The cornerstone is actually the remains of two mockingbirds collected by Darwin himself in the Galápagos Islands. According to the BBC, differences in the specimens were the “‘catalyst’ for his transmutation theory—how one species changes into another.” They will go on display at London’s Natural History Museum next week, part of the Darwin200 celebration of Darwin’s birth.
“What is fantastic about these two birds is that visitors will be able to see for themselves the crucial differences that Darwin saw.”
Jo Cooper, bird curator at the museum, claimed, “What is fantastic about these two birds is that visitors will be able to see for themselves the crucial differences that Darwin saw.” Because Darwin “knew” from an earlier visit there was only one mockingbird species in South America, the differences in these specimens led him to conclude that “all mockingbirds in the world had descended from a common ancestor, because they shared a number of similarities with each other,” the BBC explains. The report adds, “This ultimately led Darwin to the conclusion that all organisms on Earth had common ancestors.”
Naturally, we beg to differ. Darwin extrapolated too far, since he didn’t understand what sort of adaptive changes (e.g., different beak lengths and shapes, and other information-depleting changes, which we observe in nature) are genetically possible and commonly occur within a kind and those which aren’t (e.g., entirely new anatomical structures and other information-adding changes we don’t observe).
As we’ve seen with the two items above, just because an animal population has undergone change (as in the case of the Lerista genus of skinks) or shares a common ancestor (as in the case of deep-sea octopi, possibly), this is not evidence that all forms of life are the result of changes from one common ancestor.
Here’s an analogy. Let’s say upon Shakespeare’s death, historians discovered in his estate a large box containing all his manuscripts. Now, some of the manuscripts are nearly identical with the exception of a few minor differences—these are actually manuscripts for the same play, but in different stages of editing—such as several copies of Romeo and Juliet with a few words changed in various places, or perhaps two scenes reversed. These are all descendants, in a sense, of the original rough draft (the “common ancestor”) of the play, and they “evolved” over time into the final product.
"All forms of life must share a common ancestor despite their differences."
In fact, several manuscripts for each of Shakespeare’s plays and poems are discovered. The historians organize them in piles—for instance, a pile of variants of Hamlet, and a pile of versions of Macbeth. Now what if one historian comes along, picks up two versions of Julius Caesar, and—based on the similarities between those two manuscripts—claims every single manuscript of play and poem in the collection are all variants of one original short story? Is it any less reasonable to think that King Lear and Shakespeare’s sonnets are variations on the same work than it is to think that fir trees and turkey vultures share a common ancestor?
Absurd as it seems, that was the logic and evidence behind Darwin’s idea: these two mockingbirds share a common ancestor despite their differences; thus, all forms of life must share a common ancestor despite their differences.
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