Anyway that could emply that all life on Earth has the same designer.
One could argue that a "creator" possibly intervened to ignite the first life form on the planet, however there are a great deal of facts that would have to be explained away that would not well support a "designer" using a common theme so to speak to have created multiple species simultaneously.
You would first have to explain away the fact that all organisms harbor a lot of "junk" DNA that has long ago ceased to express. Left over baggage from times past.
You would have to explain away why a creator would design things with flaws or inferior traits. A great example of this is the eye. For one, the human eye from a bio-mechanical perspective is very poorly designed. The receptors are facing the wrong way, upside down, and the way the ocular nerve is connected creates a large blind spot that is only filtered out by our brains. A common flaw throughout all vertebrates. You would have to explain why there are over 50 genetically distinct ocular strains across various species branches, some performing much better than others in regards to focus, ability to sense color, ability to sense a broader light spectrum, etc.
You would have to explain away the genetic breadcrumbs laid down in our DNA that outlined the branches of the phylogenetic tree that record the trail of speciation.
You would have to explain away how there is very little correlation between the size and complexity of a species DNA sequence and the actual observed sophistication of the species. A Japanese flower Paris japonica has 49 billion base pairs (roughly 50 times the size of the human genome).
You would have to explain why all life on this planet only uses left handed amino acids to build proteins.
You would have to explain away evolution in general, which is demonstrated in our DNA, the fossil record, radiometric dating in conjunction with plate tectonics and geological stratification. The mechanism that forms our most fundamental understanding of biochemistry and the processes of organic life, which without that understanding our current technical advances in bio-engineering and medicine would not even be possible.
It all boils down to Occam s Razor really.
We can speculate all day whether we were "seeded" by aliens, but lets face it, all that does is shift the problem. Something would have had to occur for their start at some place and some time. It's a fun idea to entertain. Certainly plausible, but there is no compelling reason to believe it over other theories outside of personal preference.
As far as cave paintings of aliens, I've seen many. The problem with those is the same with interpreting any piece of art from any time. You tend to to see what you are looking for. Some might see helmets. Others might see a tribal headdress or mask. Where one imagines a spaceship, another might see only a canoe. Makes for good fodder for science fictions novels, but not compelling evidence that we are being visited.
...oh wait, don't they do that already?
