Is universal common descent a scientific fact?

Here's where you're going horribly wrong.

I suggest you take this all in.

http://en.wikipedia.org/wiki/Observation

You don't have to observe the actual event to know it happened. You can observe the evidence that it leaves behind. That still constitutes observable fact.

How would murderers ever be convicted if we had to see them in the act?

We convict them based on evidence.

And BTW, the first "cell" evolved about 1 billion years after the first self-assembling replicating molecule, which is where it really began.

Edit;

Oh, the Cambrian explosion thing again....

You mean the "sudden" appearance of many (but far from all) body plans that took place over more than 50 million years. That the one?

And the one that showed evolution of the life forms that pre-dated it by 40 million years, the Edicaran Fauna?

Is that the "explosion" you mean?

Edit 2.

Yes, it's a fact. If it weren't it would not be the case that all, and I mean ALL, life on earth uses adenine, thymine, guanine and cytosine in DNA, (+ Uracil in RNA) and the same 20 amino acids; alanine, arginine, argasparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, hisisoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine, to make protiens, when thousands of similat chemical compounds could do the same job.

It's a fact.

Get over it.

Edit 3.

You didn't ask a question about the Cambrian explosion. You made an assertion. An assertion that shown you know very little about the Cambrian explosion.

Yes. And we've observed common decent through the fossil record, and in DNA, and in common structures.

It's still a fact.

So, we need to prove every case, even though many other cases are well-documented? No, science doesn't work like that.

The Precambrian explosion did not have those body parts in most animals. Trilobites, for instance.

“The ancient allegories in Genesis 1 - 11 were never meant to be read as exact histories, and certainly never as science texts. This has been well documented by respected theologians such as Augustine and Thomas Aquinas. The overwhelming majority of Christians (and Jews) know that the earliest books of Genesis were handed down from generation to generation by oral tradition, before the Hebrew people developed a system of Phoenician writing, around 1000 BCE.

The American fundamentalist movement, is a fairly recent development, and goes against all true Biblical scholarship. Those who profess to take the Old Testament "literally" without even bothering to research the Exegesis and Hermeneutics only publicly display their ignorance, and manage to make all Christians appear delusional in the process.” Mariel

It's more probable than saying "God did it."