快猫短视频

The first large, multicellular organisms seem to have appeared on Earth a billion years earlier than we thought 鈥� 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 blob-like creatures called the Ediacarans appeared. They were thought to be the earliest beings to vaguely resemble today鈥檚 plants and animals.

However, Chinese researchers overturned this idea in 2016 when they reported that looked remarkably like primitive seaweed. They were found in mudstone in the Gaoyuzhuang rock formation in China鈥檚 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, but others said they were just colonies of bacterial cells.

Many also questioned why complex life would have emerged at this time in Earth鈥檚 history. The Mesoproterozoic era, which began 1.6 billion years ago, has been labelled 鈥�boring鈥� because it was apparently so uneventful.

Oxygen burst

Now, at the Chinese Academy of Geological Sciences and his colleagues believe they can make sense of the find. Their research suggests that the Mesoproterozoic era was more action-packed than thought, because of a previously unknown shift in oxygen levels.

There was no oxygen in Earth鈥檚 air until about 2.4 billion years ago, when photosynthetic organisms released enough for some to linger. But oxygen remained far below today鈥檚 level for hundreds of millions of years after.

The team studied the composition of ancient rocks in Yanliao basin, which was submerged under the ocean during the Mesoproterozoic era. The iron, carbon and rare earth elements in different layers gave an indication of the relative oxygen levels in the basin鈥檚 seawater over time.

The analysis revealed that oxygen levels rose at the beginning of the era, not long before the Gaoyuzhuang organisms lived. How much it increased could not be calculated, but it may have been enough to support larger organisms that need more oxygen to survive, says Zhu.

In addition, the extra oxygen would have been able to diffuse deeper into the ocean, says team member at the University of Leeds, UK. 鈥淭he Gaoyuzhuang organisms appear to have lived quite deep in the water column,鈥� he says. 鈥淭hey evolved in 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.鈥�

Mysterious origins

The next step will be working out why this oxygenation event occurred, says Poulton.

One possibility is that the extra oxygen was produced by a kind of photosynthetic bacteria that first evolved about 2.5 billion years ago. Although these bacteria only increased oxygen levels to about 0.1 per cent of modern levels at first, an environmental trigger during the Mesoproterozoic may have expanded their population and allowed them to pump out more oxygen.

For example, if there was a sudden increase in heat or rainfall, land masses may have started eroding faster and releasing 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 more complex animals, which didn鈥檛 appear until 541 million years ago, during the Cambrian explosion. This evolutionary transition may have required an even bigger increase in oxygen levels, says Poulton. 鈥淭here is evidence that oxygen rose to near-modern levels coincident with the Cambrian explosion,鈥� he says. No one is sure why.

The birth of animals

The complex structures of the Gaoyuzhuang fossils, plus the discovery of a preceding oxygenation event, strongly suggest they are multicellular organisms rather than clumps of bacteria, says at the University of New South Wales, Australia.

However, it is difficult to know if they represent the first multicellular organisms big enough to be seen with the naked eye, says Walter. 鈥淚t鈥檚 a very spotty palaeontological record when you go back that far, so you have to be cautious.鈥�

In 2010, a team reported in Gabon that also looked a bit like multicellular organisms. The fossils were up to 12 centimetres in size and were shaped like flat discs with ruffed edges 鈥� unlike any species alive today. But they are less well-preserved than the Gaoyuzhuang fossils and look more like marks left by bacterial colonies, says Walter.

The other main contenders for the oldest multicellular organisms are : centimetre-sized, tube-shaped fossils that appear in the fossil record about 1.8 billion years ago. However, some experts also think these are clumps of bacteria, not multicellular beings.