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Comment by throwaway894345 | Hacker News Reader

I hear this a lot, but I don't understand in what capacity it is true. Why can we say "Birds did not evolve from dinosaurs they are dinosaurs" but not "Humans did not evolve from fish they are fish"?

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Birds are part of what’s called the Dinosauria clade. Clades are monophyletic meaning they encompass all the descendants of a common ancestor and all share common traits.

Humans and fish on the other hand can’t be grouped into a clade. That’s because it you trace back the common ancestor you’ll find other monophyletic groups descending from those ancestors.

Think of a clade as the end of a branch and all its leaves... whereas if you go far enough down you’ll find other branches that shoot off in other directions far off, even though they’re in the same main trunk.

https://en.m.wikipedia.org/wiki/Clade

This seems like a rephrasing of the claim "birds are dinosaurs". "Birds are dinosaurs because birds are dinosaurs". Maybe the answer is "it's arbitrary; scientists decided that birds belong in the group 'dinosaurs' even though 'dinosaurs' and 'birds' are clearly distinct groups to everyone else"?

Well, they're distinct groups to scientists too. Nobody is saying dinosaur and bird are interchangeable terms. All birds are dinosaurs, not all dinosaurs were birds.

No one is saying that someone is saying that the terms are interchangeable :). The issue is that it’s not clear why “birds are dinosaurs” is correct but “birds evolved from dinosaurs” is not.

I don't think "birds evolved from dinosaurs" is incorrect per se, but it evokes a common misconception of what dinosaurs are, so you get people responding "birds are dinosaurs."

Consider by analogy "humans evolved from primates." Well ya, but humans also are primates.

I don’t think it’s a misconception at all, but rather science has its own definition and lay people have another. The definitions are useful to their respective camps. People who correct laypeople for using their definition are boring pedants.

I don't think that works. The lay persons definition is necessarily derived from the scientific one. "Dinosaur" wouldn't exist as a word or concept without scientific study.

Sorry to keep bringing up the same analogy, but do you also think it would be pedantic to correct "humans evolved from primates" to "humans are primates"?

The analogy doesn't apply because "primates" doesn't have a colloquial definition that excludes humans. "fish" or "reptiles" would be better examples. "Humans didn't evolve from reptiles; they are reptiles". I don't find it pertinent that the colloquial definition wouldn't exist without scientific study.

Fish and reptile aren't comparable terms to dinosaur. They started as colloquial terms and (as colloquially used) refer to groups based on traits, not phylogeny. Humans actually are not evolved from the phylogenetic group closest in content to "reptiles."

I guess you could argue the colloquial definition of dinosaur is similarly trait based, but even then... an ostrich is practically a small toothless T. rex.

I think the bird -> dinosaur relationship is more like human -> primate.

IIRC "fish" is kind of a fuzzy term. Like we would colloquially describe our common ancestor with frogs as a fish, but phylogenetically they would be closer to us than to some other "fish." Or something.

Humans can be classified as lobe-finned fish, so it wouldn't be wrong to say that humans are fish.

My question was more along the lines of "is it wrong to say 'Humans didn't evolve from fish because humans are fish'"? This is by all appearances the same argument the GP and others make when they say "Birds didn't evolve from dinosaurs; they are dinosaurs".

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