For 3.5 billion years the planet had been populated exclusively by bacteria. Then, suddenly, in the Cambrian there was an incredible explosion of life. In a short period of time, all forms of animal life that we know today – arthropods, molluscs and chordates – evolved or appeared . At this time the appearance of chewing, and therefore predation, which gave the trigger to an arms race that transformed the world. The first prototeeth belonged to Protohertzina , a creature very similar to the arrow worms that are part of today’s oceanic plankton. Their appearance became the earliest armor, exoskeletons , of which Maikhanella is one of the earliest examples, with their tiny cap-shaped shells constructed from clusters of spines.
Plankton began to invade the Cambrian seas, transforming the food chain of the entire planet and serving as food for the immense diversity of animals that had just appeared -and that many would end up becoming extinct-. Among the invertebrates that inhabited the seas we find beings as strange as the one that bears the description in its own name, Hallucigenia , whose surreal appearance has led paleontologists crazy since its discovery in 1911. Although the head and tail cannot be clearly distinguished , the front part is thought to have had a long neck with two or three pairs of clawless appendages, but this is unclear. It is also thought that it had a single very simple intestine that ran through its entire body. Apparently it moved along the seabed eating the remains of other dead creatures, although there are those who venture that it could have fed on the soft tissues of sponges.
Another strange creature was a flat, leaf-like being about 5 cm long and very much like a worm: Pikaia gracilens . It is not known whether it had very primitive eyes or was blind as it moved slowly through the sediment layer. With a tiny mouth and no chewing part, what makes this being interesting is that we could be facing an ancestor of vertebrates , since it seems to have a primitive backbone. Although it is not the only one: in 1999, in Changjiang, in the Chinese province of Yunnan, two vertebrate fossils were found, Myllokunmingia and Haikouichthys , with an age of 530 million years. It is believed that the former had a cartilaginous skull and although some parts of the digestive system have been preserved, this has not happened with the mouth or with any part of its tail. The second is believed to be the oldest known jawless fish (agnate).
The Cambrian explosion produced arthropods with legs and compound eyes, worms with feathery gills , and swift predators that could grab prey with their primitive teeth. Biologists have argued for decades what caused this evolutionary burst. Surely there was not a single cause, but several. Among those factors that could influence is the strong increase in atmospheric oxygen that caused the increase in photosynthetic organisms. This could facilitate the development of molecules such as collagen, a structural protein in many animal tissues. There is also a greater volcanic activity , which together with terrestrial erosion, triggered the amount of calcium dissolved in the sea and, with it, opened the door to the appearance of the first shells and exoskeletons. But there is one climatic phenomenon that seems to have played a significant role in the entire evolution of life on our planet: the phenomenon known as Snowball Earth , an event in which the entire planet froze.
Curiously, the two Snowball Earth phenomena that the planet has gone through occurred just when the two great advances in the evolution of life on Earth took place. The first, 2,200 million years ago, with the appearance of cells with a nucleus, eukaryotes, those with which the vast majority of living beings are made. The second, 550 million years ago, with the Ediacaran fauna and the great Cambrian explosion, when animals first emerged. Could it be that we, and all complex life forms, owe our existence to a totally frozen Earth?
