Billion-Year-Old Algae and Newer Genes Hint at Land Plants’ Origin

An unearthed fossil and genomic discoveries are filling important gaps in scientists’ understanding of how primitive green algae eventually evolved into land vegetation.

Dana Najjar | NAUTILUS

In life, the fossilized algae from a billion years ago may have looked as they do in this artist’s conception. The foreground of the image shows how filaments of the algae might have become embedded in the seafloor and preserved. Dinghua Yang

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 papers from early 2020—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.

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.”

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