Why is it thought that certain primitive fish diverged from vertebrates before the globin gene was first duplicated?

Question

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The fact that these fish have only one globin gene instead of multiple globin genes suggests that they diverged from other vertebrates before the first duplication of the globin gene. This is because the duplication of the globin gene is thought to have occurred later in vertebrate evolution and is responsible for the presence of multiple globin genes in most vertebrates today. Therefore, the absence of multiple globin genes in certain primitive fish indicates that these fish diverged from vertebrates before this gene duplication event.

Why Is It Thought That Certain Primitive Fish Diverged from Vertebrates