AT THE bottom of the Mediterranean Sea, just south of Greece, there is a lake. Complete with a delicate shoreline and an inviting deep blue surface, the L鈥橝talante basin looks almost like a lake on land. But this is an inhospitable place. Its waters are about as salty as water can be and never mix with the layers above 鈥� making it completely devoid of oxygen.
It was a shock, then, when biologist Roberto Danovaro scooped up samples from the bottom of this briny pool and found a thriving community of microscopic animals living there. The discovery went against everything we thought we knew about animal life and its reliance on oxygen.
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Some biologists still think Danovaro must have made a mistake. Yet his is just the most exotic in a series of discoveries that are chipping away at our long-held beliefs about the importance of oxygen to complex life. We thought anything beyond the simplest cell couldn鈥檛 do without it. But now we鈥檙e finding that some animals can carry on more or less happily with the merest whiff of the stuff.
It鈥檚 a finding primed to upset our tidy story of why complex life evolved on Earth. , a palaeontologist at the University of Cambridge, puts it bluntly: 鈥淎tmospheric oxygen has nothing to do with it.鈥� He reckons there is a much simpler explanation 鈥� one that鈥檚 been staring us in the face.
For the first 80 per cent of our planet鈥檚 history there was barely any oxygen, and no complex life either. That has always seemed perfectly logical. Organisms that use oxygen to respire can squeeze as much as five times more energy from the sugar in their food than those that use sulphur or other substances. That makes oxygen a gateway gas to a vibrant world where organisms have enough energy to become anatomically interesting. Plants are a different and more complicated story, but the consensus has been that multicellular life with specialised tissues 鈥� animals in other words 鈥� only evolved when there was enough oxygen.
The link seems to make sense geologically too. When we reconstruct the past concentration of oxygen in Earth鈥檚 atmosphere from geological samples we get a story in which the gas was hardly around for aeons, then rose a tiny bit before flatlining again. It only jumped up substantially about 800 million years ago 鈥� not long before we see the first fossils of complex life (see 鈥淏reath of life鈥�). To many geochemists, that cannot be a coincidence.
But not everyone feels this way, and there has been plenty of grumbling. As early as 1982, Bruce Runnegar at the University of New England in Australia estimated that some of the earliest animals in the fossil record might have had . He calculated that ancient 1-metre-long worms needed so little oxygen that they could potentially have lived billions of years before that jump in concentration 800 million years ago.
Those were worms, and theoretical worms at that. But in 2014, Daniel Mills and Donald Canfield at the University of Southern Denmark in Odense set about investigating how much oxygen real primitive animals need. They began by collecting living breadcrumb sponges from well-oxygenated water off the Danish coast, a relevant test case because sponges are one of the earliest animals to have evolved. Then the researchers carted them back to their lab and did their best to suffocate them.
Over several days they reduced their oxygen supply, first to 70 per cent of current atmospheric levels, then 50 per cent, then eventually to 5 per cent. Even then, , with one even seeming to grow slightly (PNAS, vol 111, p 4168). It suggested that Runnegar鈥檚 calculations pointed the right way: simple animals could have coped with ancient oxygen levels. 鈥淪ponges require vanishingly small amounts of oxygen. It鈥檚 a fact,鈥� says Butterfield.
鈥淚 think there鈥檚 a distinction to be made here between eking out a living and thriving,鈥� says , a geochemist at the University of California, Riverside. Some of today鈥檚 simpler animals might be able to endure low oxygen levels, but for all we know that makes for a grim existence. Lyons thinks it is unlikely that evolution would have selected for animals that could barely soldier on.
A comfortable sponge
The crucial question, then, is how comfortable the sponges are. To find out, we need a sense of how they fare biochemically when they don鈥檛 have much oxygen, says , a biologist at the University of D眉sseldorf in Germany. That means studying their mitochondria, the seat of cellular energy production.
Mitochondria use oxygen to help make a molecule called ATP, which cells use as a fuel. That is how our own cells work, but some animals have developed ways of coping when oxygen runs low. Their mitochondria can conduct a super-efficient form of oxygen-free sugar fermentation to stay alive. A carp, for example, for four months if its pond gets covered in ice. But as far as we know it can鈥檛 carry on indefinitely. If the sponges exist in a similar stand-by mode it lends weight to Lyons鈥檚 misgivings.
Mills is busy investigating what is going on inside the sponges鈥� mitochondria in an attempt to clarify things. 鈥淲e actually don鈥檛 know yet,鈥� he says. But fermentation is not the only way ancient animals might have thrived without oxygen. Take protists, the earliest form of our own eukaryotic lineage, which . Although protists have only one cell, their insides are far more complex than bacteria. Some of them seem to have powered this extra complexity with modified versions of mitochondria called hydrogenosomes, which work in the absence of oxygen. Might more complex eukaryotes have done the same?
That is where L鈥橝talante might be relevant. Danovaro鈥檚 team, from the Polytechnic University of Marche, Italy, became interested in the site partly because it is such unique environment. Danovaro began studying it in 1998 out of curiosity about what might live there.
More than he bargained for, was the answer. When he and his team lowered a weighted scoop and pulled up sediments from the lake bottom they found multicellular animals from a little known group called loriciferans. Assuming contamination, Danovaro returned to the site twice more 鈥� but kept on finding them.
He also found plenty of other animals, including nematode worms and tiny crustaceans. But these seemed to be rotting carcasses, and he concluded they had probably drifted down from oxygenated waters above, settling on the surface of L鈥橝talante before eventually sinking into it.
