Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


11.1  Specialization and Symmetry

Despite the fascinating possibilities suggested above, a compact multicellular condition will probably develop on many worlds with highly organized life.* This is primarily because multicellularity permits specialization.

Specialization permits a division of labor in the primitive cell colony. Some cells become expert at, say, moving the body around. Others, free to ignore motion, may perfect their skill at digesting foodstuffs. The division of labor allows cells to do what each does best, and the aggregate efficiency is vastly increased.

It is the business of life to accumulate information and complexity. The specialized, highly differentiated organism is the bearer of far more information than a more homogeneous or generalized entity. The specialist metazoan has greater complexity.

This is not to say that complexity per se necessarily has survival value. But solutions to complicated environmental crises may require complicated solutions. As Carl Sagan observes, a solution involving many molecules will frequently be qualitatively superior to one which involves only a few.20 There does seem to exist a general evolutionary trend towards increased information storage, greater complexity, and higher levels of biological specialization.

By "specialization" we usually mean "the utilization of specific structures which perform given functions within the body." These structures are called organs.

Organs, while specialized in function, need not be specialized in structure. It is entirely possible that extraterrestrial lifeforms might have what is often referred to as "distributed organs." Instead of neat packages of similar cells, distributed organs would be scattered throughout the body. There are precedents for this on our own world.

For instance, insects inhale oxygen like other animals and yet they have no lungs. Unlike the localized organs found in humans, mammals and other animals, the insect "lung" permeates the entire body. The insectile structure is crisscrossed, like a piece of Swiss cheese, with a network of tracheal tubules and microscopic holes. Air reaches the interior cells only by diffusion. The insects’ "lung" is a distributed, nonlocalized "organ."

Peter Ritner has suggested that a creature’s brain might be of a distributed nature.1550 We can imagine an enormous blob of alien protoplasm suffused with more or less generalized nervous tissue. The organism’s brain and body, like the breathing apparatus of the insect, would be coextensive.

The number of organs of each type is highly variable. The squid, for instance, has two kinds of hearts -- one for arterial blood and one for the venous. Many terrestrial creatures have two or more eyes, multiple anuses and stomachs, pairs of arms and legs, and so forth.

Sometimes, organs combine several functions at once -- such as the human mouth. There is no reason to believe that ETs will have exactly the same combinations as us.

They may have separate organs for eating, drinking, breathing and speaking. The dolphin, for example, eats through its mouth, breathes through its blowhole, and "speaks" through its "ears." The land snail’s lung opens onto a passageway quite distinct from its food canal, and sea cucumbers breathe through their rectums. (This is called "anal respiration.") Brachiopods can only vomit their solid and liquid wastes, because they have a "blind intestine," a kind of alimentary cul-de-sac.

Then there is the ant lion, which is incapable of voiding any excrement at all. This is because all digestion occurs outside of its body, by the powerful saliva it injects into its victims. After the prey’s innards have turned to soup, the ant lion sucks the victim’s body clean of the predigested pap, leaving a dry husk behind. There are no wastes.

And the members of phylum Nematomorpha (long worms) have no mouths at all. All nutrients are "eaten" by direct absorption through the skin. Aliens can be no less strange in their organ arrangements.

There is also the possibility of multiple-brained extraterrestrial animals. After all, Brontosaurus and Diplodocus, two species of giant dinosaurs, carried an enlarged section of neural tissue in the hip region -- larger than the brain itself! The volume of this "sacral enlargement" in Stegosaurus, another fossil animal of grand proportions, was perhaps twenty times larger than the brain in the cranial cavity in the head. Further, Homo sapiens (a species not yet extinct) tenders a curious "split brain." The left and right hemispheres are two coordinated organs with distinct functions and reactions to stimuli.502 Most vertebrate brains have a similar dual construction.

In addition to varying degrees of specialization and organ distribution, most extraterrestrial multicellular lifeforms will display some sort of symmetry. Symmetry refers to the basic geometrical layout of an organism, the way its various parts are positioned with respect to one another, its shape.

Probably the first to appear in evolution was radial symmetry. The radiolarians -- small, spherical lifeforms -- are ideal examples. The main body is distributed around a single point in a radiating fashion.

The cylinder shape is another form of radial symmetry, in which parts are arrayed around a line instead of a point. Many sponges and coelenterates display this pattern.

However, virtually all of the metazoans may be regarded as having bilateral symmetry. In this design, an imaginary line drawn from head to tail bisects the animal into two similar pieces. That is, the organism may be divided down the middle by a plane which results in two mirror-image halves. Humans are organized in this fashion, as are all mammals and most chordates. Bilaterality, if we can trust the fossil record of a single specimen planet, must have great selective value because it has been re-invented countless times in most phyla of the animal kingdom.

Of course, body symmetry reflects lifestyle. Primitive sedentary or attached animals are usually radial, a body configuration that permits exploration of the environment in all directions. Conversely, animals that must pursue their food more actively need a more mobile, streamlined shape. The bilateral cylinder is ideal for efficient unidirectional movement through viscous media such as water. And it is characteristic of the most successful of Earth’s fauna, including all of the vertebrates and most of the invertebrates.

Based on the evidence of literally millions of terrestrial animal species, we conclude that highly-organized mobile aliens will most probably have bilateral symmetry if they have any physical form at all.


* Terrestrial organisms display remarkably little variation in the sizes of the individual cells, most falling within a single order of magnitude of scale. However, colder planets may spawn lifeforms with somewhat smaller cells than ours. This is because there is less thermal disruption at the molecular level at lower temperatures.63 But how small can cellular building blocks be? The 1000 Angstrom "pleuropneumonia-like organism," or PPLO (the smallest living thing on planet Earth capable of independent metabolic activity), is probably extremely close to the theoretical limit of roughly 400 Angstroms.1440,2395 An alien analogue to a human being, if constructed of cells this small, would weigh a mere fifty milligrams and would stand only five millimeters tall.


Last updated on 6 July 2013