This is one of the problems with images of evolutionary trees. They indicate descent, but not time. How long did it take a lobe finned fish ancestor to make it to human (or any other mammals)? What other branches are along that line? I’m not a taxonomist, but I find these types of questions interesting. Somehow these diagrams appear to show direct lines, when in reality, it is much, much more complicated. Really - how much evidence do we have that lobe finned fishes are the ‘ancestors’ of mammals? Perhaps they were a dead end (Coelacanth), and perhaps mammals arose from another mechanism we do not know.
Just spit balling!
Well, there’s always the possibility that everything we think we know about evolutionary mechanisms and relatedness among organisms could be wrong. Science is always open to revision. But the new model would have to explain nature better than the current one does.
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I have a suspicion that confusion often arises from the definition of “more closely related to”. I think that most people probably think “more closely related to” means “shares more genetic information with”. As an example, ancestry websites will often let you sort matches by relatedness, measured in shared genetic information. You can also ask a question like “are you more closely related to your first cousin or your great-great-grandnephew” and see what kinds of answers you get.
However, in phylogenetics and evolutionary biology a different definition has been adopted, as was described above, and also in a Perspectives article in Science:
But what does it mean to be “more closely related”? Relatedness should be understood in terms of common ancestry—the more recently species share a common ancestor, the more closely related they are.
Following this definition, in the example above you are more closely related to your great-great-grandnephew because the common ancestor (your parents) is more recent than that with your first cousin (your grandparents). However, you are expected to share about 12.5% of your DNA with your first cousin and only about 3% with your great-great-grandnephew (see coefficient of relationship).
In short, I would argue that some of the blame is on the evolutionary biologists for assigning a definition to “more closely related” that doesn’t really match with how many people think.
(Yes, this conflates evolutionary trees with family trees, but that doesn’t change the point.)
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:) According to this site , Humans share a significant portion ( 60%) of their DNA with bananas.
https://thednatests.com/how-much-dna-do-humans-share-with-other-animals/
I don’t know how authoritative it is; still, I am surprisingly happy to claim kinship with bananas!
(Note: I copy pasted this from an old discussion about wolves and coyotes )
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We actually have quite a bit of fossil evidence of the period of time when a lineage of lobe-finned fishes evolved into the tetrapods. Similarly, we have a lot of evidence of the emergence of synapsids, and the therapsid lineage of synapsids that mammals are descended from. It turns out that a casual interest in evolution and the fossil record isn’t enough to really comprehend how much we have learned – every time I look into some group I’ve not paid enough attention to, it’s amazing how much there is I’ve never heard of. I agree that these trees hide a huge number of complications, but that’s for two reasons – first, because they wouldn’t be as useful if we tried to put them in, and second, because despite how much we know, there’ll always be a lot of the details that are truly lost to time.
I would take a bit of exception to the term “dead end” – I know some biologists have used it, but it’s problematic. It assumes that there’s a goal to reach – the sort of teleological thinking we have to constantly watch out for. Our two species of Coelecanth didn’t stop evolving - they’re modern species, still evolving along their own lineages.
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Yes, I agree with your objection of the term “dead end” - I should have known better than to use it. Evolution has no goal. This is something I firmly believe in. The ‘success’ of humans is no more of value than the success of Coelacanths or Horseshoe Crabs. We value mammalian evolution because it is the life most like us. As we move ‘further away’ the less important - to humans - it becomes to us. It’s one of the beefs I have with Animal Rights philosophy (although I am not up to date on any new developments). Sentience, and the ability to feel ‘pain’ takes great importance, where non sentient life is somewhat dismissed as unimportant.
I agree with this too. I guess what I was trying to convey was that most diagrams don’t give the ‘distance’ between ancestors and modern organisms. Multicellular life took a long time to evolve from single cell life, but this is not adequately reflected in the diagrams we see.
I view such simple trees as more like an electrical schematic diagram — not to scale but showing the important pathways and junctures.
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As far as I remember, Reptiles are a paraphyletic group. That means they do not have a common ancestor. Therefore, Pangolins cannot be related to Reptiles.
It is for sure paraphyletic, because birds are not included in commonly used taxonomy, but that doesn’t mean group is polyphelitic and doesn’t have one ancestral group.
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The example with teleost fishes is a complicated one as there was a fish specific genome duplication (3R) which could have an influence on the whole phylogeny.
This tree you showed here simply might not reflect the real relationships meaning that real position of the coelacanths and “fishes” might actually look different.
Right. If you include birds within Reptilia it should be a monophyletic group based on what we know.
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The concepts are a little complicated and I have to double check at times if I’m using them correctly.
One way to think of it is that a lineage or a clade is a shrub. If the shrub is monophyletic it includes the root base and all the branches. In other words it encompasses the whole plant.
If it’s polyphyletic, there are two or more root bases but the branches are all tangled together, appearing to be one plant. It may be hard to tell there is more than one shrub present.
If it’s paraphyletic, some of the branches in the shrub are recognized as something different (a separate shrub) even though they are really part of the same plant.
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Good analogy. If I understand correctly, would the Phylum Arthropoda be paraphyletic? I have always believed (not based on any extensive research) that it was polyphyletic.
Don’t know. My understanding is that there are competing hypotheses about what should be included or excluded in Arthropoda or Panarthropoda. Potentially the group could be both polyphyletic and paraphyletic depending on how it’s defined.
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If you’re willing to play with your sense of scale, any polyphyletic group could be considered paraphyletic, since it’s generally agreed that a universal common ancestor existed.
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That’s true. If you go back to the original life form, everything that came along later is monophyletic. Just one enormous diverse clade called Life.
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What is the most current view? We were taught insects came from crustaceans and chelicerata were a separate group with probably separate ancestor, don’t remember if myriapods were close with first group, only that in old views they were close to insects (I slept through most of invertebrate lectures in university 
).
I had almost no course work in invertebrates — sad to say — so I’m pretty ignorant about classification schemes for these animals.
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That depends on what version of “Arthropoda” you’re referring to.
If by “Arthopoda”, you mean Mandibulata + Chelicerata (aka Euarthropoda, including ants, crabs, centipedes, and spiders), then no, it is actually a natural monophyletic group. Generally, this is the most common definition of “Arthopoda” used today, and this is also the definition used on iNat. Euarthropoda is usually classified along with Tardigrada and Onychophora into a larger clade called Panarthropoda, but this group is sometimes alternatively called Arthropoda sensu latu. This group is also most likely monophyletic, but a minority of studies do suggest that Tardigrades may be closer to Nematode roundworms.
However, technically the original usage of the term “Arthropoda” from 1848 included both Euarthropods and Tardigrades, but not Onychophorans (or even Pentastomids, which are a type of Euarthropod), which would make it paraphyletic, because Euarthropods are closer to Onychophorans than to Tardigrades, according to most studies.
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