QUESTION = Why wouldn't life on another habitable planet look similar to Earth's?

GENERATION = There's really no point in explaining why life shouldn't look very similar on another planet. All we know is that it should, and that we haven't found any evidence of it. The obvious answer is that there *isn't* a reason to. Even if there were, it would be unlikely that alien life would evolve to look more similar to Earth - that's just not how biology works, and in fact we don't even have enough data to say what "right" is. The only thing we do know is that there is no reason to expect life to look that way, so it would probably stay that way. There are plenty of other kinds of life that look just like Earth, and even if those other kinds didn't, there's no reason to assume they wouldn't look similar to Earth. We've also got no clue whether life *as we know it* could evolve in a new environment, with entirely different lifeforms. It's just a matter of *how* we know it works. In our present universe, every other natural lifeform *we know of* operates the same way - gravity is what makes planets form. 

RETRIEVAL #0 = Astrobiology---other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe of organic macromolecules. Indeed, it seems likely that the basic building blocks of life anywhere will be similar those on Earth, in the generality if not in the detail. Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique. If life has had a comparable impact elsewhere in the Solar System, the relative abundances of chemicals key for its survival – whatever they may becould betray its presence. Whatever extraterrestrial life may be, its tendency to chemically alter its environment might just give it away. Section::::Life in the Solar System. People have long speculated about the possibility of life in settings other than Earth, however, speculation on the nature of life elsewhere often has paid little heed to constraints imposed by the nature of biochemistry. The likelihood that life throughout the universe is probably carbon-based is suggested by the fact that carbon is one of the most abundant of the higher elements. Only two of the natural atoms, carbon and silicon, are known to serve as the backbones of molecules sufficiently large to carry biological information. As the structural basis for life, one of carbon's important features is that unlike 

RETRIEVAL #1 = Flammarion (Martian crater)---places, especially craters. This is exactly what would appear if a large lake had slowly evaporated. Moreover, since some layers contain gypsum, a sulfate which forms in relatively fresh water, life could have formed in some craters. 

RETRIEVAL #2 = Superhabitable planet---compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere probably prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet. Also, it has a higher concentration of clouds, and abundant rainfall. Probably the vegetation is very different due to the increased air density, precipitation, temperature, and stellar flux. For the type of light emitted from the K-type stars, plants may take other colors than green. The vegetation would cover more regions than vegetation here on Earth, making this visible from space. In general, the climate of a superhabitable planet would be warmer, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences, in contrast to Earth that has inhospitable areas such as glaciers, deserts and tropical regions. If the atmosphere contains enough molecular oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases, but would require some adaptation to the increased gravity, such as an increase in muscles and in bone density, etc. Section::::Abundance. Heller and Armstrong speculate that the number of superhabitable planets can far exceed that of Earth analogs: less massive stars in the main sequence are more abundant than the larger and brighter stars, so 

RETRIEVAL #3 = Circumstellar habitable zone---conservative habitable zone. Geothermal energy sustains ancient circumvental ecosystems, supporting large complex life forms such as "Riftia pachyptila". Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone. Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the "Milnesium tardigradum", which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. In addition, the plants "Rhizocarpon geographicum" and "Xanthoria elegans" have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. The fungi "Cryomyces antarcticus" and "Cryomyces minteri" are also able to survive and reproduce in Mars-like conditions. Species, including humans, known to possess animal cognition require large amounts of energy, and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earth analogs in the CHZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen 

RETRIEVAL #4 = Ramaria---shown that "Ramaria" is not monophyletic, and that the characteristic coralloid shape has likely evolved several times from different ancestors. Section::::Species. BULLET::::- "R. abietina" BULLET::::- "R. acrisiccescens" BULLET::::- "R. acutissima" BULLET::::- "R. aenea" BULLET::::- "R. africana" BULLET::::- "R. albidoflava" BULLET::::- "R. albocinerea" BULLET::::- "R. alborosea" BULLET::::- "R. altaica" BULLET::::- "R. ambigua" BULLET::::- "R. americana" BULLET::::- "R. amyloidea" BULLET::::- "R. anisata" BULLET::::- "R. anziana" BULLET::::- "R. apiahyna" BULLET::::- "R. apiculata" BULLET::::- "R. araiospora" BULLET::::- "R. arcosuensis" BULLET::::- "R. argentea" BULLET:::: 

RETRIEVAL #5 = Faint young Sun paradox---deposits. A primary sink for carbon in the Earth atmosphere is the carbonate-silicate cycle. It is however hard for CO to build up in the Martian atmosphere in this way because it would likely condense out before reaching partial pressures necessary to produce a sufficient greenhouse effect. An alternative possible explanation posits intermittent bursts of powerful greenhouse gases, like methane. Carbon dioxide alone, even at a pressure far higher than the current one, cannot explain temperatures required for presence of liquid water on early Mars. Section::::On other planets.:Venus. Venus's atmosphere is composed of 96% carbon dioxide, and during this time, billions of years ago, when the Sun was 25 to 30% dimmer Venus's surface temperature could have been much cooler, and its climate could have resembled current Earth's, complete a hydrological cycle – before it experienced a runaway greenhouse effect. Section::::See also. BULLET::::- Cool early Earth BULLET::::- Effective temperature – of a planet, dependent on reflectivity of its surface and clouds. BULLET::::- Isua greenstone belt BULLET::::- Paleoclimatology BULLET::::- Snowball Earth BULLET::::- Carbonate–silicate cycle BULLET::::- Gaia Hypothesis 

RETRIEVAL #6 = Interplanetary contamination---continent to another may be able to outcompete the native life adapted to that continent. Additionally, evolutionary processes on Earth might have developed biological pathways different from extraterrestrial organisms, and so may be able to out-compete it. The same is also possible the other way around for back contamination introduced to Earth's biosphere. In addition of science concerns, ethical or moral issues have also been raised on accidental and intentional interplanetary transport of life. Section::::Evidence for possible habitats outside Earth. Enceladus and Europa show the best evidence for current habitats, mainly due to the possibility of their hosting liquid water and organic compounds. Section::::Evidence for possible habitats outside Earth.:Mars. There is ample evidence to suggest that Mars once offered habitable conditions for microbial life. It is therefore possible that microbial life may have existed on Mars, although no evidence has been found. It is thought that many bacterial spores (endospores) from Earth were transported on Mars spacecraft. Some may be protected within Martian rovers and landers on the shallow surface of the planet. In that sense, Mars may have already been interplanetarily contaminated. Certain lichens from the arctic permafrost are able to photosynthesize and grow in the absence of any liquid water, simply by using the humidity from the atmosphere. They are also highly tolerant of UV radiation