Some of the animals with basic animal bodies include arthropods such as insects, crustaceans, and trio-bites, cnidarians like jellyfish and corals, mollusks such as shellfish and squids, echinoderms like sea urchins and sea star, and chordates such as vertebrates.
The Cambrian exposition occurred during the Cambrian period that began 570 million years ago and ended 510 million years. According to Meyer et al. (2001), the Cambrian exposition occurred within a window of 20 to 40 million years during the Cambrian period. According to BBC, (2015), the Cambrian period began 545 million years ago and ended 495 million years ago. The Cambrian period was characterized by the sudden emergence of living organisms in the sea. Although geologists are lucky to identify some Cambrian fossils, there was no life on the land.
According to Darwin’s theory, the Cambrian exposition is based on the existence of changing environmental conditions that cause genetic modification of genes. The major cause of the Cambrian exposition is believed to be a rapid increase in the oxygen level in the atmosphere. An increase in oxygen level in the atmosphere occurred approximately 2.3 or 3 million years when numerous microorganisms released oxygen to the atmosphere through photosynthesis. The second oxygen increase event occurred approximately one billion years ago and resulted in the development of various complex organisms during the Cambrian period.
The link between the increase in oxygen levels and development of complex organisms is confirmed by various past events. For instance, the oxygen concentration is believed to have increased from 10 % to 21% in the past 425 million years. The event resulted in the evolution of vertebrates thus attaining essential features. Secondly, high oxygen concentration level in the atmosphere resulted in an increase in insect size (Koch & Britton, 2009).
According to Penumaka (2011), the earth was covered by glacier 580 million years ago thus