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Say you have your family tree for the last 400 million years or so, back to the Devonian, the age of fishes. You take the set of ancestors that are about 400 million years old (which will be across multiple generations). The question is are they all fish?

Or, is it more complex than that? Can there somehow be non-fish in that set?

Note added later: this question is not a duplicate of another question. In the context of that other question, howewever, my question can be stated as "Is Dawkins correct in stating that my 185th million ancestor was definitely a fish, with no exceptions?"

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Yes they all were because there weren't any land vertebrates, vertebrates were all fish at 400 million years ago.

It is a close call based on fossil evidence from 395 million years ago, there are footprints of 3 meter tetrapods from littoral/seaside fossils. https://www.sciencedaily.com/releases/2010/01/100107114420.htm

The earliest land vertebrate skeleton is from 375 million years ago. https://en.wikipedia.org/wiki/Tiktaalik

enter image description here

It is technically still a fish, so probably yes, the ancestral species of all land vertebrates were still fish. There are various experimental early quadrupeds, some with 6 fingers, which are cool reading.

They still exist in the form of lungfish.

Here are images of the earliest land vertebrates

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  • $\begingroup$ It is worth noting the problems with the term "fish" . Depending on how you use the term humans can also be fish. $\endgroup$ – John Oct 11 at 2:59

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