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I'm just curious - it seems generally accepted that birds are dinosaurs. If that's the case, then can we call all mammals reptiles? Can we go even further and call humans amphibians? And since single-celled bacteria were the precursor of all life, isn't just anything a bacterium?

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    $\begingroup$ The simple answer is: Yes (in the sense that one is descendent of the other). More complex answer would involve the definitions of paraphyletic, monophyletic etc., which are more or less arbitrary names given by consensus of the scientific community (in a way, like if pluto is a planet or a dwarf planet). Doesn't change any facts, just the names we call it. (And the school marks you get for hitting recently changed keywords.) $\endgroup$ – Gyro Gearloose Mar 2 '16 at 20:28
  • $\begingroup$ Can't cite it, but I've heard of recent doubt if mammals are descendants of dinosaurs or rather of some other reptiles. $\endgroup$ – Gyro Gearloose Mar 2 '16 at 20:34
  • $\begingroup$ You might want to have a look at If dinosaurs could have feathers, would they still be reptiles?. $\endgroup$ – Remi.b Mar 2 '16 at 21:16
  • $\begingroup$ @GyroGearloose The simple answer is "no". Birds are not dinosaurs, but the descendants of dinosaurs after at least 65 million years of evolution. This distinction is used so that we all know what each other are talking about. I'm voting to close the question because at the minute the question is a mixture of semantics, rhetoric, and reductio ad absurdum; it's subjective. $\endgroup$ – James Mar 3 '16 at 2:52
  • $\begingroup$ Why would birds being dinosaurs mean mammals are reptiles?? $\endgroup$ – rg255 Mar 3 '16 at 10:50
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Reptile is not a clade - it is a paraphyletic group because it does not contain all descendants (i.e. birds) of a common ancestor. Humans are amniotes like birds and reptiles but while birds and reptiles are sauropsids, mammals are synapsids. Humans are not amphibians either because amphibia is another paraphyletic group which is distinct from amniotes. Possibly some amphibians and amniotes share a common ancestor. Amphibians, sauropsids and synapsids can be grouped together under tetrapoda. So both humans and frogs are tetrapods but humans are not amphibians.

However, birds are dinosaurs and belong to a very specific sub-clade of dinosaurs called theropods. Both dinosauria and theropoda are monophyletic groups - i.e. they contain all the descendants of a common ancestor.

                        

Extending this to bacteria. Yes you would group all living things (eukaryotes and prokaryotes) and can call them cellular living organisms. However, animals (or any other eukaryotes) are not bacteria (they have diverged a lot). Moreover, extant bacteria are not precursors for all life. The precursor is called Last Universal Common Ancestor but we don't know much about it (at the moment it is a hypothetical organism). Eukaryotic evolution itself is a big mystery. So it would be preposterous to call all lifeforms as bacteria.

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  • $\begingroup$ Well Reptilia and Reptiliomorpha are clades (which contain mammals if I am not mistaken). $\endgroup$ – Remi.b Apr 2 at 20:34

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