What happens, however, if we discard religious doctrine and biological dogma, and consider the biosphere from an objective perspective? Do the conditions for terrestrial life indicate that humankind is really paramount? Is life on earth no more than an accident? If, on the other hand, there is a Creator, is He or She more interested in beetles, as J B S Haldane suggested? If not humans or beetles, what other sort of life has greater claim to see the earth as its natural home?
These are big questions, and we cannot give definitive answers to any of them. But a case can be made that neither beetles, nor humans, nor any of today's other animals or plants are the paramount life form. That honour goes to microbes.
Their case is threefold. First, one-celled microbes were the earliest life forms on the planet, changing their physical nature and habitats long before the emergence of the first multi-cellular creatures and long, long before early humans arrived. Second, during the four million years or so of our own presence, micro-organisms in the form of bacteria, viruses, moulds and protozoa have influenced the environment and events far more profoundly than we have. Microbes have not only made animal life possible (for example, by generating oxygen and recycling nitrogen to provide the plants we eat). They have also shown the versatility to live in a vastly greater range of habitats than we could ever contemplate. Third, and in consequence of that adaptability, micro-organisms will prosper long after we have disappeared. Microbes rule the world.
The origin of terrestrial life is, of course, inherently unknowable. Several theories are current, the most plausible still being that originally proposed by the Russian biochemist Alexandr Oparin in 1924. This envisions the earliest single cells (microbes) emerging during billions of years from a 'primeval soup' of carbon-containing substances. There is experimental support for this theory. Whatever the truth, our knowledge of evolution strongly points to microbes as the planet's earliest inhabitants. Only last year, William Schopf, of the University of California, Los Angeles, described members of eight previously unknown groups of microbial fossils in rocks in Western Australia. These organisms lived on earth 3,465 million years ago.
Of the second plank of the argument, it is true that every part of the natural world and every aspect of human society is affected, for good or ill, by microbes. They provide all our daily food and that of all other animals - whether by maintaining soil fertility, by breaking down cellulose in the rumens of cows and sheep, or in countless other ways. Microbes were the original source of the world's oil supplies. They are the active agents in sewage disposal, and degrade much toxic industrial effluent. Epidemics caused by micro-organisms - from smallpox, plague and cholera to newcomers such as Aids - have changed social history and affected military campaigns far more decisively than generals or politicians.
Most impressive of all is the fact that the microbial world, in contrast to animal and plant life, extends to all corners of our planet, however inhospitable. Lichens (tiny fungi and algae living together) grow in the extreme cold and dessication of sandstone in the Ross Desert of Antarctica. In the same region, microbes flourish at the bottom of lakes permanently covered in ice. In contrast, bacteria can thrive not only in the hot springs of Yellowstone Park, but also in super-heated oil reservoirs 3,000 metres below the North Sea. Some of them grow most quickly at temperatures well above 100C.
Extremes of temperature are not the only conditions that certain microbes have learnt to tolerate. Others can cope with high levels of salinity or acidity. Some bacteria are resistant to radiation. Some tolerate exposure to carbolic acid. Others even break down chemicals such as polychlorinated biphenyls (PCBs), now banned from industrial use because they are not generally biodegraded in the environment.
Moreover, many characteristics of this sort can be transferred between bacteria very quickly compared with animals or plants. Microbes have a much more diverse sex life than humankind. All can move genes 'horizontally' between cells, in addition to transmitting them vertically from one generation to the next as in animals and plants. Genes conferring resistance to antibiotics have spread in this fashion, making many disease-causing bacteria insensitive to those drugs. But the overall lesson is that microbes have a unique capacity to adapt.
Whatever the future scenario for planet earth - a new Ice Age, runaway global heating, a catastrophic collision from outer space - micro- organisms are superbly equipped to survive and prosper. Microbes were not only here before us, it is they, not humankind, that will inherit the earth.
The author's 'Power Unseen: How Microbes Rule the World' is published this month by W H Freeman/Spektrum.