There is a problem with even believing that life on Earth is native or unique to Earth, considering the Earth formed from Solar leftovers that accreted from that stellar matter, which is itself leftovers of a stellar nursery of gas and dust of the Milky Way. It's possible that the life we know as "terrestrial" indeed began forming on Earth, but when you consider the source of materials...it's not so black and white.
The very words "alien life" are misleading and imply the Earth is some sort of closed system. It certainly is not, and those terms really come from a pre-space exploration (1950s etc) mentality when the Earth was one thing and the rest of space was something else.
It's really all the same ****, to put it bluntly.
Sugar formation, as with much of the complex molecular chemistry in space, occurs in the solid phase on or in a variety of ices that coat interstellar grains. Most land-based chemistry occurs in the liquid phase, primarily with water, thus the widely accepted theory that life on Earth began in a "primordial soup." While these two chemistry scenarios are very different, the end results can be very similar. The possibility of early impacts from comets bringing both the water supply and the sugars and other compounds that are life's building blocks to the Earth's surface has been a scientific theory for years. Once those sugars were free from their interstellar freezer and were steeped in the Earth's warm waters, they could have easily combined with other carbon compounds and eventually formed RNA and DNA.
I always thought that they were some kind of sea urchin.
My intent was to address OwlMatt's belief that aliens are not real. I subscribe to W. Rabbit's ideas about it all being the same ****. Rabbit's pretty smart when he talks science. It's his comfort zone. If only he would stop talking about other stuff.
I should have been clearer. I never meant to say that it is impossible that there is other life somewhere in the universe, only that I am dismissive of stories about extraterrestrial life coming to Earth.