Multicellularity introduces a new element into the natural order of things. The snake has entered the  bacterial Garden of Eden. We can no longer live forever. But in exchange for leaving immortality behind, a vast array of evolutionary pathways opens up, different ways for multicellular organisms to live, in an ever-increasing number of different environments. Each of these new environments poses a different set of challenges, and only those organisms that can adapt to these changing conditions will survive to carry on the species. And all of the amazing diversity we see in nature, all of the millions of different ways to be a living thing, represent the many ways in which organisms have solved those basic environmental challenges.

We are accustomed from birth to look at plants and animals as very different sorts of beings, that somehow animals are a different order of creation from plants. But if we look under the surface, if we think about what plants and animals really are, in the most basic and fundamental sense, we might find that we are more alike than we think. All multicellular organisms, whatever their environments, share a common set of evolutionary problems. And the differences we see between them are a result of the different evolutionary strategies they have used to solve those problems.

All organisms face the same basic challenges:

1) Find and digest food

2) Find a mate and reproduce

3) Avoid being eaten while you are doing number 1 and number 2

4) Maintain a balance between the fluids in the body and the salts dissolved in them (osmotically stable environment)

5) Circulate nutrients from one part of the body to the other

6) Remove waste products generated by metabolism (especially nitrogen compounds)

Plants and animals have adopted very different strategies to solve these problems. And different groups of animals have come up with some solutions that are truly radical. The possible solutions, however, are not infinite. Any engineer can tell you that the number of solutions to an engineering problem is finite. The basic laws of physics and chemistry are not repealed when we put up a building. If you push something hard enough, it will fall over!

For example, there are three very fundamental modes of existence that an organism can adopt:

1) Sessile or Motile
2) Aquatic or Terrestrial
3) Small or Large

Sessile (attached) organisms are usually radially symmetric. Radial symmetry means the animal can be folded along any plan into mirror image halves. Like a wagon wheel. Bilateral symmetry means that there is only one plane that can divide the organism into mirrored halves, like a wagon. The Phylum Cnidaria is a large group of organisms that are sessile for all or part of their existence.  Sea anenomes, for example, or coral polyps live out their lives attached to the same spot. Radial symmetry in these organisms is probably a fundament adaptation to a sessile existence. Your awareness of your environment is omnidirectional. You can sense and get food from any direction.

The down side is that when danger threatens, you've got nowhere to go. Cnidarians solved this problem by evolving a variety of stinging cells, loaded with nasty little microscopic harpoons, which they can use to stun prey and attack predators. You also have to find a way to disperse your young when you reproduce. They can't simply walk away. So, many sessile animals have motile larvae.

Sessile animals, like sea anenomes, don't have to invest in complicated structures like legs or wings in order to move about and look for food. But being sessile limits them to one type of food source, the kind that just happens to float by. Sessile animals are usually filter feeders or suspension feeders. Some higher organisms, especially the echinoderms (sea lilies, starfish) have gone back to a sessile mode of existence, and in the process have lost their bilateral symmetry, returning to a more primitive radial symmetry.

Motile organisms are usually bilaterally symmetric, a group which includes most higher animals. This is a much more efficient shape for moving through the environment. Animals in motion can actively seek out food and mates, and run away from predators. Animals in motion generally have a specific direction. And if you are moving forward, it makes good sense to concentrate your awareness of your environment in that direction.

So bilaterally symmetric animals become cephalized. They develop a head end, where the sensory organs are located, as well as the brain those senses are wired to. That is why vertebrates have a central nervous system, and sea anenomes do not. Forward motion allows different parts of the body to become specialized for different purposes, with senses and awareness at the anterior end, and functions like excretion and reproduction at the posterior end. Such organisms also have a dorsal or top surface (remember the dorsal fin of the shark), and a ventral surface, or, to use its scientific name, the tummy.

Consider the second mode of existence, being an aquatic organism or a terrestrial one. One of the few things we know for certain about the earliest history of life is that it began in water. Most probably in the sea, as the salt content of every cell in our body suggests. The great leap from water to land required a radically different set of adaptations, problems both plants and animals had to solve:

1) Desiccation
2) Gravity
3) Excretion

Desiccation also poses a problem for reproducing on land. When it came time to reproduce in the water, you could just dump all your gametes overboard, and let the currents do the rest. The larvae would develop in a nurturing saline bath, the ultimate womb of the ocean. But terrestrial organisms must find a way to keep their gametes from drying out. Aquatic organisms can rely on external fertilization. Terrestrial organisms have to develop some sort of internal fertilization to guard against desiccation.

