Earth Science: Geological Eras – the Evolution of Animals
Animals—including human beings—trace their origins back to the oceans of the Cambrian period more than half a billion years ago when the first multicellular organisms that fed exclusively on other living things appeared.
An animal is a multicellular organism that cannot produce its own food molecules through photosynthesis (p. 181) as plants do. but instead it gains energy by feeding on other living things. This development in the Cambrian period opened up new resources and opportunities, leading to today’s great diversity of animal species, which share typical characteristics such as sexual reproduction, a nervous system, and muscle tissue.
The basic physical structures of modern animals were evident 500 million years ago. Physical, embryonic, and genetic traits are all considered in order to trace their evolutionary tree. The history of animal species contains four major milestones. The first crucial step was the appearance of “true” body tissue-previously animals had only simple, saclike bodies, such as the porous sponges.
The development of muscles and connective tissue was a prerequisite for specialized functions that would evolve over time, such as directional movement and breathing.
The second milestone was the development of bilateral symmetry (two halves of the body mirror each other) with an identifiable head, in contrast to radial symmetry (symmetrical in a ray pattern from the center), for example, jellyfish. This is the quality that enabled early animals to move purposefully in certain directions.
The third stage was the development of a fluid-filled cavity between the internal organs and the body wall, which is found in all “higher animals” except flatworms. This allowed the internal organs to function irrespective of the motion of the body as a whole. The fourth milestone was in the embryonic stage of development.
Vertebrates and echinoderms began to develop differentiated mouth structures from secondary openings off the blastopore (an initial opening in the embryonic cavity). This distinguishes them from mollusks and arthropods, in which this first opening becomes the animal’s mouth.
The transition from water to land
The ancestor of all tetrapods (four-limbed animals) was probably a lobe-finned fish that was similar to coelacanths living today off the Comoro Island’s coast. Its muscular pectoral and pelvic fins, supported by a bony skeleton, enabled it to crawl to shore.
Yet, without developed lungs and the ability to keep its skin moist, it had to return frequently to the water. Moving onto land may have been a survival strategy resulting from the need to abandon one shrinking body of water for another. Thus, individuals able to travel greater distances over land had a better chance of survival.
BASICS
ANIMALS INHABIT nearly every environment on Earth, although many continue to live in water Mammals, birds, amphibians and reptiles, along with insects and arachnids, have permanently conquered the land