Not really true, since it is obvious that all (known about) life on earth shares a common ancestor.  The only logical  conclusion is that this ancestor came about under circumstances we don't yet understand.  We have "experimental proof" that life began on its own, with a sample size of 1.  That is decidedly different from "no scientific evidence."

Assuming anything else is assuming more than is necessary.  Invisible pink elephants (not the visible ones) might be responsible, but the most logical assumption is that some natural phenomenon is responsible.

There are people looking for other "shadow" trees of life not related to our own, but all evidence so far seems to point to only one tree of life.

I don't quite understand people wanting to shit on the idea that life can come from non-life.  The only reason for this is because of a religious doctrine, but if God created the entire universe then surely he is by definition also the creator of life, right?  They go on and on about "irreducible complexity" while the entire universe is there yelling "and what do you call this, then!?"