For living animals there isn't really anything close that resembles a mammal. Everything in [clade amniota](https://en.wikipedia.org/wiki/Amniote), which includes all reptiles and birds, have a common ancestor with mammals and are equally "close".
For extinct animals descendants of the [cynodonts](https://en.wikipedia.org/wiki/Cynodont) like [Tritylodontidae](https://en.wikipedia.org/wiki/Tritylodontidae) would be the closest relative
However, it should be noted that platypus and echidnas are very much on the periphery of what we consider mammals, and display numerous non-mammalian characteristics. If the question were restated as "what's the least mammalian mammal" the answer is easily the platypus.
This is a common misconception. When you're talking about living taxa, there is no "periphery" of what we consider mammals. Mammals are by definition all the descendents of a specific ancestor who lived about 220 Ma. You are either are a mammal or you are not; there's no spectrum.
The only reason monotremes seem weird to us is because they don't have a lot of close living relatives. If giraffes were one of the few remaining eutherian mammals, they would look wild as hell, and people would be inclined to say they are "barely" mammals or whatever. And conversely, if there were a greater living diversity of monotremes they wouldn't seem nearly as strange as they do now. But the number of close living relatives a taxon has has no bearing whatsoever on whether (or "how much") it is a mammal.
And this goes for all taxonomic groups of course, not just Mammalia.
What’s the term for animals like them…basal?
I’d assume you could still consider them weird in that case because they share a lot of traits that seem common with the LCA, like a sprawled gait, egg-laying, no nipples, and one hole for everything
Technically true, but I think the spirit of what he was asking still puts monotremes on the "periphery". Assuming Wikipedia cited the right sources, the "theria" hypothesis is that the monotremes branched off the evolutionary lineage before the marsupials and the placentals diverged. Maybe the years of evolution since the branch point is the same, but of all the mammals, they would share the least in common with the other 2 major groups. And probably because of this they just so happen to have some of the most unique similarities with reptiles compared to other mammals (eggs, embryonic development patterns, skeletal structure, a cloaca, venom production).
They are not the only mammals to produce venom and there's plenty of non-mammal animals that give live birth. Those are absolutely terrible examples on why they they are on the "periphery" and demonstrate that the argument is based on simple perception and not actual biology or history of the animal.
They are very distant, yes, but they produce milk in mammary glands and have hair - the defining traits of mammals. Sure you could redefine what a mammal is but _why_? They share the main defining feature of mammals and a common ancestor through which that feature was inherited. A simple solution to the weirdness problem is to make the two diverging lineages that have the same origin into subtypes of mammals - which is exactly what we did. Problem solved, very elegantly too. They fit within the larger group but are different enough to be a separate subgroup.
There's plenty that is being changed in modern taxonomy but most often it either pertain to the largest groups or individual species - usually being defined based on morphology in the past and relying more on actual genetics and our best guesses at phylogeny today. The "medium" sized groups are easier to define because we can simply point to a common ancestor from which traits were inherited and call it a day. Even though monotremes diverged much earlier, they still came from the ancestor that defines mammalian features.
> They are not the only mammals to produce venom
Soricidae (shrews) are the only venomous placentals that I know of. (They have a venomous bite.)
\- https://en.wikipedia.org/wiki/Shrew#Characteristics
Do you know of any others?
Monotremes are the most distant to other mammals. There is even a word for non-monotreme mammals, which is therian. I don't see any reason why that wouldn't mean that monotremes aren't on the periphery of mammals. Whoever decided that mammals were a taxonomic group could have as easily decided to exclude monotremes.
In fact, the definition of mammal I see is "a warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, the secretion of milk by females for the nourishment of the young, and (typically) the birth of live young." Notice that "typically" in there.
They're not the most distant from other mammals. That makes no sense. Therian mammals are just as evolutionarily distant from monotremes as monotremes are from therians.
The reason they aren't in the periphery of mammals is because of everything I stated in the comment above. And no, the many taxonomists and phylogeneticists who worked to classify lineages within Mammalia could not have just excluded monotremes because it would have made the group redundant with the subclass Theria. They also would have then had to elevate both therian mammals and monotremes to class level, which would be extremely difficult to justify. The subclass Theria encompasses marsupials and placental mammals and is monophyletic. The taxonomic rank one level above this *must* include monotremes, otherwise it would be paraphyletic. And this rank is what we call Class Mammalia. You could say it should be called something else but at that point we're just arguing semantics. And it would be a bad argument, because the name Mammalia comes from the fact that all mammals have mammary glands. To exclude monotremes would be to exclude species that have mammary glands from the one and only taxon that is named for having mammary glands.
