Cambrian Explosion

Cambrian organisms

The Cambrian explosion, also known as the Cambrian radiation, is the name given to the rapid, almost simultaneous first appearance of most modern animal phyla in a very short geological time interval at the beginning of the Cambrian period. It began around 538.8 Ma (million years ago) and lasted around 5-10 million years. The basic anatomical characteristics of modern animals first appear in many organisms known from the Terreneuvian sediments. Significantly fewer fossils are known from the geological times before the Cambrian than from the periods after it, so the entire period from the formation of the Earth to the Cambrian explosion 538.8 million years ago is called the Precambrian. The mass appearance of various types of animals from this moment makes it possible to mark boundaries between geological units with greater accuracy and more easily determine their age.

Before the Cambrian explosion

The first multicellular organisms appeared about 2.1 billion years ago and for a long time were clonal organisms. Proterocladus, possibly the earliest green algae, existed around 800 Ma. The time of the emergence of multicellular animals is still unknown. In rocks about 700 million years old, traces of the presence of sponges have been found, which are probably the oldest multicellular animals. Attached and free-living animals gradually evolved over the next 100 million years. A relatively diverse fauna is already known from the deposits of the Ediacaran period, including large species, but of primitive anatomy. Many of those animals cannot be confidently assigned to any modern phyla, although some researchers suggest that jellyfish, mollusks and primitive arthropods could already exist prior to the Cambrian. Traces of damage on some Precambrian animals suggest that the first predators appeared then.

Since Precambrian rocks are rare and usually highly deformed, the fauna of that time is poorly known, and some authors suggest that another biological radiation occurred in the Proterozoic, that produced the Ediacaran biota.

Cambrian fossils

Wiwaxia

Anomalocaris

Opabinia catches a Hallucigenia

The beginning of the Cambrian is associated with the appearance of a variety of burrowing animals and small (millimeter-sized) shelled organisms. A little later, the appearance of larger species began, among which modern echinoderms, worms, mollusks, arthropods and primitive chordates can be recognized. Many new forms of life differ from earlier species in the presence of specialized external organs, of which the most important were limbs for crawling and gathering food, antennae for touch, and eyes, which made it possible to better explore the environment and notice prey or predators from afar. Carnivores appeared among both bottom-dwelling and swimming species. Even then, some animals (especially molluscs and arthropods) acquired strong outer coverings that protected them from blows and bites. Some, like Wiwaxia and Hallucigenia, could defend themselves with spines. When attacking, predators like Anomalocaris and Opabinia used prototype claws and the first teeth to tear and crush the body of their prey. In many bilateral species, the oral opening was located on the underside of the anterior edge of the body, and numerous lateral appendages still remained as archaic features.

Burgessia

Among the various Cambrian groups, trilobites stand out and remained the most numerous arthropods for the next 80 million years. The first trilobites are considered markers of the still unnamed second Cambrian epoch. Invertebrate graptolites and conodonts also appeared in the Cambrian. Fossilized footprints are a sign that some animals (probably Burgessia-like or Emeraldella-like arthropods) already walked on land in the Cambrian, although the first terrestrial species appeared much later, when the amount of breathable oxygen in the atmosphere increased. About 520 million years ago, the first vertebrates, Haikouichthys and Myllokunmingia, appeared among chordates. However, for a long time they remained small detritivores.

Already in the Cambrian, noticeable extinctions of early species occurred, although in general they did not affect global evolution. The combination of various evolutionary processes of the Cambrian period gradually led to the following and even bigger biological radiation, known as the Great Ordovician Biodiversification Event.

Causes

There are theories according to which modern types of animals appeared before the Cambrian, but consisted of soft tissues or were small and therefore were not found. The Cambrian explosion occurred billions of years after the origin of life, and researchers are still debating the reasons and a late time of its beginning. Various authors offer two most common explanations.

The first possible reason is the appearance of predators. The biological race for survival that followed this event continues to this day and is the main evolutionary mechanism. Some animals began to adapt for attack, acquiring sharp teeth and claws, while their victims developed defensive features, including shells, spines and camouflage coloring. Many new ecological niches were probably occupied in attempts to escape predators as well as the first attempts to adapt to life on land.

The second reason is related to conditions on the planet, which for a long time prevented the evolution of multicellular life. Among the limiting factors the high temperatures, peculiarities of the chemical composition of the seas as well as great glaciations can be named. A likely favorable factor for evolution was a decrease in tectonic activity and a concomitant decrease in ocean temperature.