About 600 million years ago, a singular group of organisms appeared in the fossil record that was unlike anything that predated or followed its reign. Bacteria and other microorganisms appeared around 3.5 billion years ago, with algae at 1.5 billion years, and single-celled forerunners of animals following in the next billion. Why was life so small? A simple lack of oxygen in the ocean prevented large organisms from evolving. Bigger was almost never better during the Precambrian era.
About 750 million years ago, however, the single supercontinent of Rodinia began to break apart, with drastic consequences for the Earth's equilibrium. Mountain ranges were formed, ocean currents were created - and the ocean was flooded with more oxygen than it had ever contained before, in part because of multiple ice ages that froze and then melted concentrations of water. At this very beginning of macroorganism development, a series of strange and primitive organisms would arise. A few of them would be the progenitors of mollusks, jellyfish, and other modern organisms. Many of them, however, followed a path of development that placed them almost entirely outside the realm of macroorganic life on Earth as we know it, because of the bizarre set of circumstances under which they existed.
Predators, in the realm of the early Cambrian before the Tommotian and main Cambrian "explosions" of life, simply did not exist - but there was now an oxygen-rich environment that could support more complex systems of life. Thus, organisms could economically begin to "spread out" both individually and in the sense of a biosphere, developing larger as well as more advanced and specialized forms that would interact with their microbiotic fellow-travelers. For the first time, larger organisms would outnumber smaller animals. Mollusks and worms existed, but dwelt in the shadow of creatures such as Phyllozoon.
Phyllozoon has absolutely no modern equivalents that paleobiologists can identify. A foot-long organism divided into zig-zagging segments, this strange creature probably resembled a wide, translucent belt - whether it moved freely in the water, or was anchored to the sea floor, is debatable. Some theorize that it contained algae or bacterial colonies within its chambers, using sunlight or chemicals absorbed from the water to fuel the creatures inside it as a self-contained power plant. One thing is for certain: no organism, from the Late Cambrian on, can be related to Phyllozoon. It is as alien to Earth's current life as a creature from another, albeit similar, planet.
The species Phyllozoon is not alone; it belongs to a group of creatures known as Vendobionts that existed only within the Early Cambrian. Classified by some as an extinct phylum of Kingdom Animalia, by others as related to photosynthetic algae, and by the inventor of the term, German paleontologist Dolf Seilacher, as completely separate from defined kingdoms, Vendobionts ruled the world from 600 to 540 million years ago. Individual species are known only from fossilized impressions left in ancient rock in areas ranging from southern Africa to New Zealand where fossils are laid down in quick fashion. Their greatest concentration, however, can be found in the silver mines and hills of Ediacara (Ee-dee-AK-ara) in Australia, and it is for this location that their era is named.
When the fossil beds of this area were first studied in the 1940s, scientists proclaimed that they had found the roots of all modern life, identifying the remains as belonging exclusively to complex worms, crustaceans, jellyfish, and other organisms with contemporary descendants. The segmented Spriggina, named for Australian geologist Reg Sprigg, was placed on the family tree of the common earthworm, along with the similarly constructed but much larger Dickinsonia. Other remains were classified in the same fashion, and paleontologists presented the Ediacaran fossils as the budding that led to the "Cambrian explosion" in the fossil record that was, at the time, seen as the mysterious origin of all complex life on the planet.
This story was tidy, but could not exist for long under direct scrutiny. Seilacher, examining the fossils for years, announced in 1983 his conclusions: most of the Ediacaran fossils - what he called the "Vendobionts" - had nothing to do with modern animals. Even the simplest sponge, Seilacher pointed out, has simple organs: a mouthlike part specialized for ingesting food and excreting waste. The Vendobionts, on the other hand, seem to have had almost no body-area specialization at all when they were compartmentalized, which many were not. For animals their size, they were more than merely unusual in this surprising lack of organs: they were unique, and probably not related to modern life at all.
The Ediacaran fossils, now viewed in this light, become a new sort of find: a snapshot that mixes our own ancestors with their alien cousins. Storm surges in shallow waters, volcanic ash, and other natural phenomena buried ancient animals and bizarre biological artifacts alike. Kimberella, a small and mobile bottom-feeder, left trails that link it with modern sea snails, alongside clusters of the cylindrical and stationary Vendobiont Ernietta. Worm-like Spriggina crawled on the same rocks where the large but very simplistic Rangaea was fixed, its feathery extensions probably doing little more than waving in the currents and absorbing whatever algae and bacteria flowed through its body. Big, immobile, and entirely defenseless creatures were the order of the day.
With the Cambrian "explosions" came change. Mobile worms with sophisticated nervous systems chased and attacked their food on the ocean floor. Hard shells developed to fend off this new predator class, and from there, primitive claws evolved to pick apart stubborn prey. The Vendobionts, as simple as they were large and unable to compete with the complexity of animal survival mechanisms, died out almost as quickly as they arose. Scientists still debate the exact ties between the Ediacaran species and living animals, but some see the Vendobionts as the window as to how life might have developed had the early billennia of an oxygen-rich environment occurred under different circumstances. They may, some say, be the closest we have to a good view of possible extraterrestrial life.
National Geographic, Vol. 193, No. 4. April 1998. "Life Grows Up", p 100.
The series of excellent writeups contained at Cambrian Explosion.