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The early birth of complex life
THIS WEEK
The early birth of complex life Big organisms appeared much earlier than thought, fuelled by oxygen Alice Klein
BOTTOM: MAOYAN ZHU; TOP: RICHARD BIZLEY/SCIENCE PHOTO LIBRARY
THE first large, multicellular organisms seem to have appeared on Earth a billion years earlier than we thought – and their evolution may have been driven by a surge in oxygen. Until recently, the prevailing wisdom was that life made the leap from simple, microscopic cells to large, complex organisms about 600 million years ago. This is when strange marine creatures called the Ediacarans
“It is unclear why the fossil organisms took more than a billion years to evolve into complex animals” appeared. They were thought to be the earliest organisms to vaguely resemble today’s plants and animals. However, Chinese researchers overturned this idea in 2016 when they reported 1.6-billion-year-old fossils that looked like primitive seaweed. They were found in mudstone in the Gaoyuzhuang rock formation in China’s Yanliao basin. Their finely preserved structures revealed closely packed cells, arranged in elongated shapes up to 30 centimetres long and 8 centimetres wide. The fossils ignited vigorous debate. Some experts believed they were the first true plant-like organisms. Others said they were just colonies of bacterial cells. Many also questioned why complex life would have emerged at this time. The Mesoproterozoic era, which began 1.6 billion years ago, has been labelled “boring” as it was apparently so uneventful. Now, Zhu Xiangkun at the One of the 1.6-billion-year-old fossils of “seaweed” 6 | NewScientist | 28 April 2018
Chinese Academy of Geological Sciences and his colleagues say they can make sense of the find. Their research suggests that this era was more lively than thought because of a previously unknown shift in oxygen levels. There was no oxygen in Earth’s air until about 2.4 billion years ago, by when photosynthetic organisms had released enough
The Ediacarans were thought to be the first complex life on Earth
for some to linger. But levels were far below today’s for hundreds of millions of years after. The team studied ancient rocks in Yanliao basin, which was under the ocean in the Mesoproterozoic era. The iron, carbon and rare earth elements in different layers indicated the oxygen levels in the basin’s seawater over time. This showed that oxygen levels rose at the beginning of the era, not long before the Gaoyuzhuang organisms lived (Nature Geoscience, doi.org/cnqr). How much it increased is unclear, but it may have been enough to support larger organisms that need more oxygen to survive, says Zhu. The complex structures of the
Gaoyuzhuang fossils, plus the preceding oxygenation event, suggest they are multicellular organisms and not bacteria, says Malcolm Walter at the University of New South Wales, Australia. The extra oxygen may also have diffused deep into the ocean, says team member Simon Poulton at the University of Leeds, UK. “The Gaoyuzhuang organisms appear to have lived quite deep in the water column,” he says. This is “a very calm environment, below the depth where storms would have reached, and where there would have been stable levels of oxygen and plenty of nutrients to feed on”. The next step will be working out why this oxygenation event occurred, says Poulton. One suspect is a particular kind of photosynthetic bacteria that evolved about 2.5 billion years ago. Although they only increased oxygen levels to about 0.1 per cent of modern levels at first, an environmental trigger 1.6 billion years ago may have expanded their number, allowing them to pump out more oxygen. For example, if there was a sudden rise in heat or rainfall, land masses may have begun eroding faster and released more nutrients into the sea, providing a feast for the bacteria, says Poulton. However, he says this is just speculation for now. It is also unclear why it took more than a billion years for the Gaoyuzhuang seaweeds to evolve into complex animals, which didn’t appear until 541 million years ago, during the Cambrian explosion. This evolutionary leap may have required an even bigger rise in oxygen levels, says Poulton. “There is evidence that oxygen rose to near-modern levels coincident with the Cambrian explosion.” No one knows why. ■
The early birth of complex life Big organisms appeared much earlier than thought, fuelled by oxygen Alice Klein
BOTTOM: MAOYAN ZHU; TOP: RICHARD BIZLEY/SCIENCE PHOTO LIBRARY
THE first large, multicellular organisms seem to have appeared on Earth a billion years earlier than we thought – and their evolution may have been driven by a surge in oxygen. Until recently, the prevailing wisdom was that life made the leap from simple, microscopic cells to large, complex organisms about 600 million years ago. This is when strange marine creatures called the Ediacarans
“It is unclear why the fossil organisms took more than a billion years to evolve into complex animals” appeared. They were thought to be the earliest organisms to vaguely resemble today’s plants and animals. However, Chinese researchers overturned this idea in 2016 when they reported 1.6-billion-year-old fossils that looked like primitive seaweed. They were found in mudstone in the Gaoyuzhuang rock formation in China’s Yanliao basin. Their finely preserved structures revealed closely packed cells, arranged in elongated shapes up to 30 centimetres long and 8 centimetres wide. The fossils ignited vigorous debate. Some experts believed they were the first true plant-like organisms. Others said they were just colonies of bacterial cells. Many also questioned why complex life would have emerged at this time. The Mesoproterozoic era, which began 1.6 billion years ago, has been labelled “boring” as it was apparently so uneventful. Now, Zhu Xiangkun at the One of the 1.6-billion-year-old fossils of “seaweed” 6 | NewScientist | 28 April 2018
Chinese Academy of Geological Sciences and his colleagues say they can make sense of the find. Their research suggests that this era was more lively than thought because of a previously unknown shift in oxygen levels. There was no oxygen in Earth’s air until about 2.4 billion years ago, by when photosynthetic organisms had released enough
The Ediacarans were thought to be the first complex life on Earth
for some to linger. But levels were far below today’s for hundreds of millions of years after. The team studied ancient rocks in Yanliao basin, which was under the ocean in the Mesoproterozoic era. The iron, carbon and rare earth elements in different layers indicated the oxygen levels in the basin’s seawater over time. This showed that oxygen levels rose at the beginning of the era, not long before the Gaoyuzhuang organisms lived (Nature Geoscience, doi.org/cnqr). How much it increased is unclear, but it may have been enough to support larger organisms that need more oxygen to survive, says Zhu. The complex structures of the
Gaoyuzhuang fossils, plus the preceding oxygenation event, suggest they are multicellular organisms and not bacteria, says Malcolm Walter at the University of New South Wales, Australia. The extra oxygen may also have diffused deep into the ocean, says team member Simon Poulton at the University of Leeds, UK. “The Gaoyuzhuang organisms appear to have lived quite deep in the water column,” he says. This is “a very calm environment, below the depth where storms would have reached, and where there would have been stable levels of oxygen and plenty of nutrients to feed on”. The next step will be working out why this oxygenation event occurred, says Poulton. One suspect is a particular kind of photosynthetic bacteria that evolved about 2.5 billion years ago. Although they only increased oxygen levels to about 0.1 per cent of modern levels at first, an environmental trigger 1.6 billion years ago may have expanded their number, allowing them to pump out more oxygen. For example, if there was a sudden rise in heat or rainfall, land masses may have begun eroding faster and released more nutrients into the sea, providing a feast for the bacteria, says Poulton. However, he says this is just speculation for now. It is also unclear why it took more than a billion years for the Gaoyuzhuang seaweeds to evolve into complex animals, which didn’t appear until 541 million years ago, during the Cambrian explosion. This evolutionary leap may have required an even bigger rise in oxygen levels, says Poulton. “There is evidence that oxygen rose to near-modern levels coincident with the Cambrian explosion.” No one knows why. ■