[alconservative]

It appears that the universe is billions of years old and that evolution (in a general sense) happens, but how does it appear that all terrestrial life evolved from a common ancestor? Where is the actual evidence for this? It could very well be that there was more than one starting life form, or maybe just one, we simply don't know. I think the assumption of a single common ancestor just comes from a materialistic philosophy, not from any actual evidence.

[Antonello]

It appears that the universe is billions of years old and that evolution (in a general sense) happens, but how does it appear that all terrestrial life evolved from a common ancestor? Where is the actual evidence for this? It could very well be that there was more than one starting life form, or maybe just one, we simply don't know. I think the assumption of a single common ancestor just comes from a materialistic philosophy, not from any actual evidence.

DNA evidence, at least, supports it.
From PBS.org:

	3. Are all species related?
	Yes. Just as the tree of life illustrates, all organisms, both living and extinct, are related. Every branch of the tree represents a species, and every fork separating one species from another represents the common ancestor shared by these species. While the tree's countless forks and far-reaching branches clearly show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common ancestor from some point in evolutionary history. For example, scientists estimate that the common ancestor shared by humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor, but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although humans share far more genetic material with our fellow primates than we do with single-celled organisms, we still have more than 200 genes in common with bacteria. (Emphasis added) It is important to realize that describing organisms as relatives does not mean that one of those organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of any other living species. A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.

[AntiGuv]

It could very well be that there was more than one starting life form, or maybe just one, we simply don't know. I think the assumption of a single common ancestor just comes from a materialistic philosophy, not from any actual evidence.

I don't have time to give a more complete answer right at the moment, but the position that I tend to favor is that the last universal common ancestor was an ancestral gene pool - i.e., multiple discrete organisms that shared information via lateral gene transfer, parasitic insertion, etc.