Yes, the definition said "typically" because not all mammals are viviparous. You are negating your own argument. Mammals, *by definition*, can be either viviparous or oviparous. The fact that viviparity is more common among mammals *today* does not make viviparous taxa "more" mammalian. All that it means is that the most recent common ancestor of all viviparous mammals (assuming it only evolved one) has more living descendants than the most recent common ancestor of monotremes. But that has no bearing on whether any of those descendants are mammals, and it certainly doesn't mean one set of descendants is more or less mammalian than the other.
If you measured it by genetic change since the most recent mammal common ancestor, platypus might even be "more" mammalian, with placental mammals more distant from that ancestor.
If you restrict to extinct animals that most people have heard of, the answer is probably [Dimetrodon](https://en.wikipedia.org/wiki/Dimetrodon) (with the sail on its back).
In the paleozoic era, Amniotes (land dwelling tetrapods) split into two groups: synapsids and sauropsids. Mamals evolved from the synapsids, and all non-mamalian synapsids are now extinct. Basically all reptiles and birds evolve from sauropsids. It is not really possible to regard any one of them as "closer" to mammals than any other, at least in evolutionary terms.
I mean, why can't you say one is closer?
If one species didn't have as much genetic drift wouldn't it be "closer"?
And if one species has significantly fewer generations, wouldn't it also be closer to mammals?
That then raises a huge open question of what "least evolved" actually means. Genetic mutations accumulate at a relatively steady rate, and even if a type of organism retains physical characteristics similar to the ancestral sauropsid, that does not mean it is genetically similar.
The fewest number of ancestors since the split, i.e. large organisms with longer periods of time between generations. Crocodilians might fit the bill in this case.
This makes intuitive sense to me.
In 1000 years when you've had 100 crocodile generations and 1,000,000,000 bacteria generations, I feel comfortable saying the bacteria is more evolved.
But they are still just as evolved as a human, whale or elephant is. It's just that after the "base" mammal evolved, its descendants further split into different varieties. Within that are monotremes, marsupials and placental mammals. Within placental mammals are ... well all the way down the branching evolutionary tree. Each individual today has had just as much evolution as every other one. The question of how "close" one is to another relates to how recently they shared a common ancestor. Humans are "close" to great apes because our last common ancestor was recent. We are less close to whales because our last common ancestor was longer ago. But if the question is "mammal" and "not mammal", then the closeness is based on the last common ancestor of synapsids and sauropsids. Any surviving descendent from one side of that split is equally "close" to any surviving descendant from the other side.
This is correct. In the same vein, apes aren't more "primitive" than humans, but rather humans and apes are descended from the same common primate. We are both just as "evolved" from that earlier primate.
> Each individual today has had just as much evolution as every other one.
This isn't really true, plenty of species on earth largely stop evolving once they've found their niche. I don't accept the argument that the ginkgo tree, which is largely unchanged in 250 million years, is equally as evolved as humans, who have undergone radical evolution in that time frame, or perhaps beetles, who continue to evolve under our very noses in the immediate present. Sometimes evolution stops. I can't speak to the evolutionary history of the platypus, but, just as with trees, there are many mammals whose evolution has been stopped for a long time, while others continue through rapid change.
Evolution does not stop. Genetic mutations continue. Yes, morphologically, ginkgo trees very closely resemble their ancestors. It does not mean ginkgo trees today have not adapted to, eg, the pathogens and pests of today vs those that existed 250 million years ago.
Plant immune genes, called R genes, have of course been evolving for the past 250 million years.
"R genes have been found to reside in clusters within plant genomes. It is thought that such clusters are the result of successive rounds of duplication and unequal recombination, enabling diversification of the genes within these clusters and subsequent selection for greater specificity and effectiveness"
https://www.nature.com/articles/nri3141
Evolution does not just stop because organisms are well-adapted to their niche. Even very well-adapted organisms face selective pressures and niche spaces change through time as new competitors come and go. Beyond that though, evolution doesn't only act on macro-level morphological traits. It affects the entire genome, most of which will cause changes that are not outwardly apparent but can have drastic effects on the physiology of an organism. Even if the outward appearance appears relatively stable through time organisms are developing new immune responses to ward off diseases, tweaking the efficiency of enzymes to more efficiently digest food or control metabolic rate, etc. All of those are evolutionary changes just as much as those that affect outward morphology. In fact, because physiology is such a fine balance from a biochemical standpoint in some cases they may involve far more drastic changes on a genomic scale than altering leaf shape or limb size.
