We have no clue.
Those who claim that we do know how life arose have muddied the difference between science and speculation. Read anything on the topic and uncertainty terms like 'may,' 'could,' 'might,' 'it is possible,' and 'perhaps,' ... show up on a regular basis. We are continuously discovering more about cells and organisms - even the simplest life forms are more complicated than anything we imagined in decades past - just ask your anatomy teacher. But real scientists are optimistic about finding life on other planets.
Many think the argument is insurmountable - if we can accidentally synthesize amino acids, life isn't far behind. That's quite a leap of logic.
We really can trace DNA back and we're pretty sure we can genetically connect all the living organisms we have tested. Miller's experiment from 1953 and many since have shown us a lot about the synthesis of chemical compounds necessary for life as we understand it. We know a lot. But it is science hypothesis when scientists talk about the first form of life from which all of life here originated. We have difficulty proposing genetic pathways for the past creation of new cell machinery. We are trying to fill in the gigantic gaps by stitching together little bits of knowledge as we find them.
The author of this NOVA article writes "Harvard's Andy Knoll is content studying a puzzle that may never be solved: Exactly how did life on earth begin?" This HowStuffWorks.com article explains the minimum we need for life and it's a lot of complicated stuff. And here's another article that raises more questions than it answers on the topic
At least they're realistic.
Don't feel bad if you're a rabid defender of biology - Life is amazingly complicated. Physics has the same problems. From whence came the universe? How does gravity work? Is string theory real? Geology and climate science are no exceptions. Our best minds laboring for centuries sometimes find more questions than answers. That's the nature of the science beast.
Once we find exoplanets, however, we immediately ask if they can support life. What does an exoplanet need? What is necessary to incubate life? If we assume early earth conditions, Mars can't have life since it doesn't have oceans, or maybe Mars could hold its life deep within.
I found an oft-referenced article named "Requirements and Limits for Life in the Context of Exoplanets." There is a simpler version for the rest of us HERE.
Some are cautiously optimistic about the prospects of life on other planets like Carl Sagan from the original Cosmos series. Listen for Sagan's careful language of uncertainty. Watching science TV without a critical ear allows us to muddle that we know and that which is hypothesis.
Others think that time and cosmic accidents make us rare. Unique. The lone intelligent life in the Milky Way Galaxy. John Gribbin's Alone in the Universe makes the case that, although lower life forms may be common, higher life forms like us are ridiculously uncommon. Here is the Wall Street Journal book review.
Humans have existed for an infinitesimally small time compared to the age of the earth
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Lines too thick to show scale of human life
and we're pretty good at destroying ourselves. To expect to find other radio-capable societies in our galaxy during our time is a great improbability. Sorry. If we really are the sole intelligent life in the universe, space-based science fiction just got really boring.
We think that living planets need to be in the Goldilocks zone for the 'just right' temperature. But other moons or planets of various sizes with seas or heavy carbon dioxide cloud cover might work too. Many think life is possible in many places; even likely. It's interesting to imagine, exciting to think about, sobering if we are alone. We may never know.
Question the equation at 5:00
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