Origins and Evolution of Animals

The First Animals
The Development of True Tissue
The Evolution of Bilateral Symmetry
Coelomates and Deauterstomes
Vertebrates
The Shift to Land
Mammals
Evolutionary Mysteries


The First Animals

The first animals originated between 1.2 billion and 670 million years ago from protist protoanimals. The cells within these protists were organized into two or three primary layers; cells in outer layers specialized in movement, while cells in inner layers specialized in the actual maintenance of the organism and formed a primitive digestive cavity. The Trichoplax adhaerens, a member of the Placazoa phylum, is the oldest and simplest known animal species; it not only gives us an idea of what the earliest animals may have looked like, but also puts in perspective how far the animal kingdom has come. Comprised of only several thousand cells, the cells of the adhaerens are only organized into two layers (think pita bread). It has no mouth, and is only three millimeters across.

Trichoplax adhaerens
Trichoplax adhaerens


The Development of True Tissue

The first landmark evolutionary split in lineage within the animal kingdom occurred between the Parazoa and the Eumatazoa. Parazoa are not always considered “true animals”, because they have no true tissues. In fact, sea sponges, the only surviving Parazoa, have so little real tissue that “a sponge can be passed through a sieve and reassemble on the other side with no damage to its overall cellular function” ( science Classification_Lab/Eukarya/Animalia/). Though sponges have no symmetry, tissue, or organs, it’s important to note that they do share some of the traits characteristic to animals; all sponges have a stage of embryonic development, and their cells are specialized and layered.

Sea Sponges, only surviving parazoa
Sea Sponges, the only surviving parazoa


The Evolution of Bilateral Symmetry

The next great divergence in the Animal Kingdom was between radially symmetric animals and bilaterally symmetric animals. In animals with radial ancestry, body parts are arranged regularly around a central axis within the body. All animals with radial ancestry preside in the phylum Cnidaria. These animals include jellyfish, corrals, and sea anemones; all of these animals have tentacles, developed tissues, and a firm but deformable hydrostatic skeleton. Cnidarians were the first animals to develop nerve cells, and in comparison to sponges, their cells interact in a more coordinated fashion and carry out specific tasks.

Animals of bilateral ancestry have a right half and left half that are mirror images of each other. Just about all other animal phyla are organized with this kind of symmetry.



Sea Anemone
Sea anemone, an example of radial symmetry

Coelomates and Deauterstomes

Animals of bilateral ancestry made another great leap in the development of body cavities, or coeloms--- fluid-filled spaces within the organism lined in unique tissues, which hold organs in place within these cavities. The first phylum to show any signs of coeloms was the roundworm. The roundworm is known as a pseudocoelom; it has body cavities, but these cavities are not fully surrounded with special tissue. Because coeloms better secure the organs within an organism, the evolution of the organ system coincided with the development of body cavities. Roundworms are also the simplest animals with a complete digestive system. Because organs were able to become more complex, coeloms gave rise to larger and more complicated animals. After the roundworm, all other animals were coelomates.

Through the course of time, coelomates diverged and began to differ widely in embryonic development, leading to major differences in body plans. This divergence eventually led to the split between protostomes, which include Arthropods, Annelids, and Mollusks, and deuterstomes. The two deuterstomes phyla are Echinodermata, and Chordata--- the phylum that encompasses fish, amphibians, reptiles, birds, and mammals.


Vertebrates

It is unclear what animal vertebrates originally evolved from. Scientists speculate that vertebrates arose from tadpole-shaped chordates, sometime in the Ordovician period. These chordates were predatory and relied on speed. Their speed was greatly increased by the adaptation of an endoskeleton, which muscles work against to initiate motion.

Other crucial adaptations coincided with and were closely related to the development of the backbone. Predatory fish developed jaws from a series of structural supports from the gill slits. Jaws opened up vast feeding opportunities, and triggered a coevolutionary arms race between predators and prey in the ocean. This coevolution favored the development of a nerve cord that expanded into the brain, and the development of sensory organs. Evolution also favored fins, the beginning of segmentation (creation of limbs) in chordates, and precedent to wings, legs and arms amongst vertebrates today. Fish also began to rely less on gills and more on lungs. Placoderms (Silurian period) are among earliest fish to exhibit jaws and fins.


The Shift to Land--- Amphibians, Reptiles, and Birds

The evolutionary shift to land occurred at the end of the Devonian period. The key ancestor of land dwelling chordates is the lobe-finned fish--- fish with crude lungs and large fins that incorporate fleshy extensions from their body. The evolution of the lobe-finned fish into amphibians most likely evolved in times of drought, when lobe-finned fish utilized their capacity to breathe oxygen and tried to waddle from pond to pond. Descendents of lobe-finned fish continued to develop lungs, as well as circulatory systems and other key adaptations to life on land.

Descendants of amphibians, reptiles were the first vertebrates to escape dependency on life in the water. Adaptations of tough, scaly skin, internal fertilization, the ability to conserve more water in their kidneys, and the development of amniote eggs five reptiles the ability to live solely on land. Lineages of small reptiles evolved into two more classes of chordates. One of them was birds, which diverged from a lineage of small reptiles that ran on two legs sometimes in the Mesozoic Period. Similar complex social behaviors and some similar physiological abilities (such as the ability to adjust body temperature) directly indicate bird ancestry.


Salamander, an amphibian Eagle, a bird
Salamander, an example of an amphibian
Eagle, an example of a bird

Mammals

The other class the evolved from reptiles was mammals. Mammals developed over 200 million years ago from therapsids, a link between small reptiles and mammals and the ancestor of mammals. Early mammals of the Jurassic period were mouse-sized, had hair and modified jaws and teeth, and had legs that were more directly under the main trunk of their body. After the extinction of the dinosaurs, mammalian carrying capacity increased, and mammals were given room to evolve. Due to convergent evolution, three dif kinds of mammals developed: pouched mammals, egg-laying mammals, and placental mammals.

Tree dwelling mammals start developing into monkeys and apes about 36 million years ago. Because of these animals perilous lives in the trees, evolution favored larger brains that could better understand motion, depth, shape, color. Adaptations of refined hand movement, and skeletal modifications promoting bipedalism also helped early monkeys survive in the trees. Primates continued to evolve. 25-5 million years ago, the first homonids evolved and spread across the globe.


Koala, a pouched mammal Chimp, a mammal
Koala, a pouched mammal
Chimpanzee, a placental mammal

Evolutionary Mysteries--- Segmentation, Echinoderms, and More

When discussing the evolution of the Animal kingdom, it is important to acknowledge that much of this evolution remains a mystery. A classic example of this is the evolution of segmentation, or the development of limbs and appendages. Among protostomes, Mollusks are not segmented, while most arthropods and various annelids are. Among deuterstomes, chordates are segmented, while the phylum Echinodermata does not contain segmented organisms. Because only some protostome phyla and only some deuterstome phyla are segmented, scientists are currently debating how the trait of segmentation occurred. Did segmentation develop before protostomes and deuterstomes diverged, and was then lost by non-segmented phyla? Or did segmentation evolve separately in the protostomes and deuterstome lines after the split occurred? Neither hypothesis has been proven.

Another mystery in animal evolution is the Echinoderms. Echinoderms are a mix of radial and bilateral features, whereas most of the other coelomate phyla are solely bilateral. Biologists are currently unable to decipher how these coelomates could share traits with jellyfish, anemones, and much less complex animals. And of course, there is the actual mystery of the origin of animals itself--- the original ancestral protist is still unknown.



Timeline


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