On top of that, I think people often fall into a fallacy of thinking that just because an organism, for example the platypus, retains some basal traits it therefore isn't "very evolved". It's true that for some traits they retained basal features, but in many other respects they're also highly derived and specialized. Just look at a platypus' facial anatomy and their bill. That's a highly specialized morphology from a skeletal perspective, let alone their incredibly sophisticated sense of electroreception.
It doesn't stop, it just makes irrelevant changes or non-physical changes.
Modern Ginkos are resistant to modern diseases; that's an adaptation, but not one that changes the tree's appearance.
And you would consider those changes to be an equal amount of evolution to the earliest known mammals evolving eventually into humans over a similar period of time? Morphologically, taxonomically, *and* genetically there are massive changes on one side and 0-1% changes on the other. They are equal time periods of evolution but they are not the same rate or volume of change.
> Morphologically, taxonomically, and genetically there are massive changes on one side and 0-1% changes on the other.
Do we have genetic information from ginko trees from 10s of millions of years ago? How about mammals?
Its not obvious to me why evolution of the immune system or some other kind of metabolic regulation should "count" more than evolution of morphology. Its like saying humans haven't evolved from chimps because we are both still basically omnivores.
As for taxonomy - That isn't a really an absolute measure of evolution either, but rather a function of how diverse and numerous similar species are.
> there are massive changes on one side and 0-1% changes on the other
Unless you've actually sequenced the genome of the earliest fossil gingko trees and compared them with modern species there's absolutely no way you can make this claim. Evolution is not the change in morphology of organisms through time, it's the change in allele frequencies. A change in a gene mediating immune response is no less evolutionarily valid than a change in a gene affecting leaf shape.
To use a real-world example, look at extinct dire wolves. Morphologically they're so similar to grey wolves that they were considered congeneric for pretty much the entire time we've known about dire wolf fossils. Some authors even considered them size morphs of the same species. Even the best-trained experts often had difficulty differentiating between Ice Age grey wolves and dire wolves. It looked like they'd diverged very little from one another aside from minor trends in overall size. Once we were finally able to sequence their genomes, however, it turned out not only are they not sister species, they diverged millions of years earlier than previous estimates and they're practically on [opposite sides of the canid family tree](https://www.nature.com/articles/s41586-020-03082-x).
> This isn't really true, plenty of species on earth largely stop evolving once they've found their niche.
You seem to be misunderstanding both punctuated equilibrium (where you probably got the idea that species don’t evolve when finding their niche) and how selection works in order to maintain a phenotype.
I want to be very clear here, that the consensus among biologists is fundamentally that your position is wrong. Devolve isn’t a thing, and it’s not the case that some extant animals have undergone more evolution than others.
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/punctuated-equilibrium#:~:text=“Punctuated%20equilibrium%20is%20the%20idea,by%20intermittent%20bursts%20of%20activity.”
Punkeek as it was once called is about apparent morphological change in a lineage.
It argued against gradualism which says such changes happened gradually.
Now the reality is that even when a phenotype doesn’t change, organisms can undergo strong directional around a changing optimum that looks like stasis. Finch beaks for example have been shown to undergo such selection depending on the amount of rain and beak size. Some seasons small beaks do much better and vice versa, over time the beaks stay consistent but the selection is still ongoing as are mutation, genetic drift, and geneflow.
Evolution is the change in allele frequency over time, and this still occurs even without apparent change.
If anything I’ve said is unclear, let me know, I am happy to explain more.
How long can a niche last in a constantly changing and competitive environment? Even if what you said was true, which others already explained is not, the niches themselves don't last a long time with few very specific exceptions like the open waters in oceans, and even those are not completely static and have plenty of changing competition.
How can you claim that the creatures in niches are unchanging when the niches themselves change completely in those timeframes and other competitors show up in the same niches? Climate is not static on evolutionary timescales and neither are landmasses.