Some primitive plants get around this problem by relying on a thin film of water, like dew, to give their gametes a moist place to swim through. Such plants, like mosses and ferns, are limited to moist environments. As are some animals, like amphibians, which must return to the water for at least part of their life cycle. In a very basic sense, many terrestrial organisms have never actually left the water. They live in the thin films of water that cling to moist places, like the tiny pore spaces between grains of moist soil.

Organisms also had to evolve new ways to protect their embryos from drying up on land. Animals evolved the amniotic egg, sealed in a shell and bathed in nutritious liquid. Amphibians must return to the water to lay their eggs, but reptiles can lay their eggs anywhere. Higher plants evolved the seed, a tiny time capsule filled with food and sealed against the elements. The reptilian egg and the seeds of gymnosperms allowed organisms to break the last link with their aquatic heritage.

Gravity is another basic fact of life that is not a very big deal in the ocean, but of paramount importance on land. Aquatic organisms rely on the natural buoyancy of water to support their weight. In general, they don't need to invest much energy in support structures. Unless, of course, they need to move very rapidly, like vertebrates. On land, gravity requires a support system. Plants developed the root-shoot system, roots holding you in place while the stiff tissues of the stem lift your body up into the air. Animals on land developed sturdy skeletal systems, whether internal, like our own (endoskeleton), or external, like that of an insect (exoskeleton).

Getting rid of wastes is not a big problem in the ocean - dump it overboard. Waste material is generally excreted in a solution of water, and is usually high in nitrogen compounds. Aquatic organisms usually excrete nitrogenous wastes in the form of ammonia. Ammonia requires large amounts of water to dissolve in, but if you're floating in the ocean - no problem! On land, however, organisms have to conserve water, in any way they can. So terrestrial animals excrete liquid wastes as urea. Even urea requires a fair amount of water to dissolve. Birds couldn't fly with a full bladder. So birds rely on uric acid to get rid of nitrogenous wastes, which uses very little water (the white part of bird droppings). Excretory systems themselves pose certain critical problems. The water that carries off the waste stream also takes with it essential salts that the organism must replace. So animals have developed excretory organs like nephridia, simple tubes through which the wastes pass and in which precious salts can be recovered.

Related to all of these basic environmental challenges is the problem of size. If you remain small, you can rely on simple diffusion to absorb nutrients and excrete wastes. This is true for both plants and animals. Increasing size brings increasing control over your environment, and allows for greater complexity. But larger and thicker organisms can no longer rely on diffusion. Cells that are too far away from the surface will starve to death, or drown in their own poisons before they can be carried off. And to make matters worse, the surface area across which gases, nutrients, and wastes must be exchanged rapidly decreases as you get larger.

As organisms become larger, their volume increases much more rapidly than their surface area. Cells become farther removed from the outside at an exponential rate. Consider a spherical creature. The formula for the surface area of a sphere? (4 pi r²). The formula for the volume of a sphere? (4/3 pi r³) The animals volume increases as a cube of its radius, but its surface area only increases as the square of the radius. So as it gets bigger, more and more volume is covered by less and less surface area.

Organisms have solved this problem in several ways:

    1) Folding the respiratory, digestive, and other surfaces to increase the amount of surface area that can be packed into a limited space (lungs, brains, intestines)

    2) Being very thin or very flat

    3) Developing vascular systems - Plants and animals have solved this problem in a basically similar way. They rely on a network of tubes that runs throughout the body of the organism, a vascular system. These closed tubes can circulate water and nutrients, and carry off wastes.

    4) Developing coelomic systems, fluid-filled cavities that can be used to circulate materials and hold the internal organs, along with a variety of other useful functions. (fr. Greek koiloma = cavity)

Kingdom Animalia includes 35 phyla and over 1 million species. If all the various worms and insects were finally found and described, the number of named animal species might grow as high as 10 million species! Most of these animals, about 95-99% of all known species, are invertebrates, animals without backbones (a term coined by Lamarck). And most of these are different types of aquatic worms! Animals are diploid, eukaryotic and multicellular. All animals are heterotrophic. And all animals respond to external stimuli.  All animals are motile, moving about at some point in their life cycle. Only animals can fly. J.B.S. Haldane once called animals “wanderers in search of spare parts.” All animals reproduce sexually by forming haploid gametes of unequal size, the egg and the sperm. The gametes fuse into a zygote, which develops into a hollow ball of cells called a blastula. In most animals, this hollow ball folds inward to form a gastrula, a hollow sac with an opening at one end, the blastopore, and an interior space or blastocoel.

Most authorities now agree that all animals share a common ancestor, most probably some type of colonial protozoan called a choanoflagellate, perhaps a protozoan version of Volvox. We can divide the entire animal kingdom into two subkingdoms. The Subkingdom Parazoa contains the sponges, and one or two obscure groups of rudimentary animals. Parazoa literally means “animals set aside”. These animals that are so strange, so unlike all other animal life, that we tuck them away in their own little group.

