Life Elsewhere?

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Unknown artist, Planet Quest,
JPL, Caltech, NASA

Larger illustration.

Life found on other planets or even
around other stars may be based
on similar (if spread by impacts
or interstellar dust) as well as
very different biochemical
processes (if developed solely
based on local conditions).

What is life and did it develop elsewhere?

According to physicist and astrobiologist Paul Davies, the scientific community has not yet reached consensus on a strict definition of life. Many biologists agree, however, that living organisms should be able to do the following:
NASA's Phoenix Mars Mission (which will seek for organic evidence of ancient or current Earth-type life down to three feet below Mars' surface upon landing on May 25, 2008) argues that life has six common properties:
- Order: Molecules in living things are arranged in specific structures;
- Reproduction: Living things have the ability to reproduce their own kind. Simple life forms, such as bacteria, reproduce by dividing and making almost exact replicas of themselves. More complex organisms reproduce sexually, so that their offspring have genetic material from two individuals. Offspring with traits from both parents have a greater chance of survival because they are better able to adapt.
- Growth and Development: Living organisms grow and develop in patterns determined by heredity, the traits passed to offspring by parents.
- Energy Utilization: Living things need to capture and use energy, a process known as metabolism. An example of such a process is photosynthesis, whereby plants convert sunlight into energy.
- Response to Stimuli: Living organisms respond to changes in their environment.
- Evolutionary Adaptation: Living things evolve in such a way that future generations are adapted to unique situations in their surroundings. For example, the hammerhead shark, considered to be perhaps the most highly evolved species of shark, has superior vision and sensory perception due to its hammer-shaped head. Organisms that cannot adapt to a changing environment decline or become extinct.
Under these types of definitions, viruses (and prions) which rely on larger lifeforms to perform metabolism and reproduction are not technically alive, but the status of possible nanobacteria (under 200 nanometers) has yet to be resolved.

In recent years, the idea that life originating on one planetary body can somehow disperse through space to other planetary habitats has gained support. Within the Solar System, it is possible that life developed in the first accommodating habitat -- perhaps in the early days of a possbily watery Venus -- and was spread by the ejecta of asteroidal and cometary impacts to neighboring planets. Indeed, if some microscopic life can survive long passages through space (despite the cold, radiation, and vaccum), then perhaps Earth's lifeforms originally developed outside the Solar System. If that's true, then it's also possible that microbial life from our Solar System may have already spread to other star systems. It is also possible that alien microbes are falling through Earth's atmosphere every day but are not well adapted to common environmental niches and so do not compete very well for survival, except in biological niches less hospitable to native lifeforms. (More discussion at NASA and panspermia.org.)

Cyanosite -- NASA image of Chroococcidiopsis

Dividing Chroococcus sp., a type of cyanobacteria,
photosynthetic microbes that also produce oxygen.
While "primitive," Chroococcidiopsis survives in
extremely dry, cold, and salty environments -- even
potential ejection from Mars without sporulation
(also Horneck et al, 2008).

It is possible that life on Earth has developed independently more than once, and that some lifeforms rely on biochemical processes different from those already known. It is also possible that these "exotic" lifeforms persist on Earth today. If so, they may be difficult to detect (Paul Davies, December 2007).

Biochemical Alternatives

Many biologists believe that all Earth lifeforms that have been studied in detail thus far almost certainly descended from a common origin. Apparently, organisms found thus far on Earth share a similar biochemistry and rely on a highly similar genetic code (relying on DNA, which possibly developed from RNA and even earlier precursors) which enables biologists to sequence their genes and position on a single tree of life. Some (including Paul Davies), however, argue that the procedures used to analyze newly discovered organisms are too customized to detected life as already known to biologists and so lifeforms relying on alternative biochemical processes may commonly exist but escape detection.
Some alternative biochemistries include:
- Mirror life - In known lifeforms, large biologically important molecules have a strict orientation, where amino acids are left handed and DNA is a right-handed, double helix.
- Exotic amino acids - With rare exceptions, all familiar organisms found on Earth use the same 20 amino acides to build proteins, but chemists have synthesized many others (e.g., isovaline and pseudoleucine which also have been found in meteorites).
- H2O2 life - Hydrogen peroxide and water in intracellular fluid may help microbes survive in very cold and dry conditions, and some biologists speculate that NASA's Viking probes may have detected such life on Mars in the 1970s (see Life on Mars?).
- Arsenic life - Although poisonous to life that relies on phosphorus, arsenic can fill the biochemical role that phosphorus plays in known lifeforms because it mimics phosphorus so well. A team of scientists at the U.S. Geological Survey now believes that photosynthesis based on arsenate (using arsenic with four oxygen atoms attached as an energy source) evolved in Earth's arsenic-eating bacteria at the same time or even before "normal" photosynthesis, and that a similar mechanism may once have fuelled life on other planets -- i.e., Mars, Jupiter's moon Europa, etc. (more discussion from Nora Schultz, New Scientist, August 14, 2008; Tina Hesman Saey, Science News, August 14, 2008; and Oremland et al, 2008).
- Silicon life - More radically, alien lifeforms could be based on silicon instead of carbon because atoms of both elements contain four electrons in their outermost orbital, and so silicon atoms can also be arranged in rings and long chains for the backbones of biological molecules. However, silicon is less abundant in the universe and its structures are much less stable and much more reactive than carbon's, particularly in the presence of oxygen where it produces a solid (more discussion from the NASA Astrobiology Institute and astronomer David Darling).

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