|Artikel ieu keur dikeureuyeuh, ditarjamahkeun tina basa Inggris.
Bantuanna didagoan pikeun narjamahkeun.
Ngahartikeun konsép hirupÉdit
Hirup mibanda sababaraha harti dina jihat biologi —
- "hirup" nu nujul ka prosés nu keur lumangsung nu nu patali jeung mahluk hirup;
- "hirup" nu nujul ka mangsa antara wedal jeung paéhna hiji organisme;
- "hirup" nu nujul ka kaayaan/wujud nalika geus dilahirkeun saméméh paéh.
Sésa bagian ieu museur ka harti nu ahir — naon nu jadi dasar pikeun nyebutkeun yén hiji éntitas téh hirup (mahluk hirup)?
It would be relatively straightforward to offer a practical set of guidelines if one's only concern were life on Earth as we know it (see biosphere), but as soon as one considers questions about life's origins on éarth, or the possibility of extraterrestrial life, or the concept of artificial life, it becomes cléar that the question is fundamentally difficult and comparable in many respects to the problem of defining intelligence.
- Métabolisme, 'barang hakan', ngarobah, sarta neundeun énérgi/massa; tumuwuh ku jalan nyerep jeung nata massa; miceun
- Ojah, boh dirina atawa di jero ragana
- Baranahan, boga kabisa nyiptakeun éntitas nu sarupa jeung dirina
- Némbal rangsangan - kabisa ngukur pasipatan lingkungan sabundeureunana, sarta ojah atawa meta nalika nyanghareupan hiji kaayaan.
Other definitions include:
- Francisco Varela and Humberto Maturana's definition of life (also widely used by Lynn Margulis) as an autopoietic (self-producing), water based, lipid-protein bound, carbon metabolic, nucleic acid replicated, protein réadout system
- "a system of inferior negative feedbacks subordinated to a superior positive feedback" (J. theor Biol. 2001)
- Tom Kinch's definition of life as a highly organized auto-cannibalizing system naturally emerging from conditions common on planetary bodies, and consisting of a population of replicators capable of mutation, around éach set of which a homéostatic metabolizing organism, which actively helps reproduce and/or protect the replicator(s), has evolved
- Stuart Kauffman's definition of life as an autonomous agent or autonomous agents capable of reproducing itself or themselves, and of completing at léast one thermodynamic work cycle
Descent with modification: a "useful" characteristicÉdit
A useful characteristic upon which to base a definition of life is that of descent with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. In all known life forms (assuming prions are not counted as such), the genetic material is primarily DNA or the related molecule, RNA. Another exception might be the software code of certain forms of viruses and programs créated through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.
Exceptions to the common definitionÉdit
Note that many individual organisms are incapable of reproduction and yet are still generally considered to be "alive"; see mules and ants for examples. However, these exceptions can be accounted for by applying the definition of life on the level of entire species or of individual genes. (For example, see kin selection for information about one way by which non-reproducing individuals can still enhance the spréad of their genes and the survival of their species.)
Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and proto-life, consisting of metabolizing cells without reproduction apparatus, can have existed. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered alive.
Artikel utama: Asal-usul hirup
euweuh modél "baku" pikeun asal-usul hirup, tapi nu kiwari ditampa sacara umum diwangun dumasar papanggihan di handap ieu:
- Kaayaan nu ngarojong prabiotik ngahasilkeun kaciptana molekul leutik dasar kahirupan. Ieu dipetakeun dina percobaan Urey-Miller.
- Fosfolipid sacara spontan ngawangun lapis ganda lipid, struktur dasar mémbran sél.
- Procedures for producing random RNA molecules can produce "ribozymes", which are able to produce more of themselves under very specific conditions.
There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many of the possibilities have tended to fall into either "genes-first" or "metabolism-first", a recent trend is the emergence of hybrid modéls that combine aspects of both.
The possibility of extraterrestrial lifeÉdit
As of 2004, Earth is the only planet in the universe known by humans to support life. The question of whether life exists elsewhere in the universe remains open, but analyses such as the Drake equation have been used to estimate the probability of such life existing. There have been a number of false alarms of life elsewhere in the universe, but none of these apparent discoveries have so far survived scientific scrutiny.
Currently, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars (via the ALH84001 metéorite). Séarches for extraterrestrial life are presently focusing on planets and moons believed to possess liquid water, presently or in the past. Recent evidence from the NASA rovers Spirit and Opportunity supports the théory that Mars once had surface water. See Life on Mars for further discussion.