Humans as organisms can’t be isolated from other life forms, since we need a variety of them as food. We require plants to convert sunlight into food energy, to convert our exhaled carbon dioxide into oxygen, and to convert nitrogen into proteins. In a closed environment we require organisms to break down our wastes. Although all of these processes can, on a limited scale, be accomplished synthetically, only the natural processes provided by other living organisms are economically viable on a large scale. The life forms of habitable planets are, in most cases, as varied and plentiful as the life forms on Earth, but since they evolved independently, each species is totally different from any that exists on Earth.
Some general statements can be made about all corporeal life. In all bio systems, be they carbon nitrogen- water based, sulfur-chorine-hydrofluoric based or whatever, there is a basic food chain. The chain begins with life forms that convert sunlight to chemical energy. These forms are eaten by other creatures who may in turn be eaten by still others. Each life form occupies an ecological “niche” or position in the food chain. In general, there are more individual life forms, or at least more masses of life forms, lower on the food chain than higher.
More and less complex forms of life can be found in all life systems. The more complex are sometimes said to be more “evolved,” but, in fact, some very simple life is the product of as many stages of evolution as homo sapiens. “More complex life forms” is not really a precise, scientific term, but is used here to mean that a particular species has evolved a higher degree of structural specialization. Mammals and flowering plants are among the most complex life forms of the animal and plant kingdoms on Earth.
The basic classification schemes used on Earth have been found to be useful in studying and classifying the life of the colonial planets. The primary classification categories are the kingdoms: plants, animals, protists, and synergists (viruses). Within kingdoms are grouped numerous “phyla.” Some phyla such as tracheophytes (vascular plants) and chordates (animals with spine-like nervous systems) appear common to most of the colonial worlds. Each phylum is divided into several classes. Angiosperms (flowering plants) are a class of tracheophytes; mammals are a class of chordate. Similarities exist between classes of the various colonial worlds, but the differences are far too great to consider the classes identical. For example, many of the colonies have warm-blooded flying animals resembling the Earthly class aves (birds). These “birds” may bear their young alive or they may not have feathers, yet having feathers and laying eggs are two important characteristics of the group of animals classified as aves on Earth. The similarities between life forms on Earth and on the colonies sometimes shock pioneers, but they should come as no surprise. The laws of physics apply throughout the known universe. Chemical elements found on Earth are the same elements that constitute all matter. Biological phenomena can’t violate these laws. In addition, since certain forms of biological organization (such as cell structure) are more efficient, they win out over less efficient forms conceivable within our physio-chemical framework. Life evolves in similar patterns on each of the different worlds, yet the individual species, their genetic organization, biochemistry, habits, etc., are unique.
Because the life forms of other planets are quite alien, they are potentially very hazardous. The greatest hazard springs not from large, carnivorous creatures, but from microorganisms such as protists and viruses. Such creatures cause most of the diseases on Earth, but Earth’s population has some immunity to most of them. Humans and our domestic animals have no immunity to the microorganisms of another planet. Diseases there have the potential for wiping out an entire colony in a very short time. Fortunately, during the last 500 years, microbiology and immunology have become well understood and fewer devastating diseases have struck in the colonies than were originally anticipated. Scientists speculate that this is because few parasites native to the colonial worlds can adapt to Human hosts. Rather than exposing us to harm, our alien body chemistry may offer the best source of protection from disease. Pioneers have dealt with most serious viral outbreaks which have hit the colonies by using life support systems to maintain the victims until medical scientists isolate the virus and produce antitoxin. In the last 200 years of colonial development, deaths from viral epidemics have been few. Yet danger from these outbreaks still exists, particularly in the newer colonies. The microbiology laboratory remains as vital to new colonists as their computer and power reactor.