Animals are multicellular organisms that acquire nutrients from the environment by engulfing and digesting food.
The establishment of the multicellular animal phyla and the conserved features of their respective bodyplans was achieved within a relatively brief window of geological time between the Ediacarian (635-542 mya) and the Cambrian (542-488 mya).
The emergence of animals occurs relatively late in the history of the planet´s development - occurring approximately three billion years after unicellular life first originated - and the entire diversity of the animal kingdom has occured within the last one-sixth of life´s evolutionary history.
Interestingly, multicellularity has evolved many times during the earths geological history but seldom does a shift in the unit of natural selection occur as it did for plants, animals, and fungi.
Indeed the boundary between colonial multi-cellularity and eumetazoan is the starting point for understanding the initial physiological organization and subsequent evolutionary principles underlying the emergence and diversification of animals.
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The modern assemblage of animals was preceded by and subsequently displaced the Edicarian fauna which was the first generation of animal multicellularity - an evolutionary cul-de-sac that would cease to exist as an assemblage by the beginning of the Cambrian and who´s last putative representatives would disappear well before the Cambrian came to a close.
Recent research indicates that ecological factors played an important part in the transition to multicellularity.
The origin of metazoans is preceeded by several global glacations and shifts in atmospheric and ocean oxygen levels.
It is thought that these and other events may have provided the necessary pressures to result in a shift in the unit of natural selection - form the selection of individuals within a colony; to selection of the colony as an individual.
For much of the twentieth century, the Burgess Shale (505 mya) served as one of the chief sources for fossils from the Cambrian period (, , ).
Discovered by Charles Doolittle Walcott in 1909, the Burgess Shale is a Cambrian black shale deposit located in Yoho National Park in British Columbia containing exquisitely preserved remains (Konservat-Lagerstätte) of some of the earliest animals.
The Burgess Shale contains representative fossils from most major extant phyla as well as some representatives of phyla that no longer exist today.
Recently several extensive fossil beds have been discovered in China that shed new and improved light upon the Cambrian period.
The Maotianshan Shale of Chengjiang (525 to 520 mya) and the Kaili Formation (513 to 501 mya) in the Guizhou Province of China have yield excellent soft tissue specimens of many early Cambrian animals.
The Maotianshan Shale is between 10 and 20 million years older than the Burgess Shale improving our resolution of the early Cambrian period.
These impressive fossil finds serve as an excellent starting point for trying to reconstruct the evolutionary pathway leading from the establishment of animals to the emergence of vertebrates and humans, but before we proceed it is necessary to touch upon a few preliminary points so as to properly orient ourselves to the task at hand.
Leo W. Buss, in a short, insightful, and articulate book called The Evolution Of Individuality ( ), detailed how the origin of multicellular animals involved a shift in the level of selection from the unicellular individuals to the cell colony as whole (i.e. a multicellular individual).
In this process the cells would have to inhibit each others natural tendency to reproduce independently - and instead temporally organize the timing and placement of cells so as to induce the cells to differentiate and assume specialized roles within the colony.
Indeed, Mitotic reproduction under conditions where cell-surface interactions leading to reproductive inhibition are absent can be seen in cancers where inhibitory mechanisms on cellular reproduction within the soma fail, leading to an unhealthy proliferation of certain cells at the expense of the body as a whole..
At the origin of animals, the once free-living cells came under the influence of somatic selection pressures exerted during the lifetime of the colony and gradually came to be tightly integrated within the organism.
The animal embryo and organism from this perspective is primarily studied as if were a motile eukaryotic cell colony with sub-populations of progressively differentiated and specialized cell groups coordinating their interactions (within the interior milieu of the body - the cellular micro-ecology) such that they act as an adaptive whole with a sense of unitary identity (within the animals macro-ecological niche).
The evolution of the individual cell populations in the body is driven by adaptive somatic selection pressures within the physiology (cellular ecology) of the bodyplan and the evolution of the animal species is driven by the adaptive natural selection of favored individuals within respect to the large-scale ecology.
As we will see, biological organisms can be described as a set of nested dissapative structures organized as somatic selective systems embedded within their environments - each system undergoing a variety of forms of selection.
Examples of somatic selective systems we will encounter in our effort to understand the evolution of animals:
Degeneracy is the driver of innovation and stability within the context of an ever changing ecology.
An interesting property of an evolving somatic selective system is the capacity of the system to accumulate and then mask, variation that is not deadly but may be potentially developmentally or physiologically destablizing under the normal conditions of existence.
The masking agents mainly act to buffer the normal developmental ontogeny of the organism from genetic mutations that destabilize development.
By selectively releasing the masking agents in times of environmental pertubation, the system is capable of drawing on an immediate reservoir of variation that can aid the systems attempt to adapt to the pertubation.
Over time some of the masking agents associated with variation that is adventitious in response to cyclical pertubations are capable of forming stress or shock response systems.
Canalization is the habitual recruitment of a degenerate reportoire of sensorimotor behaviors in response to hedonic and ecological conspecifics.
All of the major phylotypic bodyplans were established early and have enjoyed various degrees of success.
The phyletic diversity of each phylum has fluxuated internally as well.
It is interesting to note particular bottleneck periods corresponding to mass extinction events.
Over a century ago the great Russian evolutionary biologist Elie Metchnikoff challenged Ernst Haeckel´s Gastrea Theory of animal origins on the basis of discontinuities in the physiological viability and evolutionary continuity of his proposed gastrula organism as the origin point for animal evolution ( ).
Metchnikoff´s stress on the need for physiological viability in all proposed transitional stages leading from colonial unicellular organisms to multicellularity and onward throughout the evolution of the animal bodyplan is noteworthy.
It is important to remember that - at the origin of animals, the individual cells of the metazoan body retained the hereditary patterns of activity of their once free-living ancestors; including, but not limited to intracellular digestion (still seen today in macrophages), cell-cell signaling mechanisms (cell-surface mechanosensors, receptors, and secretory substances), as well as the tendency for continuous growth and mitotic reproduction.
Between 1873 and 1878, Elie Metchnikoff developed his theory of Parenchemella/Phagocytella Theory of animal origins and presented critical arguments against Ernst Haeckel´s Gastrea Theory as an account of the origin of animals.
Metchnikoff´s careful studies of the embryos of poriferans, considerations of mechanistic continuity, and physiological viability lead him to postulate an intermediate stage between single-celled organisms and Haeckel´s Gastrea.
Key to Metchnikoff´s conception of the origins of animals was consideration of the digestive modes of unicellular organisms versus metazoans and how the transformation from intracellular digestion to extracellular digestion occured.
The development of extracellular digestion provides a major selection force in favor of the transition in the unit of selection from the individual cell to the entire colony of cells as a whole.
Precise cell to cell communication during animal development is essential for the formation of morphologically complex organ systems and bodyplans.
For metazoans, all of these emerge from the division of a fertilized egg whose progeny subsequently organize themselves and further differentiate to form a viable multicellular organism.
Understanding the nature of bodyplan evolution requires that we understand the embryological development of the organism and how heritable alterations in the developmental sequence affect it´s final morphological phenotype.
The major developmental signalling pathways predated divergence of cnidarians and bilaterians.
The several developmental intercellular signalling pathways are conserved thru out metazoan development:
Most porifera are organized as filter-feeders possesing Collar cells with flagella that aid in pulling water and food through the matrix, where amoebocytes and collar cells absorb food.
The three dimensional structure of poriferans is maintained by spicules formed in the extracellar spaces.
Poriferans are generally classified according to the of canal system they employ: