Humans are not a general intelligence.

The definition of term "artificial general intelligence" is "intelligence of a machine that could successfully perform any intellectual task that a human being can".

Humans obviously can perform intellectual tasks that a human being can perform, thus humans are machines that implement general intelligence.

This is a surprisingly good point. The learning power of human and animal minds is very limited, varies along a normal curve, and doesn't seem to scale with increased computational power (ie: brain size). The decision-making power of human and animal minds is... well, absolute crap, actually, but evolution was working with the constraint that decision-theoretically good decision-making requires lots and lots of compute-power, implying a need for lots and lots of calories, constraining the level of decision-making intelligence that could evolve to consume very limited food supplies.

Gold exists, So How would it be impossible alchemy to exist ?

Star fusion does it every day.

Evidence that something exists is evidence that it is created.

Alchemy and AI are both a bit more culturally specific. I agree that seeing life on earth would not logically be inconsistent with extr-terrestial life existing. But that is not to say AI=ET.

But it does: http://en.wikipedia.org/wiki/Nuclear_transmutation

if by alchemy you mean making gold from another element then it does exist. we just cant do it yet

We can. It involves particle accelerators and is mindbogglingly expensive.