As anybody who has read my diaries knows, I spent my professional life as a field biologist, taxonomist and ethologist, specializing in arthropods. I developed that interest from an early age and although I got distracted by interests in astronomy and geology, as well as chemistry, that early tendency prevailed and I followed it to an academic career.
Arthropods are fascinating, but than again few organisms are boring, once you get to know them. The phenomenon that we call "life" is in reality so strange (even if we find it to be common in the universe) that it defies definition. We used to talk about "life force," but that idea has been rejected as being of little use as it explains virtually nothing. Indeed, it seems as though life follows from the complex structure of molecules allowed by carbon chemistry, but not by other elements, save perhaps in lesser degree by silicon. Still, the origin and early development of living organisms remains shrouded in a fair amount of mystery. We know that some form of primitive bacteria was in existence by 3.5 billion years ago and that these tiny non-nucleated organisms eventually developed into complex multicellular lifeforms, including ourselves.
We also know that Walt Whitman was right in ways he could not have imagined when he wrote the lines
"I believe that a leaf of grass is no less than the journeywork of the stars"because the very heavy elements required for the chemicals of our bodies, and those of grasses and all other life, were forged in the explosions of stars - supernovae in fact.
This brings us to a number of conundrums that despite our best efforts we have been unable to completely solve.
1. What is life?
2. How did it originate?
3. How did multicellular organisms with complex internal organs originate?
4. What is the basic unit of life? Is it the species? The population? The individual? or perhaps the gene?
5. Is life common in the universe?
6. Is there intelligent life in the universe other than on earth?
7. If there is, where is it?
What is life? is a question long tackled by scientists and philosophers. Still we really do not know the answer. Scientists have long argued over, for example, whether viruses are living, or are just somewhat complex aggregates of molecules with the ability to reproduce. How seemingly inert molecules could become animated and produce bacteria, leaves of grass, amoebae. elephants and us, is best described as a natural property of carbon chemistry, but we have no easy definition of what living means.
The origination of life is certainly a difficult question and it has been answered in a number of ways by scientists, philosophers and theologians. We are pretty sure that simple life had been produced on earth about 3.5 billion years ago. Because the DNA of humans shares a sizable proportion with corn plants and everything else living, we are fairly certain that all life is related and thus a line of descent from bacteria to us is fairly certain. It is unlikely that God, the gods or a goddess produced life, but of course we have no absolute empirical data. Some have hypothesized that life was seeded by extraterrestrials or came from spores traveling in the void of space or on comets or asteroids. However this just leaves the question of where and how life originated (as it had to have somewhere) still up in the air and is as easy to examine scientifically as the idea that some god breathed the universe out, and with it life.
Once you have bacteria there is another problem - how did bacteria form multicellular organisms, complete with nervous, circulatory, respiratory, digestive and endocrine systems? Actually this may not be too difficult to envision as we have examples in the obviously bacterial mitochondria and chloroplasts as cell organelles, but the organizational level developed through embryogenesis is still pretty mind-boggling in its complexity. Getting the cells of a multicellular organisms to form tissues and organs that have to cooperate to survive seems a nearly impossible task, yet it happened. We still do not understand how the radical metamorphosis in insects, echinoderms, and crustaceans, among others, evolved, although there have been some rather odd ideas about this recently.
As to the basic unit of life, I was taught that it was the individual phenotype that was the target of selection. However the species seems to be the basic unit of taxonomy, as other classifications are difficult to always justify. Members of a species are composed of potentially interbreeding individuals and populations. Still, even the species is a shaky concept, partly because evolution by natural selection is ongoing, not static. Another problem is the existence of many organisms that do not reproduce sexually. These include vegetative reproduction by plants, cellular division in protists, parthenogenesis (as in some of our racerunner lizards in New Mexico) and selfing, as in some tapeworms. Things are not quite as cut and dried as we often think.
Life in other parts of the universe, given the billions of stars and planetary systems that exist there, seems very likely. As unique as the development of life on earth seems, there are way too many planets already discovered for me to think them all barren.
Intelligent life is another matter completely. While I doubt that it is unique to earth, it may still be fairly rare. We have no reason to think that other intelligent life forms would be like us, the average Star Trek stereotypes not withstanding (they did have some non-humanoid species, but many looked a lot like us and seemingly could often interbreed with our species.) They could be totally aquatic and they could be intelligent without inventing our sort of technology. There is no way to be sure until we actually meet some.
As to why we cannot detect such intelligent life if it had similar technology after many years of listening with radio-telescopes, my theory (also thought of by many others) is that technological civilizations just do not survive that long before they destroy themselves by war, overpopulation, and environmental degradation. As near as I can tell (and I would like to be proven wrong) warp drive is an illusion, although I like a space novel as much as any science fiction nerd, and travel by worm hole would be a disaster to an organized being. Well, maybe tomorrow the alien space ship will land and I will be shown to have been entirely too pessimistic, but somehow I do not think so.
As near as I can see, life like that on earth is a natural, if relatively uncommon, event. We are all of star stuff. We are all the journeywork of the stars. That is magic enough for me and I would like to keep it, if it is at all possible. I would like my grandson to be able to see a living tiger and to marvel at the leaves of grass!
There are many references on the subjects I have very superficially discussed here. The subjects are vast and opinions vary widely. Those who wish to delve further will find that they will need to devote a fair amount of time to even get a smattering of the material available.