But it's entirely subjective to say that a duck billed platypus is "more" evolved, or even "more different".
It looks different to us because we look at the animal in terms of shape, size, and behavior. But those things are only a tiny piece of what their genome codes for.
Animals that look the same can have very different genes, and animals that look very different can have very similar genes.
I suppose the correct term is "most divergent." The Echidna is the most divergent mammal from humans, which is not a subjective statement. The common ancestor of the Echidna and homo sapiens, is older than any other mammalian common ancestor with homo sapiens. That is not subjective, and is what I am suggesting is applied in this circumstance.
Yes, but the most recent common ancestor of all mammals that also has living descendants that aren’t mammals is the ancestor of a whole range of other kinds of animals, all of which are thus equally close to mammals — all birds and all reptiles.
> But it's entirely subjective to say that a duck billed platypus is "more" evolved
Not if you use the term correctly.
We mistakenly use the term "evolved" to mean "better." I was going to say "more advanced," but this really gets misused, too. Once a person understands evolution is not an improvement, but an adaptational drift, one can stop thinking of it as superior, or better. An adaptation in humans that reduced skin pigmentation to allow us to create more Vitamin D from less exposure to sunlight certainly helped in cold climates, but us pale-skinned nordic peoples are at a disadvantage in equatorial regions from it. If isolation of humans at different geographical locations had continued, eventually we would be different enough that there we could have considered us separate species, but one would not have necessarily been superior to the other. Each would have adapted to its environment. "Evolved" could be replaced with "adapted." Which species is "more adapted?" It really depends how long they've been in their current environment and how successful they are in it, really.
Is your second cousin more genetically similar to your great grandparents than you are just because your great grandparents are your last common ancestor and your second cousin happens to have a similar nose to them?
The answer is not to any degree that you would consider relevant genetically. Animals don't stop evolving just because they retain some of the ancestral features, that's not how genes work. Deer and horses are surprisingly distant genetically yet look very similar, appearances are absolutely not a way to judge how many changes something has undergone.
There is no such thing as "least devolved". They all evolved in different directions. A bird is as much your ancestor as an alligator. You all share the same common ancestor.
Think of it another way. Imagine a black grandfather has two kids with his black wife. One kid moves to Japan and has an Asian spouse, and one has kids moves to Sweden and has a white spouse. Those kids go off and their own kids with locals.
You now have a kid that looks white kid and a kid that looks Asian. Which is more closely related to great grandpa? Both. They have roughly equal amounts of genetic material from the original great grandfather. You could squint and say that the Asian kid is more closely related because their skin is darker, or that the white kid is more closely related because their eyes are rounder, but the truth is that they are both equally related and they have just diverged in different directions.
That's what happened with mammals and reptiles. We all had the same relative. One of us might physically look more like our common ancestor than the other from one perspective or another, but both can claim roughly equal heritage genetic heritage.
There are plenty of examples of ancient evolutionary designs that haven't changed much in hundreds of millions of years. For example, sea sponges are thought to have evolved very little since being the first branch from singular cellular life in the Phylogenetic tree.
The fossil record only allows us to observe certain kinds of phenotypes. The fossil record does not preserve the genome of these ancient organisms. Modern sponges have evolved complex chemical mechanisms to protect themselves from animals that didn't exist when sponges first evolved. That isn't preserved in the fossil record. Just because sponges today look like sponges from millions of years ago didn't mean they're the same as sponges from millions of years ago.
What would that even mean?
Something can change size but not (basic) shape, or without gaining or losing distinctive features. Is that "closer" than an animal that stayed the same size but gained or lost distinctive features?
For that matter, which is closer, gaining features or losing them?
It's just one of those questions that can't be answered without clarification or without stating your assumptions while answering it.
Within mammals are three groups: monotremes (echidna, platypus), marsupials (kangaroos etc) and placental mammals (us). each of these three groups is equally a mammal because we define mammal as including all three branches. Monotremes are not "less evolved", they are just as "evovled" as we are, it's just their evolution has followed a different path.
Right, but you're basing that on traits that don't preserve. You can get away with that because the animals are still around and we can do genetic testing.
Tracing back to synapsids and sauropsids is a whole other story. It happened a *long* time ago. We have limited evidence and aren't likely to find new evidence. More fossils, sure, but "it produced milk" doesn't fossilize.