But the loriciferans were different: they looked pristine. And they acted as if alive, taking up a fluorescent dye absorbed only by creatures with a working metabolism. When Danovaro looked at the animals under a microscope, he saw structures the size and shape of mitochondria but without their telltale internal architecture 鈥� hydrogenosomes in his estimation.
鈥淓ither the animals are alive or being eaten from the inside by bodysnatchers鈥�
That鈥檚 not enough to convince those in the know. 鈥淚dentification by sight of which organelle is involved 鈥� mitochondria or hydrogenosome 鈥� isn鈥檛 easy, if possible at all,鈥� says biochemist , who studies hydrogensomes at Utrecht University in the Netherlands. It鈥檚 through their behaviour that hydrogenosomes are properly identified, which means studying the loriciferans鈥� biochemistry while they are alive.
That鈥檚 rather an ask. It鈥檚 not even clear that the loriciferans can survive the journey from their high-pressure habitat to a ship. Danovaro鈥檚 fluorescent dye experiments suggested they might, but aren鈥檛 definitive.
Then of the Woods Hole Oceanographic Institution in Massachusetts had an idea that might at least settle whether the loriciferans were living in the lake. 鈥淥ur game plan was to recover specimens that looked intact and then try to extract messenger RNA from them,鈥� she says. This ferries genetic information around cells and breaks down rapidly, so finding it would show the animals were alive recently.
Edgcomb and her colleagues visited L鈥橝talante to give it a try, but quickly ran into a snag. They had opted to use a remote-controlled submersible called Jason to take samples rather than Danovaro鈥檚 crude scoop. But the salty lake was so dense that the sub couldn鈥檛 dive into it. Like the collection of plastic bags and tin cans that the sub鈥檚 camera saw accumulated on the lake鈥檚 surface, 鈥淛ason was too buoyant,鈥� says Edgcomb.
Cutting her losses, she set about exploring the oxygenated mud around the shores of the lake. In 2015, she reported that , lending support to the idea that they merely fall into the pool after death. What about the fluorescent dye that supposedly shows the animals were alive? Edgcomb thinks it could be explained by bodysnatcher bacteria, which don鈥檛 need oxygen, eating the dead loriciferans from the inside out.
For his part, Danovaro stands by his original claims, though he admits . Another mission to L鈥橝talante with the right equipment to catch living loriciferans on camera would help settle things. But for now the question of whether animals can thrive, rather than just survive, without oxygen remains in stalemate.
Whatever the truth about the loriciferans, more people are beginning to accept that what Butterfield calls the 鈥渙xygen explains everything鈥� narrative fails to tick all the boxes in its description of why complex life evolved. With conclusive evidence in short supply, Butterfield thinks it鈥檚 time to consider an alternative. 鈥淭he reason that animals appeared so late in the day is very simple,鈥� he says. 鈥淭hey are extraordinarily complex and it took a long time for evolution to discover them.鈥�
Evolution has invented multicellular organisms relatively few times, says Butterfield 鈥� and what he calls 鈥渢ruly complex鈥� multicellular life has evolved just twice, giving us animals and land plants. Plus, animals were the only branch of the eukaryotic tree to turn multicellular as oxygen levels began to climb 800 million years ago. If a rise in oxygen was the only barrier to their evolution, why didn鈥檛 more eukaryotic branches follow suit? The atmosphere has been rich in oxygen for hundreds of millions of years, after all.
Even the 鈥渃oincidental鈥� rise in oxygen at the time of the first animal fossils looks different through the prism of evolution, says Butterfield. He points out that animals are profound ecosystem engineers. So filter-feeding sponges, for instance, would have pumped enormous quantities of water through their bodies every day, feeding on the plankton it contains. With fewer plankton sucking up the oxygen in the ocean water, more of the gas would be available to power complex organisms on the seafloor 鈥� which is where early animals lived and breathed. Put simply, Butterfield reckons the sponges could have , not the other way around.
There is, however, an elephant that may yet trample on Butterfield鈥檚 idea: the whopping uncertainty over what oxygen levels actually were in the distant past. Our reconstructions are based on ancient marine sediments and fossil soils. Analyse their chemistry and you can deduce what was going on in the atmosphere when they formed. Except they might not always be reliable. The soils, for instance, were almost certainly home to microbes that would have influenced how much oxygen was stored. The consensus is that around 800 million years ago oxygen concentration was somewhere between 1 and 20 per cent of what it is now. But some calculations suggest it was as high as 40 per cent.
In an effort to help pin things down, Lyons, together with Noah Planavsky at Yale University and Chris Reinhard at the Georgia Institute of Technology, have tried using a shiny new method involving chromium. How much of this metal is stored in rocks depends on how much oxygen was in the atmosphere when they formed. But instead of clarifying the picture, the trio have further fogged it up. Their chromium record indicates that between 1.8 billion and 800 million years ago, 鈥� lower than anyone thought possible.
鈥淚t鈥檚 a controversial dataset,鈥� admits Lyons. Yet if it contains any truth, it鈥檚 a boost for the traditional view of oxygen limiting the evolution of complex life. Few would argue that even the simplest animals could have thrived on such meagre supplies.
Even though the story isn鈥檛 yet resolved, the disagreements mask a deal of common ground. 鈥淚鈥檇 like to think there鈥檚 a shift towards a more nuanced view of the oxygen story,鈥� says Butterfield. And this nuanced view has ramifications far beyond the evolution of life on Earth. A whiff of oxygen in an exoplanet鈥檚 atmosphere was long seen as a hint that complex life is more likely to be found there. These recent results give the lie to that more than ever. So if we do find an inhabited planet one day, will it be veiled in a cloud of oxygen? Don鈥檛 hold your breath.
This article appeared in print under the headline 鈥淥ut of breath鈥�
