Oxygen is an Evolutionary Constant as well as a Constraint.

2.5 billion years ago— one of the first animal was ancient sponge appeared and they needed oxygen.

Over its 3.5 billion years, life on Earth has evolved more than a quadrillion-fold increase in size from its microscopic beginnings. How and why did body size evolve so dramatically?

According to Timothy Lyons Professor of Biogeochemistry, “We know, for example, that oxygen was likely very high a bit more than 2 billion years ago – but there were no animals,” he says. “Evolution wasn’t ready.”

According to the evidence the team collected, life has experienced two big growth spurts. For the first 1.5 billion years of its history, life stayed small — barely increasing in size from its beginnings. Then, around two billion years ago some lineages rapidly evolved to be roughly a million times larger than the largest organisms had been before.

All animals around today split from a common lineage, about 650 million years ago. A new fossil find pushes back the start of the evolution of multicellular animals. The fossil of a tiny, ancient sponge is 600 million years old, according to the paper published in Proceedings of the National Academy of Sciences, meaning it predates that explosion. That puts it in a time period earlier than many scientists expect to find sponges and could shake up what we know about the timing of animal evolution.

It is generally accepted that the first animals were ancestral sponges, although there is much debate about the early sequence of events. The Earth is nearly four billion years old and for much of that long history, life was basically just bacteria, plankton or algae—small, simple creatures. But about 570 million to 530 million years ago, something sparked and suddenly (in geological time, anyway) the complex, multicellular life we see around us started appearing during the Cambrian explosion. Sponges were a major part of that evolutionary flowering of complexity. Sponges similar to the world’s first animals can survive in water containing extremely low levels of oxygen. The finding challenges the standard view that the evolution of animals was delayed by a lack of sufficient oxygen for them to breathe – and fits with a theory that the first animals may have helped raise oxygen levels.

Daniel Mills of the University of Southern Denmark in Odense, and his colleagues, collected breadcrumb sponges (Halichondria panicea) from oxygenated waters in a Danish fjord. They kept the sponges in an aquarium and gradually removed the oxygen. Even with 200 times less oxygen than is currently found in the atmosphere, the sponges survived until the end of the study, 10 days after oxygen levels finished dropping. If modern sponges can live with little oxygen, early animals probably could too.

Hypocrisies of variation in oxygen levels on early Earth evolution. Earth’s oxygen unlocked the evolutionary potential. Many scientists consider the creatures, called rangeomorphs, some of Earth’s first complex animals, although it’s impossible to know exactly what these organisms were, the researchers said. The creatures prospered in the ocean during the late Ediacaran period, between 580 million and 541 million years ago, just before the Cambrian era. They’re often considered to be some of the first animals to evolve on Earth, but they share little in common with modern critters — they look more like plants. But according to a new study, their reproductive techniques were astonishingly complex — and familiar, too.

Scientists also noticed that there was a big delay — hundreds of millions of years — between the evolution of the key innovation and the increase in body size. What was holding life back in the interim, constraining lineages from evolving larger body sizes once they had the basic structures necessary to do it? The answer that immediately occurred to the team was oxygen. The two, big body size jumps lined up almost perfectly with times that Earth’s oxygen levels increased dramatically! The scientists suspect that these changes in Earth’s oxygen unlocked the evolutionary potential of many lineages allowing them to diversify and evolve larger body sizes. Most life on Earth needs oxygen in order to produce usable chemical energy in the cell, which is necessary for growth and reproduction — so it makes sense that oxygen levels might limit the body sizes that organisms can evolve.

Interestingly, these shifts in Earth’s atmospheric chemistry didn’t just influence evolution — they are also the indirect result of evolution. Four billion years ago, Earth’s atmosphere had little or no free oxygen. That changed when bacteria evolved the ability to photosynthesize, which releases oxygen as a byproduct. For example, the first jump in Earth’s oxygen levels may have been triggered by the proliferation of photosynthetic bacteria in the oceans. And this jump in oxygen levels may have, in turn, triggered the evolution of larger body sizes. Life is locked in a feedback loop with the chemistry of the Earth’s atmosphere. As we’ve seen from this story, the makeup of the atmosphere is powerful enough to trigger leaps in evolution, and as we know from our experience with global climate change, life is certainly powerful enough to change the makeup of Earth’s atmosphere.

Sponges vs jellies

In the evolution of animal life on Earth, sponges have long soaked up the accolade of being the most primitive creature ever to have existed. Now it seems that their position at the very base of the tree of animal life is in jeopardy, thanks to the humble comb jelly. The finding may force us to reconsider our understanding of early animal evolution.

“When you’re learning about how animals evolved it’s always been that the last common ancestor to all living animals was probably very simple and that once the animals had a neural system and muscles they would never lose them,” says Joseph Ryan at the US National Institutes of Health in Bethesda, Maryland.

According to this view, early sponges sit at the base of the animal tree of life, which then forks into four other groups: comb jellies, jellyfish, primitive multicellular animals called placazoa, and another group of early symmetrical animals that led to worms, insects, and ultimately us (bilaterians).

But recently, analyses of single genes had suggested that ctenophores could challenge sponges to the spot as the most primitive animals. “People didn’t believe it,” says Ryan, admitting that he, too, was one of the sceptics.

Jellyfish have a complex life cycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms. Whichever came first, comb jellies and jellyfish (and other Cnidarians) made an important step in evolutionary history: they are the earliest known animals to have organized tissues—their epidermis and gastrodermis—and a nervous system. They’re also the first animals known to swim using muscles instead of drifting with the whims of the waves. The oldest ancestors of modern day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago. Because jellies have no bones or other hard parts, finding jellyfish fossils is rare.

Genetic analyze this provided strong evidence that comb jellies are the most primitive of all animals – more so even than sponges.

More genetic analyses will be required to back up this conclusion. While some researchers contacted by New Scientist were enthusiastic about the latest study, Detlev Arendt of the European Molecular Biology Laboratory in Heidelberg, Germany, points out that Ryan and his colleagues themselves say there are other possible interpretations. They say alternate interpretations are less likely, but Arendt is not convinced.

By Damien Marie AtHope

References:

http://www.smithsonianmag.com/smart-news/one-oldest-known-animals-tiny-ancient-sponge-180954523/?no-ist

https://www.newscientist.com/article/dn25067-first-animals-may-have-lived-with-almost-no-oxygen/

http://www.livescience.com/51725-earliest-known-complex-reproduction.html

http://shapeoflife.org/video/sponges-origins

http://evolution.berkeley.edu/evolibrary/news/091101_oxygenconstraint

http://ocean.si.edu/jellyfish-and-comb-jellies

https://www.newscientist.com/article/dn24751-first-ever-animals-were-made-of-jelly-not-sponge