Life Is As Life Does
Living things grow and they reproduce. Growth is a way to generate the materials for reproduction. Reproduction is a way to make new organisms that can grow. Thus, the apparent "goal" of every organism is to fill the available world with its offspring, that is, with "self". It has been suggested that each unit of inheritance itself, each gene, is selfish in this way. It acts in such a way as to increase its chances to spread to all available individuals of a population. If other genes are helpful in this, good. If not, don't collaborate.
The mindless drive toward expansion that is the hallmark of living things is a program invented early in life history. It has proven very successful. (It was reinvented by the free market as a successful program for organizations living in the economic world.) The goal of the program can never be achieved, because organisms depend on each other for their existence. Thus, there is a "negative feedback" on the growth of every organism which keeps things in balance, sort of. Humans, lately, have been especially successful in avoiding or neutralizing negative feedback on population growth (such as disease or lack of food). As a result, they have discovered that the environment loses desirable properties as it is filled with people, and that resources become scarce. This discovery has given rise to the notion of "sustainable development", which may be translated as "growth without the negative consequences." It remains to be seen whether the mindless drive toward expansion that we share with all other organisms can be checked by intelligent internal feedback before it is halted by lack of resources.
All growth depends on the appropriation of outside matter, that is, on "eating" in some fashion. Ecologists have classified the various ways organisms eat (all in Greek, for style):
Organisms eating others (other-eaters) are called "hetero-trophs". If this happens in an oxygenated environment and oxygen is used to "burn" the food, we speak of "aerobic respiration." We humans are heterotrophs and we use aerobic respiration to maintain our bodies. (We breathe oxygen in and carbon dioxide out.) Other heterotrophs may use anaerobic respiration to process food (they strip the oxygen from molecules such as nitrate or sulfate) or they may use fermentation (they chop up the organic molecules and rearrange the parts, as does yeast). Organisms that feed themselves using sunlight to build new organic matter (self-feeders) are called photo-auto-trophs. Normally, oxygen is produced when doing this ("oxygenic photosynthesis") as described by the photosynthetic equation. A third type of making a living, other than eating others or feeding oneself using sunlight, is by using chemical gradients as an energy source for synthesis (rather than sunlight). Such organisms are called "chemo-auto-trophs". They are all microbes of a special class of bacteria in the broad sense ("archea") and have special adaptations to cope with chemically unusual environments (highly acidic, sulfur-rich, etc.). There are those that are "anaerobic" (that is, there is no molecular oxygen available) and those that are "aerobic" (that is, they use available molecular oxygen to change the local chemistry and produce energy). Among the anaerobic forms are methane and sulfide producers, for example. (Methane is formed in the guts of cattle; sulfide produces the rotten-egg smell). Among the aerobic forms are sulfide oxidizers and iron oxidizers. (The former produce the acidic runoff from mines. The latter produce rust on wet rocks.)
Growth leads to reproduction. Birds do it, bees do it, and so do microbes. We all use the same basic method: replication of information-bearing molecules called RNA and/or DNA. The details differ. Birds lay eggs. Such an egg, at the moment of fertilization, is a very large single-celled organism. Its way of reproducing is to make a bird, either a female, to make more eggs, or a male, to fertilize the eggs of others. Bees are mainly non-reproductive workers. They serve a queen deep inside the hive. The queen does all the egg-laying and the workers care for the eggs and feed the larvae. Microbes split to make new individuals. Bacteria do so mostly just by duplicating their individual genetic instructions. The new individuals are precisely identical to the previous one, except for occasional mutation. Eucaryotes (radiolarians, bees, birds, people, trees) have a more complicated reproductive cycle. Some can split if they choose to do so. The offspring are identical to the parent and to each other. This is quite common among single-celled eucaryotes. But bees and even certain lizards can produce offspring from unfertilized eggs. So even highly evolved eucaryotes can reproduce without sex. As a rule, eucaryotes use sexual reproduction. They split their DNA into specialized cells, gametes, which then recombine, generally with those from another organism, into a cell that is now different from the two original ones. This sexual reproduction ensures shuffling of genes, to try out as many combinations as possible for best results in a changing world. The constant combination and re-combination of genes defines a population. Interbreeding populations define a species. The concept of species, then, applies to the sexually reproducing organisms. The others (the vast majority of the microbes) may have other ways to exchange genetic information, to be discussed anon.