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Do We Really Need Oxygen For Complex Life?

Sea sponges can survive on astonishingly small amounts of oxygen. The discovery raises questions as to whether the first complex lifeforms could do the same, and if so why they didn’t evolve earlier. “The sponges were able to both respire and feed to oxygen levels cycling between 0.5% and 4% PAL [Present Atmospheric Levels],” Mills reports in Proceedings of the National Academy of Sciences. In one experiment a sponge survived and even appeared to grow for 24 days in such conditions, although four hours of complete oxygen loss caused cell death in another sponge. Life appeared on Earth within a few hundred million years, but for billions of years it was restricted to single celled organisms. When complex life forms emerged the timing was shortly (geologically speaking) after the dramatic rise in oxygen levels in the atmosphere (shortly followed by the oceans) 630-635 million years ago. The conclusion was obvious: the rise in oxygen was necessary to fuel the complex organisms that eventually became us. The connection has been challenged before.  Last year researchers at the University of Southern Denmark produced evidence that 2.1 billion years ago atmospheric oxygen was similar to that when the complex life forms appeared, raising questions as to why, if so, we can’t find evidence of advanced life forms from that time.
Now Daniel Mills, a PhD student at the same university has gone further. He tested how little oxygen was needed by sea sponges, considered some of the closest surviving relatives of the first multicellular creatures. There have been reports of sponges living in low oxygen waters near under sea volcanoes, but Mills didn’t need to go to such exotic locations – he plucked Halichondria panicea from Kerteminde Fjord, just meters from the university’s Biological Research Center and raised them in seawater from the fjord with both normal and drastically reduced oxygen levels. “Therefore, it is possible that the oxygen content of the atmosphere was completely permissive to the origin and early evolution of sponge-grade metazoans well before their evolutionary first appearance,” the paper notes. If this is the case, the big question is why complex lifeforms didn’t appear earlier – how much more evolved might we be with an extra few hundred million years headstart? Mills has yet to test the possibility that sponges need more oxygen to make it out of their larval stage. However, should the sponges survive this challenge attention will shift to questions of what else could have stopped complex life appearing earlier. The paper speculates that greater turbidity in the early oceans meant conditions in shallow waters often became completely oxygen depleted. Alternatively, Mills suggests,“Maybe life remained microbial for so long because it took a while to develop the biological machinery required to construct an animal. Perhaps the ancient Earth lacked animals because complex, many-celled bodies are simply hard to evolve.” Just don’t tell the Breatharians – they’ll probably decide that oxygen is as unnecessary as they consider food.
Wallabies lack other marsupials’ capacity to distinguish colors, but this doesn’t stop the members of the kangaroo family getting hooked on color-based computer games. Puzzles remain, however. The gene for the third photoreceptor in other marsupials has not been found, leading Ebeling to speculate that some other function has been doubled up to detect light at a third wavelength. Moreover, it is unclear how wallabies could have lost this receptor while quokkas kept it. There are rumors of dichromacy in certain possums and Ebeling is keen to get funding to study a variety of species. “I’d really like to test koalas,” she says, “But you can’t find an incentive to get them to play.” Instead she would like to enroll some wombats in her program, but agrees she may need a more sturdy machine. Ebeling, then based at the Australian National University, set out to conclusively test the wallaby’s eyesight. She reports in PloS ONE showing them differing colored lights, and training them to push the button lit up to be the most similar color in return for a food pellet. Pressing the wrong button led to the wallaby being locked out of the game for a period.
Reptiles and birds have four different color sensors in their eyes. Some invertebrates go even further, but most placental mammals make do with just two, probably because the ancestral species were nocturnal. Humans can thank our primate ancestors for (most of us) being able to see the color range we can. Somewhere along the line a third cone type was added to the retina, making us trichromatic. “For fruit eaters the ability to spot a red apple against green foliage, rather than get stomach ache from eating an unripe fruit, is a big advantage,”says Curtin University’s Dr Wiebke Ebeling. So what about marsupials? Having branched off the evolutionary tree from placental mammals early in the piece scientists thought they might have retained extra color sensors. Past research found that honey possums and fat-tailed dunnarts are trichormatic. Quokkas (tiny kangaroos too cute to be believed) also show signs of trichromacy. However, in 2010 Ebeling produced evidence tamar wallabies might have only two color sensors (dichromatic). Tammar wallabies are quite closely related to quokkas, while being larger and almost as cute. They also make excellent study animals, adapting well to captivity.
“The most remarkable result was the determination of the ‘Neutral Point’ which describes a single color that to wallabies looks identical to white, where the animals cannot make up their mind which switch to choose,” Ebeling says. “In the case of wallabies, this was a shade of cyan (greenish blue).” Neutral Points are restricted to dichromatic species. Ebeling also learned a lot about wallaby behavior. Although the buttons could be pressed with paws her subjects preferred to use their noses. The wallabies also got so competitive they often wouldn’t stop to eat the food reward – researchers would arrive in the morning to find the nocturnal animals with a tray full of food pellets from playing all night.  However, one wallaby had to be booted out of the program when she began pressing the buttons at random. Investigations revealed she had recently become pregnant. “We didn’t intend this,” Ebeling explained, but keeping the males and females apart was insufficient. Wallabies, like most kangaroos, can put their embryos in suspended animation, or embryonic diapause and restart the pregnancy when they are good and ready. Perhaps an abundance of food pellets inspired the wallaby it was time to get into the pregnancy proper.
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