All other animals belong to the Subkingdom Eumetazoa, the “true” metazoans (meta - zoan = animals that came “after”, as opposed to “proto” -zoans, = first animals). Eumetazoans have a definite symmetry, which sponge animals lack, and share common patterns of embryonic development. There are two branches of Eumetazoans: one includes animals like sea anenomes that have radial symmetry, and the other branch including all other animals, all of whom have (like ourselves) bilateral symmetry.

The bilaterally symmetric animals can be further divided into three grades. Grade is not a formal taxonomic term. Grades represent a level of organization. The group of all animals that fly, for example, could be called a grade. These three grades represent three basic types of body plan found in all animals. These body plans differ mainly in the presence or absence of an internal body cavity, or coelom. So what is a coelom, and how does it form?

In the embryos of all bilaterally symmetric animals, there are three tissue layers distinctly visible in the developing organism. These are the endoderm (= the skin within), which gives rise to the gut and the internal organs; the mesoderm, (= the skin in the middle), which gives rise to the skeleton and body muscles, and the ectoderm (= the skin outside), which gives rise to the epidermis or outer covering of the animal as well as the nervous system. Many primitive animals, like flatworms, have no coelom at all, just a rudimentary internal pouch, like sticking your finger deep into a ball of clay. We call such animals acoelomate, because they lack a coelom. Another group of animals, including nematode worms and rotifers, have a large body cavity that looks like a coelom, and functions like a coelom, but is actually formed in a different fashion. This pseudocoelom is a remnant of the hollow space inside the blastula, the blastocoel. We call these animals pseudocoelomates. All higher animals have a true coelom, a body cavity formed from within the mesoderm tissue layer. This cavity is lined by mesodermal membranes, and the internal organs grow and develop within this fluid filled cavity. Such animals are called eucoelomate.

If we look at the overall pattern of animal evolution, we see that all of the coelomate animals are split into two distinct groups, one called protostomes, the other called deuterostomes. This is a very ancient split within the animal kingdom, going back at least 570 million years to the early Cambrian. These groups are separated by what happens to the blastopore, the small hole that opens in the gastrula, connecting the embryonic gut to the outside. In protostomes, this opening gives rise to the animals mouth, hence proto=first, stoma=mouth. This group includes the annelid worms, the mollusks, and the arthropods. In deuterostomes, the first opening becomes the anus, and the mouth opens up later on in development at the opposite end of the embryo, hence deutero=other, stoma=mouth. This group includes the echinoderms and the chordates.

Another fundamental difference between protostomes and deuterostomes has to do with the fate of the early cells in the developing embryo. Protostomes show a pattern of spiral cleavage, in which new cells are staggered in a spiral fashion, overlapping one another like bricks in a brick wall. These cells are determinate, their fate is determined early on in development. Removing them results in an incomplete organism. Deuterostomes show a pattern of radial cleavage, in which cells appear directly over other cells, like a stack of coins. These cells are indeterminate. If you separate them at an early stage, each one can develop into a complete functioning organism. This is how identical twins are created, by cell separation at a very early stage of deuterostome development. The coelom in protostomes develops as a split in the mesoderm (schizocoels). The coelom in deuterostomes develops from outpocketing of the gut (enterocoels)

What makes the coelomate body plan so successful? The fluid-filled coelom represents a big evolutionary advance over acoelomate body plans.

1) The coelomate body plan is a “tube within a tube”. Because this tube is filled with fluid, it allows fluid circulation, even in primitive animals that lack circulatory systems.

2) Fluids (like water) are relatively incompressible. The fluid-filled coelom can therefore act as a type of rigid skeleton, or
hydrostatic skeleton. The muscles now have something solid to push against.

3) The coelom allows for an open digestive tract, with a mouth at one end, an anus at the other, and this tract can be increased by coiling within the coelom so that it is many times longer than the animal itself.

4) Animals like flatworms, on the other hand, with one opening into a hollow cavity, are limited in how fast they can eat, digest, and excrete.

5) A coelom allows digestion independent of movement. Gut action need no longer depend on the muscular contractions generated by the animals movements.

6) There is more space for the internal organs to develop, especially the gonads, and large numbers of eggs and sperm can be stored in the coelom as well.

7) And finally, the combination of a coelom and bilateral symmetry opens up an entirely new evolutionary pathway, in which parts of the body can be adapted to perform special functions. This new pathway, which has ultimately given insects dominion over all other life, is called segmentation, and well discuss it further when we talk about arthropods.
 

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