To assess the probability of intelligent life elsewhere in the universe, let's first define what is basic life, what are the differences between an inanimate object and a living creature.
The discovery of life in hydrothermal vents, openings in the deep ocean floor where extremely hot, mineral-rich water erupts from the crust, showed that it is possible for life to evolve in places without light from the sun, and in other worlds without sufficient light from the parent star. Europa, an icy moon of Jupiter, might accomodate similar organisms.
Life has been found in other extreme environments as well. Scientists discovered microcolonies of lichens called cryptoendoliths in rock samples of the Antarctic desert, where temperatures often drop to 100 degrees below zero and there is little or no liquid water. In contrast, thermophilic (heat-loving) bacteria have been found in hot springs where temperatures exceed the boiling point of water.
It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as amino acids, have been found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth's collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue.
Some planets might support complex organic molecules -- proteins and nucleic acids -- and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. There is an emerging consensus that single-celled micro-organisms may in fact be common in the universe, especially since Earth’s extremophiles flourish in environments that were once considered hostile to life. The potential occurrence of complex multi-celled life remains much more controversial. In their work Rare Earth: Why Complex Life Is Uncommon in the Universe, Peter Ward and Donald Brownlee argue that microbial life is probably widespread while complex life is very rare and perhaps even unique to Earth. Current knowledge of Earth’s history partly supports this theory: multi-celled organisms are believed to have emerged at the time of the Cambrian explosion close to 600 mya but more than 3 billion years after life itself appeared. Finally, even organisms that have adapted extremely well to their environments are mortals.
How would the physical size of intelligent alien life forms affect, or possibly even justify their existence? Could intelligent aliens exist that are diminutive (perhaps only a few centimeters tall) but still possessing complex intelligence and the ability to manipulate mass around them in ways that allow them to master their environment? What about species that would make humans appear the size of an insect?
If alien life does exist, what might it be like? Does the consistency of the size of atoms and particles in the universe suggest that the size of humanoids (either humans, or perceived extraterrestrials) is an evolutionary constant, or would intelligent life elsewhere evolve based on the influence of their surroundings, and the aptitudes that environment would require?
Would it be simple forms of life such as bacteria, viruses or algae, or more advanced, multi-cellular creatures, perhaps even intelligent beings? Would aliens be animals, plants or have characteristics of both? Would they have arms and legs and walk upright as we do? Would they depend upon vision as their primary sense or use another way to gather information about their surroundings? Would they "breathe" oxygen or some other gas?
Are technology and its applications typically relative to the physical size and characteristics of the species that harnesses them? How would a species that was far smaller than humans in stature, but more advanced than us, have advantages or disadvantages over us in a takeover attempt?
Theories about extraterrestrial life