The Cambrian Explosion of life or Biological Big Bang

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The Cambrian explosion (also know as biological big bang or Cambrian radiation ) was a period of rapid diversification and speciation of animals that lasted for about 20 million years. Before the Cambrian, life was relatively simple and uniform. There were only a few types of multicellular animals, such as sponges, jellyfish, and worms. These animals lacked hard parts, such as shells, bones, or teeth, that could be preserved as fossils.

But something changed in the early Cambrian. Suddenly, a plenty of new forms of life appeared, with different shapes, sizes, and structures. Some of these animals had hard parts, such as exoskeletons or internal skeletons. Some of these animals had eyes, mouths, limbs, and other features that enabled them to move, sense, and interact with their environment. Among these organisms, there were still filter-feeding creatures. Some organisms crawled or swam on the seafloor to seek bacteria or seaweed actively. There were also some powerful predators that preyed on other animals. These animals were the ancestors of almost all modern and extinct animals. There were also some peculiar organisms that appeared to belong to isolated branches of the evolutionary tree and couldn't be classified into existing categories.

Several representative Cambrian organisms: trilobites, Abnormal shrimps, brachiopods

One of the most iconic groups of animals that emerged in the Cambrian were the trilobites. Trilobites were arthropods, and relative to insects, spiders, and crustaceans. They had segmented bodies with three lobes (hence their name), a pair of antennae, and multiple pairs of legs. They also had compound eyes that could detect light and movement. They ranged in size from a few millimeters to over half a meter in length. Their backs and heads are covered with hard armours to protect them from predators. They can swim in the water, and can crawl or dig at the bottom of shoals. They are mainly found on the bottom of the shallow ocean or in coral reefs where they live on algae, bacteria, protist or other animal carcasses. They survived for more than 250 million years, until they went extinct at the end of the Permian period.

Anomalocaris, which means "abnormal shrimp", were similar to modern shrimp, but had strange feature. Abnormal shrimps had a pair of formidable grasping appendages in front of their mouth. The serrated teeth encircle the circular mouth and the top of these teeth can't come into contact with each other, leaving a gap in the center of the mouth. They can tear the hard exoskeleton of prey, and scientists found crushed shells in their fossils. Their head and segmented body were covered by flexible carapace, but not abdomen. The strong flexible body, fan-shaped tail and large lateral lobes on the body made them active swimmers. The compound eyes with thousands of lenses gave them excellent vision. They were over a meter long and were the largest marine creatures of that time. These ruthless hunters captured and devoured prey such as trilobites and other arthropods. They also fed on soft-bodied animals such as worms and jellyfish. abnormal shrimps were the earliest apex predators and dominators of the Cambrian oceans, and no other creature could challenge their throne.

Another group of animals that flourished in the Cambrian were the brachiopods. They were similar to modern shellfish, but there were some differences between them. The modern shellfish can move actively in the seafloor and have two shells of the same size. However, brachiopods soft body were enclosed by two shells of different size. They fixed themselves to the seafloor by a muscular stalk and filtered food particles from the water. Brachiopods were the second most diverse and abundant animal in Cambrian, with thousands of species known from the fossil record. They are still alive today, but they are much less common than they were in the past. They were already defeated by modern shellfish a long time ago.

What caused this remarkable explosion of life in the Cambrian?

This is one of the most intriguing and debated questions in paleontology. There are many hypotheses and factors that may have contributed to this event.

The fossil gap hypothesis

This hypothesis suggests that there was no real explosion of life in the Cambrian, but rather a fossil explosion. According to this idea, before the Cambrian, most multicellular animals were soft-bodied and rarely leave fossils. In contrast, many Cambrian animals had hard parts that could fossilize more easily. Therefore, the sudden appearance of new forms of life is due to a break in the fossil record, rather than a true evolutionary event.

Increased oxygen and the predation hypothesis

This hypothesis proposes that the emergence of predators in the Cambrian triggered an evolutionary arms race among animals. According to this idea, before the Cambrian, the ocean is an ecosystem made up of simple primary producers like blue green algae, and most predators were passive filter feeders that lacked the competition or predation pressure. However, in the Cambrian, prokaryote-feeding animals began to actively search and capture food. They quickly emptied the algae and bacteria that had previously occupied the ecosystem and made room for other organisms to evolve. Others evolved into more powerful predators to prey on the prokaryote-feeding animals. They evolved features such as jaws, teeth, claws that enabled them to hunt and kill other animals. This increased the selective pressure on prey animals to evolve defenses such as shells, armor, camouflage and speed. This resulted in a feedback loop of coevolution and diversification among predators and prey. The increase in oxygen levels also accelerated this process during the Cambrian period.

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