It is hard not to notice that all the life that surrounds you. There are trees that grow, insects that walk around, and birds fly in the sky. Looking at them, it is all too easy to say that this is all the life that is around you but is it? Is all the life that you see all there is?
What the science of biology says is that the majority of life that has been studied by biology is not the life that we see with our own eyes but forms the bulk of the whole biosphere and those large organisms that we see are just a small fraction of the life present in the biosphere and they are small and numerous and they live deep in the soil, in the air, in the water, and on our bodies. The life that is small and abundant can only be seen with a microscope and they are microbes.
Microbes are so small that you can see with a microscope but not only is life in abundant in the form of living microbes but microbes have had a profound influence on the evolution of life on our planet. They recycle elements such as carbon and nitrogen, produce the oxygen needed for animal life, and through a process called endosymbiosis, two different species of microbes have came together to produce eukaryotes or cells with nuclei and organelles which are the cells that form fungi, plants, and animals.
All of eukaryotes along with the prokaryotes also show a common ancestor as revealed through molecular biology vindicating Darwin’s proposal that all of life has a common ancestor and even though as far as plants and animals, many species have evolved from simpler to complex forms, some species of animals such as nematodes have evolved parasitic lifestyles taking up residence in animal and human bodies and of some lineages of species such as the coelacanth, a primitive kind of lobe finned fish, are still around, and much of past life is extinct, the majority of all of life that has been present since the beginning are microbes and indeed if much of human life as well as most species of animals were to go extinct, the likely survivors will be all the microbes of the biosphere and indeed it has been since throughout the 3.5 billion years of the biosphere, five major episodes of extinctions had occurred and there were living survivors , which included the ancestors of today’s reptiles, birds, and mammals, the true survivors up to now are the microbes.
Microbes can live in virtually any environment, ranging from mild to extreme, and because they are ubiquitous on this planet has lent credence to the idea that the first life forms we may encounter outside of earth may likely be microbes.
Within the field of astrobiology or that field of biology that studies the evolution, origin, and distribution of life on other planets in other solar systems ( I will talk about the science of astrobiology in detail in a future blog post), and since being a field of biology, this particular science depends on other fields of biology such as biochemistry, microbiology, genetics, and so on as well as other science such as geology and of course astronomy, there are theories regarding the prevalence as well as possible rarity if not absence of life throughout the universe and regarding the fact that prokaryotes, the opposite of eukaryotes which are small cells without nuclei and as you have guessed it, these are the bacteria as well as the “extremophiles” or prokaryotes which are those that can survive those harsh conditions such as acidic conditions and temperatures close to boiling.
One such model is what is called Life Unseen and as the term implies is that the most common, the most abundant life forms that are common throughout the cosmos will likely be microbes. After all, microbes have been on earth for about 3 billion years and because they can exist on any range of conditions from mild to extreme, there is no reason to suppose that if life, in the form of prokaryotes, are the most abundant life forms on this planets and since, in a previous blog “What is Life, Really” all prokaryotes as well as unicellular eukaryotes display all the seven characteristics of life, and what earth life , depend on water, carbon, and energy and since these three ingredients of life, are common throughout our solar system and since astronomy has confirmed that planets are common throughout our galaxy and in other galaxies, then the chances of biospheres that depend on water, carbon, and energy are high and it is likely given that evolutionary histories are usually if not completely dominated by contingent factors or random factors such as fluctuations in climate to asteroid impacts, then considering how prevalent these three factors then it is likely that there may be biospheres where the dominant life forms are single cells such as prokaryotes or something similar.
It is true that as for the evolution of life, the large life forms that are familiar to us such as elephants to butterflies are multicellular organisms or organism composed of various kinds of eukaryote cells and all species of the plants and animals that we can see had a long evolutionary history the surviving species the result of adaptations that allowed the survivors to live with changing environments but these life forms can only adapt to a narrow conditions such as narrow conditions of pH in the blood to a narrow range of body temperature but as you know it is different for prokaryotes which because of their small size, their ability to reproduce fast and with a genomes that are far more dynamic, can withstand any extreme temperature.
In addition, one of the reasons why earth can support life as we know it in the solar system is that the distance from the sun is in the habitable zone or the zone from the star that receives neither too much sunlight nor too little sunlight but just the right amount to allow liquid water to cover about two thirds of the earth surface and for the formation of ocean and air currents. It is because of the habitable zone that the evolution of macrobiota or the large multicellular life forms such as trees and tigers are possible but it seems that by considering only the habitable zone or what authors Irwin and Schulze-Makuch (2011) has called “stellar habitable zone” (p.11) and this is that kind of habitable zone, in which the source of energy that powers life in the form of visible light emitted from the sun, which is what allows for the evolution of macrobiota as well as microbiota or prokaryotes and unicellular eukaryotes.
As for the microbiota, it seems that by considering the stellar habitable zone only narrows the possibility of finding life on planets that orbit suns when it is likely that the evolution of prokaryotes allows for the utilization of sources of energy for metabolism and growth other than sunlight and oxygen and they would minerals from geothermal events. In fact it is likely that before photosynthesis there was chemosynthesis which uses chemicals as energy sources and indeed there are microbes that can use chemical energy such as those found in geothermal vents and of course as you would expect, microbes have been recovered deep from underground several meters below, using minerals as energy sources. This would indicate that the habitable zones may not be confined from visible light from stars but from planets and possibly planetoids or even meteorites but still prokaryotes still need energy, carbon, and water but energy flow most of all so while planets are likely to provide energy flow from their interiors , asteroids and meteorites seem unlikely since there is no known energy source from such small objects that could support a thriving colony of bacteria however organic compounds such as amino acids have been found in some meteorites samples as well as in the cores in comets.
What is small is not necessarily insignificant and what may be insignificant may not necessarly be un important for microbes have had a profound influence on our biosphere. It is best to keep that in mind.
Astrobiology(n.d). Retrieved on August 27 2015: https://en.wikipedia.org/wiki/Astrobiology
Davies, P (1999) The 5th Mircacle: The Search for the Origin and Meaning of Life New York, NY: Simon & Schuster
Irwin, L.N, Schulze-Makuch, D(2011) Cosmic Biology: How Life Could Evolve on Other Worlds New York, Springer
Sagan, D, Margulis, Lynn (1986) Microcosmos: Four Billion Years of Microbial Evolution Los Angeles, California: University of California Press
Picture Pest Bakterien (Bacteria) – 630x https://www.flickr.com/photos/picksfromoutthere/14045255442/in/photolist-np8B7A-bwHUWH-rhKBuU-pWTA41 CC BY 2.0