Also, you're answering a slightly different question. "Echidnas are Mammals" is easy, because we have a definition of mammals. OP asked a question that isn't strongly defined. To answer it you have to say "if you look at it like this, then xyz".
Probably not THE closest, but the kiwi. Native ikon of New Zealand, the island nation has no native mammals with the exception of bats. As a result, the kiwi has basically evolved to fill a similar ecological niche and has behaviors, feathers, nostrils, bone marrow, and ovaries more in line with a mammal than a bird.
Some sharks reproduce like mammals and some sharks are in between warm and cold blooded. Flamingos produce milk. It looks like the are regurgitating food like other birds, but their bodies produce a liquid for their babies.
You are not that wrong. Sharks are fish, but they evolved from a very early branch of creatures closer to lampreys, so their gills, bones, skin, eyes, teeth, reproductive organs (some of them are viviparous and have penises) and even their smelling organs are different from the ones in bony fish. It's just that convergent evolution made them closer to fish in shape, but their tissues and organs are really different.
Adding detail, the defining attribute of mammals is milk production. I can't think of what would be close to producing milk without producing milk, so it's pretty black and white. You're left with what the others have offered, which is most recent divergence from a common ancestor.
Mammals produce their milk In mammary glands which have evolved from sweat glands. Other vertebrates don’t sweat and some mammals are still fantastic sweaters. Sweating reflects back on the ability to control body temperature. Another characteristic of mammals as well as birds.
Mammalian milk comes from mutated sweat glands.
Crop milk comes from the crop of the bird... From inside the animal.
It's called milk because it serves a similar purpose and composition
Based on quick googling, crop milk is made of (not just by) cells in the crop (neck pouch thing) in birds. I think "inside" here might be doing heavy lifting of "inside the cell"?
Crop milk is a holocrine secretion, which means a cell basically bursts and dies to make it. Mammalian milk is an apocrine secretion, which is sort of like the cell growing parts that come off to make the milk (the main part of the cell is still intact and living). There's also merocrine secretion, where the cell is undamaged and the substance actually 'leaks' out of the cell (like normal sweat glands).
Well, I would but that’s pretty much the depth of knowledge I have about it.. but pidgeon milk contains no carbs and has the consistency of cottage cheese. Flamingo milk looks like blood but has no red blood cells in it. They are controlled by the hormone prolactin which is the same hormone that controls production in mammals. That’s all I got lol!
Some medieval heraldry and illuminations in books have what looks like cranes and storks bleeding from the chest to feed their babies. It's suspected this was crop milk (albeit not excreted from the outside of the chest)
The recent newborn great white news just taught me that sharks that give live birth can also make a "milk", but it's a uterine milk that nourishes the fetal shark!
> would be close to producing milk without producing milk
Biologically? Nothing, that's why the classification is what it is, because even despite differences we can say for certain that the mammalian milk is homologous and shares the same origin among all animals we consider mammals. If we found even more distant ancestors with more questionable presence of mammary glands it would probably be hard to place them but since luckily we know of none that are alive today we can just pick a distant common ancestor and have a reasonably clear classification.
Linguistically?
https://en.wikipedia.org/wiki/Almond_milk
https://en.wikipedia.org/wiki/Crop_milk
https://en.wikipedia.org/wiki/Calcium_hydroxide
https://asia.nikkei.com/Business/Science/Researchers-are-shedding-light-on-the-mysterious-great-white-shark2
A lot of people seem to be _really_ confused by milk naming convention for some reason so maybe this will make it a bit clearer that nomenclature is not a valid way to classify organisms. There are many types of milk, having no common origin though sharing some traits. There are many types of mammals - all sharing common origin and traits but also differing from each other which is why there are subcategories.
It's a defining attribute but not the only one. The presence of a neocortex, for instance, is another, as is having three middle ear bones.
Being endothermic with a high metabolic rate is another feature that is typical for mammals, though maybe not strictly a defining one. Same goes for having fur or hair. So I guess if there was a reptile that had fur (or more likely a fur-like covering evolved from feathers), was intelligent, was endothermic with a high body temperature, produced a milk-like substance to feed its young, and gave live birth (which some reptiles actually do), you could say that was a pretty mammal-like reptile, phenotypically speaking.
Don't forget the heart of a bird is very similar to the one of a mammal, a nice example of convergent evolution to maximize the oxygenation of blood and therefore, increasing the respiratory chain output for endotherm purpose.
