First, the suggestion of common ancestry is taking similarities and concluding that life came from one single life form of long long ago. There is no science for that, only a series of guesses and suppositions.
We don't need to run, we can walk first.
Suppose I showed you DNA evidence that demonstrated the genetic similarities you have with other humans. Suppose it's a paternity court case of "Who's the daddy?" in which you're skeptical a girl is actually yours. You were with the mother around a year prior to when the child was born so the math works out.,
In this we can observe, since DNA is hereditary based, the shared genetic markers between both parents in the offspring to conclude their relations. We can see the shared genes the girl has with you, resulting in a 99.99% likely match that you're the father.
This is all a hypothetical example so bear that in mind. Suppose we find that after analyses it shows you're the father. Would you doubt the results?
Now, the same type of genetic analysis can be used further out across all humans showing greater degrees of relatedness.
https://genographic.nationalgeographic.com/human-journey/
The same further can be used for extant living organisms to determine how closely they are related to each other and other species (showing common ancestry).
Phylogenetic Trees and Monophyletic Groups | Learn Science at Scitable
The same nested hierarchy that is created is evident in genetics (genetic phylogeny trees) as it is in Cladistics.
Secondly, and in my opinion more importantly, is the question of how indescribably complex and varied life today was created. The answer is, nobody knows, it all depends on one's faith-based creationist view. A single life form from which all plants and animals were created is only a supposition, a guess.
It's not a guess when you have a preponderance of empirical evidence that is consistent and points in one direction and only one direction.
The conclusion is that the creationist worldview taught in schools today, and it is a creationist theory, is based on less than concrete evidence.
Suppose I ask, what evidence would you expect to see if common ancestry is true?
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