It rolls back the clock on the confirmed existence of these algae by a staggering 200 million years. “It’s very daunting. A billion years–that’s at least five times older than the oldest dinosaurs,” said Xiao, who is a senior author on the Nature Ecology & Evolution paper that announced the discovery. “It’s before any animals. The world is very, very different from what we know today.”
Qing Tang, a postdoctoral researcher in Xiao’s lab and the first author on the study, has made it his mission to seek out billion-year-old rocks all over the world. He’s interested in one of the critical steps in the evolution of complex life: how cells became eukaryotic, with different internal compartments rather than free-floating cell contents. “I’m interested in the question of how eukaryotes evolved in the early stages on the Earth,” Tang said. “To answer that question, we have to look for very old rocks.” He excavated some from formations near the city of Dalian in northern China, where geological maps had told him he was likely to find the green-hued rocks containing fossils from that remote epoch. But it wasn’t until he got back to the lab and examined them under an electron microscope that he understood the value of what he had found: “I was very excited when I saw the first piece of this green seaweed,” he said. “These kinds of fossils are totally new to science.
Ancient as the fossilized algae are, they seem to have many of the characteristics also seen in much later green seaweeds. It isn’t just that they were clearly photosynthetic and multicellular–traits that help to define seaweeds but have murky evolutionary origins. “They have leaves, they have branches,” Tang said.
Before Tang’s find, the oldest known green algae fossil was only around 800 million years old. Because that fossil is fragmented and poorly preserved, not all scientists agreed that it was a piece of green algae immortalized in rock; the uncertainty left the early history of seaweeds in doubt. The new fossils from Dalian have weathered their billion-year existence more successfully. “We [are] more confident to interpret them as seaweed,” Tang said, “and they [are] more convincing for other scientists.”