When Life Got Big, or, a Forgotten Architecture of Life
Several times in our planet’s history, all or most of the Earth’s landmasses have been gathered together into one supercontinent. Most well-known is Pangaea, home of dinosaurs. Supercontinents form and disperse cyclically in geological time. 750 million years ago, an earlier supercontinent, Rodinia, broke up. Following this breakup came global climate change, in the form of the Sturtian (725-710 million years ago), the Marinoan (635-600 mya.) and the Gaskiers (ca. 580 mya) ice ages. The latter two were perhaps the most severe ice ages the world has ever seen, with glaciers at sea level within ten degrees of the equator. Some have termed that era the Snowball Earth, although it is likely that there were oases of liquid water in which life could survive.
Up until this point, microbial life, and especially bacteria had dominated the world. Following this period came the so-called Cambrian explosion, a period where most of the animal types that survive to this day rather suddenly (in geological terms) start appearing in the fossil record. This is when things that somewhat resemble macroscopic life as we know it begin to appear.
But the Cambrian starts around 542 million years ago. In between the end of the Gaskiers and the start of the Cambrian was a period called the Ediacara, and there exist an intriguing class of fossils from this era, the Ediacara biota, represented above by Dickinsonia costata. Although multicellular algae had existed for quite some time before, the Ediacara biota represent the first time complex, macroscopic life measured in centimeters and meters evolved on Earth. They were soft-bodied, mostly immobile creatures luckily preserved in sediment, by microbial mats or under beds of volcanic ashes. Fossils have been found on five continents. And these aren’t your ordinary fossils: they’re utterly weird, some of them almost resembling familiar life forms, others “fractal constructions unknown in our modern world.”
The climate changes following the breakup of Rodinia put extra pressure on life to evolve new forms. What evolution came up with was the marine life that evolved into the animal kingdom we know today. When the Ediacaran fossils were first discovered, they were held to be early examples of extant phyla, many of them probably a kind of “prehistoric jellyfish”. But in recent decades, this view has been challenged: the Ediacara have proven much more difficult to put into the existing hierarchies of life. Scientists no longer think all the fossils from this period fit under one umbrella; rather, each specimen is up for grabs.
Some of them are now thought to represent the dawn of animal life, related to sponges and cnidaria (corals, jellyfish, sea anemones, etc.). But we are close in time here to the hypothesized common ancestor of all animal life, and some of the Ediacara may not be rightfully classified as animals at all. The most intriguing of them may actually represent “failed evolutionary experiments”—branches of life that evolved separately from both the prokaryotes and the ancestors of today’s animal and plant life, but that eventually died out:
Perhaps the best example of an extinct high-level taxonomic group is that of the rangeomorphs, a group of colonial organisms that exhibited a modular construction of similar, highly fractal elements. These elements were combined as modules to construct frond-, spindle-, comb-, or bush-shaped colonies that filled most niches in the Mistaken Point ecosystem. Rangeomorph communities are most similar to those of modern, suspension-feeding animals, but it is difficult to relate rangeomorph morphology to any modern animals, and they appear to represent a “forgotten” architecture and construction that may represent an extinct phylum-level stem-group near the base of animal evolution. Rangeomorphs characterized the early stages of Ediacaran evolution, perhaps because their fractal growth and modular construction required less genetic complexity than was required by other animal phyla. Rangeomorphs were unable to compete with later, more highly evolved animals, and occur only rarely in younger Ediacaran assemblages and are not known from any Cambrian or younger assemblages including fossil Lagerstätten.
Right on the border between the Ediacaran and Cambrian periods, around 542 million years ago, the Ediacaran fossils disappear. Vanished from the fossil record. Perhaps they all died out due to some ecological disaster, possibly a temporary drop in oxygen levels, leaving a gaping ecological hole for the conventional animals of the Cambrian explosion to fill. Perhaps the conditions that preserved these proto-animals (which did not have skeletons) disappeared, and the creatures lived on for some time unrecorded in the fossil record. Possibly the most likely explanation is that with the Cambrian explosion, predators started appearing, and primitive, immobile and defenseless creatures such as the Ediacara biota may have been easy targets, driven to extinction by the evolutionary cousins that would replace them.