After death, their tiny bodies would have sunk to the bottom, or even stayed in place in water salty or sediment filled enough, and in rotting would have taken even more of the precious few oxygen molecules being produced by the thin layer of oxygen-producing microbes in the surface layer above them. Precious oxygen molecules, destined for the atmosphere and clear oceans, were used up instead in the rotting of a purple demon.
While rare on Earth now, this same stratified system still exists in a few places. One of the most famous is in the Micronesian island ofPalau, in the famous “jellyfish” lakes. Here, large freshwater lakes are filled to bursting with enormous, abundant jellyfish, swimming gracefully through aquamarine and well-oxygenated water. Yet some tens of feet below this crystal lens of clean, oxygen-life-filled water rests a second and deeper stratum, which is dark, and to us creatures of light and oxygen, vile to the extreme. It has little or no oxygen, but is saturated with hydrogen sulfide. And it is dark purple in color, stained by untold numbers of the same purple sulfur bacteria that kept the world unsafe and unavailable for anything needing abundant oxygen for what was to them probably not boring at all.
Our revised model for oxygen in the air and sea.
The purple sulfur bacteria and their world needs were finally sent to dank, poisonous back rooms of our world. But they were always there, always ready to take back the world they lost when oxygen finally broke through to higher levels, some 600 million years ago. They can be thought of as the evil empire. And in the Devonian, Permian, Triassic, Jurassic, and middle Cretaceous, this empire struck back, as we will see in subsequent chapters.
Eventually, the balance of sulfur photosynthesizers to oxygen producers changed in oxygen’s favor, possibly triggered by a gradual increase in the area of subaerially exposed continents. Iron eroding from the continents and washing down into the ocean would react rapidly with the sulfur, precipitating it into a heavy, sinking solid massof pyrite, keeping it out of the system. This loss would have starved the sulfur bacteria of the one element they could not do without. In addition, continental weathering and erosion generates clay minerals, which bind strongly to organic molecules and bury them in the sediments. If an atom of organic carbon is buried before something can eat it, the molecule of oxygen that was produced when it formed hangs around in the environment, raising the oxygen level and destroying H 2 S. Prompted by two snowball Earth events, each of which seemed to jack up oxygen levels by the postsnowball algal bloom, the environment reached a tipping point of some kind. After the last event, 635 million years ago, the first traces of big animals appeared. It did not take so long to evolve them after all, once hell on Earth was banished.
THE BIZARRE, FIRST MULTICELLULAR CREATURES
Most life during the now not-so-boring billion was composed of the long-running champions of life on Earth, the longest-running show of all—stromatolites. Microbes still held sway, just as they had from their first appearance on Earth. But appearing about 2.2 billion years ago a strange new kind of life form appeared. It looks like a thin black spiral, but is certainly not microscopic. Its name is Grypania , and its appearance demonstrates that life had made an important advance: the ability to live as “colonies” of cells, held together and bound by membranes. These were the first multicellular organisms.
Grypania has long been known. But in 2010 a strange series of fossils from Gabon, Africa, changed our view of things. 3 While Grypania might be a colony of prokaryotes (in this case probably bacteria), the new fossils, still unnamed, look too large and too complicated. Whatever they were, we know what they were not . They certainly were not the first animals.
The first true animals are much younger than Grypania and
While rare on Earth now, this same stratified system still exists in a few places. One of the most famous is in the Micronesian island ofPalau, in the famous “jellyfish” lakes. Here, large freshwater lakes are filled to bursting with enormous, abundant jellyfish, swimming gracefully through aquamarine and well-oxygenated water. Yet some tens of feet below this crystal lens of clean, oxygen-life-filled water rests a second and deeper stratum, which is dark, and to us creatures of light and oxygen, vile to the extreme. It has little or no oxygen, but is saturated with hydrogen sulfide. And it is dark purple in color, stained by untold numbers of the same purple sulfur bacteria that kept the world unsafe and unavailable for anything needing abundant oxygen for what was to them probably not boring at all.
Our revised model for oxygen in the air and sea.
The purple sulfur bacteria and their world needs were finally sent to dank, poisonous back rooms of our world. But they were always there, always ready to take back the world they lost when oxygen finally broke through to higher levels, some 600 million years ago. They can be thought of as the evil empire. And in the Devonian, Permian, Triassic, Jurassic, and middle Cretaceous, this empire struck back, as we will see in subsequent chapters.
Eventually, the balance of sulfur photosynthesizers to oxygen producers changed in oxygen’s favor, possibly triggered by a gradual increase in the area of subaerially exposed continents. Iron eroding from the continents and washing down into the ocean would react rapidly with the sulfur, precipitating it into a heavy, sinking solid massof pyrite, keeping it out of the system. This loss would have starved the sulfur bacteria of the one element they could not do without. In addition, continental weathering and erosion generates clay minerals, which bind strongly to organic molecules and bury them in the sediments. If an atom of organic carbon is buried before something can eat it, the molecule of oxygen that was produced when it formed hangs around in the environment, raising the oxygen level and destroying H 2 S. Prompted by two snowball Earth events, each of which seemed to jack up oxygen levels by the postsnowball algal bloom, the environment reached a tipping point of some kind. After the last event, 635 million years ago, the first traces of big animals appeared. It did not take so long to evolve them after all, once hell on Earth was banished.
THE BIZARRE, FIRST MULTICELLULAR CREATURES
Most life during the now not-so-boring billion was composed of the long-running champions of life on Earth, the longest-running show of all—stromatolites. Microbes still held sway, just as they had from their first appearance on Earth. But appearing about 2.2 billion years ago a strange new kind of life form appeared. It looks like a thin black spiral, but is certainly not microscopic. Its name is Grypania , and its appearance demonstrates that life had made an important advance: the ability to live as “colonies” of cells, held together and bound by membranes. These were the first multicellular organisms.
Grypania has long been known. But in 2010 a strange series of fossils from Gabon, Africa, changed our view of things. 3 While Grypania might be a colony of prokaryotes (in this case probably bacteria), the new fossils, still unnamed, look too large and too complicated. Whatever they were, we know what they were not . They certainly were not the first animals.
The first true animals are much younger than Grypania and
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