What common ancestry of all living things offers is the narrative satisfaction of continuity.
If you tell me that I have a common ancestor with the dump bear, the cloud of mosquitoes around the bear, and the patch of weeds he is lying in, what have you told me? That all life is connected in some way (I thought so, but the concept of common ancestry makes the idea more concrete).
An explanation is offered for the way I sense that the bear, the mosquitoes, the weeds and I all differ in the same general way from rocks and sand.
But now, the specific claim may or may not be true, even if the general intuition that makes it sound believable is.
One problem is that there is no remotely believable account of the origin of life:
So right at the beginning of the story, there is a problem. The dump bear and I and the other life forms have a common ancestor but no one knows what it is or how it came to be.
The difficulty with common ancestry of humans and chimpanzees is somewhat different. It’s the differences we need to account for, not the similarities. As I have said more than once, the claims about humans being genetically 99% chimpanzee merely demonstrate that genetics isn’t all it’s cracked up to be. When they get to 99.999ff identity, they’ll have proved that genetics is meaningless.
Hey, wait a minute.
What then becomes of the original claim about common ancestry (which sounds at least intuitively right)? Now I am being asked to believe things that sound intuitively wrong—they are unbelievable on the evidence and they cast doubt on what I take to be solid science.
Could one reason many thoughtful people doubt common ancestry be this lack of a coherent narrative? Thoughts? – O’Leary for News
PS: Common ancestry and evolution are different concepts. Picture a far-off galaxy in which there are two solar systems, in each of which life originated and evolved independently. Both systems have been evolving but they have no common ancestor.
Added: Just as evolution could occur without common ancestry, common ancestry could occur without evolution for a very long time. Suppose a species of fern is unchanged for 180 million years. It could have common ancestors with other ferns without having undergone any meaningful evolution.
See also: Picture that terrifies creationists?
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