ANIMAL KINGDOM EVOLUTION

THE MAJOR GROUPS

The roots of the animal kingdom and other kingdoms are closely intertwined prior to the origin of multi-cellular plants and animals.  We think the earliest organisms are still represented today by the bacteria and other forms lacking a nucleus in their membrane-enclosed selves.  During this stage, perhaps the first billion years of evolution, the basic biochemistry of life evolved.  The RNA, DNA, and much of the basic materials still found in subsequent organisms evolved.

A consequence of the development of photosynthetic organisms in the world, then lacking oxygen in the atmosphere, was the production of oxygen as a toxic waste product that accumulated and changed the biosphere for the remaining time on earth.  Some of the early organisms developed the ability to utilize oxygen to oxidize organic material for their energy.  They could then remain active in the absence of light while extracting more energy from food than was possible by anaerobic process alone.  

Organisms that protected their genetic material from the oxygen with a nuclear membrane could better survive as oxygen reached higher levels.  Some developed a symbiotic relationship with other organisms.  Details of these early steps are discussed by Lynn Margulis (1981, Symbiosis in Cell Evolution, W. H. Freeman and Co., New York).  The evidence that mitochondria of our cells are a result of symbiosis is very strong; perhaps cilia are derived from flagella that also came from a similar symbiotic origin.

At this stage of evolution the Animal Kingdom or its one-celled progenitors, the Protozoa, had representatives so overlapping with plants and fungi that many biologists prefer to put them in a separate kingdom, the Protista.  These early steps were developing during the second billion years of life on earth.

By the beginning of the third billion years on earth a protozoan that could change back and forth from one with a flagellum to one with pseudopodia had evolved.  Sometime the pseudopodia would develop into a collar around the flagellum.  Eventually some of these dual potential cells stuck together and developed small colonies that eventually specialized into sponges.  The single cell with the capacity for diverse structure and a mechanism for controlling it needed a few control changes in a few different cells of the colony to provide the basic material for evolution of many of the features of all animals.

The Porifera were the first phylum of animals to develop.  They diversified into many different sponge types until one group gave rise to coral-like animals as indicated by the similarity to a Middle Devonian anthozoan (Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837.).  

Recognition of this evidence of anthozoans as the first cnidarians provides a basis for a simple continuity of phyla in the early stem of animals leading to the next phylum, the Platyhelminthes which may be considered the earliest protostomes.  A simple but unconventional view is that anthozoan polyps gave rise to jellyfish ancestral to triclad planarians.  The complexity of the simple process is why I needed to write my manuscript, Evolution Insights, to make it evident.

The sponges have less well-defined tissues than phyla that follow.   But the main mass of sponge is jelly-like with a few amoeboid cells and a tangle of collagen-like fibers and is much like loose connective tissue in our own bodies.  The jelly-like mass is mostly covered with flattened cells and is perforated by many pores leading to canals and or cavities lined with choanocytes.  Choanocytes are cells with a flagellum surrounded at the base with a collar that collects microscopic food items to nourish the sponge.  Water is passed out one or a few large openings.  Most sponges have spicules.  Spicules are mineralized (calcareous and/or siliceous), often needle-like, or three-pointed and other shapes often specific to the class of the sponge.

The protostomes included all the animals above the cnidarians until the deuterostomes evolved.  The seemingly hidden origin of deuterostomes becomes simple and clear when the role of the Pogonophora is known.  The next several blogs are expected to deal with the origin of the deuterostomes.  Then it will be time to clarify the Porifera-Cnidaria-Platyhelminthes links.  Later, the origin of mollusks and arthropods from annelids will be covered.  The foggiest portion of animal evolution, Platyhelminthes to Annelida, is obscure because the intermediate steps left neither a fossil nor living close relative to my knowledge. 

The annelids seem to be the living representatives of the most ancient animals with a true coelom, a body cavity with body wall lined with a cellular layer of flattened cells.  Organs enclosed in the coelom are also covered with a similar cellular layer; the two layers often connect to form a double layered mesentery.  The mesenteries may help keep organs in position, including blood vessels and nerves servicing them.  Of the simple animals, more complex than flatworms, but still lacking both a true coelom and segmentation (or its derivative, metamerism), although having characteristics more in common with advanced animals, we find only the nemerteans.

The protostomes including flatworms, nemerteans, annelids, mollusks, and arthropods get their name from the embryonic origin of the mouth from the blastopore.  The first (proto-) opening becomes the mouth (-stome), thus their name Protostomia.  In deuterostomes a second embryonic opening or region becomes the mouth.  The deuterostomes include hemichordates, chordates, and echinoderms.

Besides mouth origin, major contrasts between major phyla of the two groups (advanced protostomes and deuterostomes) include spiral versus radial cleavage, determinate versus indeterminate cleavage, presence or absence of chitin.  A minor phylum, the Pogonophora, blurs these and other distinctions and gives good reason to be the link between the two branches of higher animals.  To me, the evidence is so good any other proposals lack standing.  

An earlier post (SCIENCE SCREW-UP NO. 1) provides reasons the currently popular view of phylum relationships is incorrect.  Most of my immediately following posts will address various aspects of the origin of deuterostomes.  

Joseph G. Engemann, Emeritus Professor of Biology, WMU, Kalamazoo.  6/20/2013