National Geographic : 2018 Mar
When liFe got coMplicated 93 element that fuels animal metabolism. Thanks to marine bacteria that generated oxygen as a prod- uct of photosynthesis, levels of the gas rose about two billion years ago but stayed relatively low for another billion years. Then, between 717 million and 635 million years ago, a series of glaciations took place, so widespread and severe that they may have frozen over the entire planet, a situ- ation some scientists call a “snowball Earth.” During that time oxygen levels bumped up again, for reasons that are still poorly understood. next? Did the end of the glaciers, an increase in available oxygen, and the evolution of more com- plex cells allow the Ediacarans to blossom, like the first crocuses of springtime? Maybe. Equally enigmatic is their relationship to life today. One eminent German paleontologist, Ad- olf Seilacher, assigned them to a kingdom all their own, distinct from the animal kingdom, because of what he called their “unique, quilted type of biological construction,” so different from most multicellular animals. The “quilted” effect seemed to offer structural stability that might have compensated for the absence of a skeleton. Maybe the quilting, and the frondy shapes, also helped maximize surface area, so they could bet- ter absorb nutrients through their skin. Nutrition would have been problematic for the Ediacarans because, so far as fossil evidence re- veals, almost none of them had a mouth. They had no gut, no anus. No head, no eyes, no tail. In some cases there was a sort of anchoring knob or disk at one end, now known as a holdfast, which gripped the sea bottom and allowed the frond to waft upward in the water. Many sea-bottom areas at that time were coated with thick microbial mats, which helped stabilize the sediments like a layer of crusty soil. But the frond wasn’t a plant—photo- synthesis couldn’t have nourished it—because many Ediacarans lived in the depths, thousands of feet underwater, where light didn’t penetrate. If they couldn’t eat and they couldn’t photo- synthesize, how did they nourish themselves? One form, a sluglike thing called Kimberella, may have scratched up and swallowed (this one did have a mouth, major advantage!) sustenance from the microbial mats beneath it. But the leading hypothesis for most Ediacarans is os- motrophy, a fancy word for a very basic process: the uptake of dissolved nutrients by osmosis, or absorption through their outer membrane. It was good enough, maybe, in a simpler world at a simpler time, but it would have been meager sus- tenance. Some scientists have focused on anoth- er fascinating aspect of many Ediacarans: their finer architecture. At a glance they look quilted, but close inspection reveals that their struc- ture is fractal. That is, similar patterns repeat ROMIP SPECIMEN 36502 The great freeze ended as volcanic eruptions spewed carbon dioxide into the atmosphere, creating an early greenhouse effect that warmed the planet and thawed the oceans. Another brief glaciation around 580 million years ago, known as the Gaskiers, may not have been global, but it put Newfoundland, among other places, in a deep freeze. These changes all preceded the earliest appearance of Ediacarans in the fossil record. Were they the causes of what happened EDIACARAN The first large, biologically complex organisms appear in the fossil record some 570 million years ago, even before the Cambrian explo- sion, during a mysterious period called the Ediacaran. Known only from impressions their soft bodies left in mud or ash they were buried in, Ediacaran organisms like this Fractofusus misrai from Newfoundland are unrelated to and unlike any animal alive today. Fractofusus’s body plan, composed of ever smaller repeat- ing branches, dramatically increased its body surface area, the better to feed by absorbing nutrients directly from seawater.