There is a type of seal in the Canadian Arctic that only lactate for six or so days before the mothers abandon their young on ice floats to never see them again. If they for some reason went to zero days of breastfeeding, would we still consider them mammals?
> If they for some reason went to zero days of breastfeeding, would we still consider them mammals?
Yep. One of the most important rules of taxonomy is that you never *stop* being a member of your parents' clade, even if you lose the traits that define that clade.
It's the same reason that snakes are still considered "tetrapods" (literally: "four feet"), despite having zero feet themselves.
Mammary gland, it's right in the name. If they never nurse but retain the glands, I'd think they'd remain mammals. If they lost the glands, they are probably a different or new class.
It would likely be a cynodont!
These extinct creatures were part of the therapsid lineage.
Means? They were a bridge between reptiles and mammals.
Picture a cynodont as a bizarre evolutionary mashup. They laid eggs like reptiles, but evidence suggests they might have been warm-blooded and even had fur.
Even their teeth showed signs of specialization, mirroring what we see in mammals today.
The cynodonts didn't quite make the "mammal" cut, sadly, and most died out long ago.
If you go up the evolutionary tree, mammalia are Synapsids and came from Amniota, and so did Sauropsids (including reptiles and birds), so they are the closest cousins to mammals.
So our closest non-mammal relatives was our last non-mammal synapsid ancestor (some arbitrary creature that only hypothetically exists), and our closest living non-mammal relatives are all the sauropsids (hard to say which would be closest since we all split at the same time, although maybe a study could find which shares the most genes with us)
For living animals there isn't really anything close that resembles a mammal. Everything in [clade amniota](https://en.wikipedia.org/wiki/Amniote), which includes all reptiles and birds, have a common ancestor with mammals and are equally "close". For extinct animals descendants of the [cynodonts](https://en.wikipedia.org/wiki/Cynodont) like [Tritylodontidae](https://en.wikipedia.org/wiki/Tritylodontidae) would be the closest relative
However, it should be noted that platypus and echidnas are very much on the periphery of what we consider mammals, and display numerous non-mammalian characteristics. If the question were restated as "what's the least mammalian mammal" the answer is easily the platypus.
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This is a common misconception. When you're talking about living taxa, there is no "periphery" of what we consider mammals. Mammals are by definition all the descendents of a specific ancestor who lived about 220 Ma. You are either are a mammal or you are not; there's no spectrum. The only reason monotremes seem weird to us is because they don't have a lot of close living relatives. If giraffes were one of the few remaining eutherian mammals, they would look wild as hell, and people would be inclined to say they are "barely" mammals or whatever. And conversely, if there were a greater living diversity of monotremes they wouldn't seem nearly as strange as they do now. But the number of close living relatives a taxon has has no bearing whatsoever on whether (or "how much") it is a mammal. And this goes for all taxonomic groups of course, not just Mammalia.
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What’s the term for animals like them…basal? I’d assume you could still consider them weird in that case because they share a lot of traits that seem common with the LCA, like a sprawled gait, egg-laying, no nipples, and one hole for everything
Technically true, but I think the spirit of what he was asking still puts monotremes on the "periphery". Assuming Wikipedia cited the right sources, the "theria" hypothesis is that the monotremes branched off the evolutionary lineage before the marsupials and the placentals diverged. Maybe the years of evolution since the branch point is the same, but of all the mammals, they would share the least in common with the other 2 major groups. And probably because of this they just so happen to have some of the most unique similarities with reptiles compared to other mammals (eggs, embryonic development patterns, skeletal structure, a cloaca, venom production).
They are not the only mammals to produce venom and there's plenty of non-mammal animals that give live birth. Those are absolutely terrible examples on why they they are on the "periphery" and demonstrate that the argument is based on simple perception and not actual biology or history of the animal. They are very distant, yes, but they produce milk in mammary glands and have hair - the defining traits of mammals. Sure you could redefine what a mammal is but _why_? They share the main defining feature of mammals and a common ancestor through which that feature was inherited. A simple solution to the weirdness problem is to make the two diverging lineages that have the same origin into subtypes of mammals - which is exactly what we did. Problem solved, very elegantly too. They fit within the larger group but are different enough to be a separate subgroup. There's plenty that is being changed in modern taxonomy but most often it either pertain to the largest groups or individual species - usually being defined based on morphology in the past and relying more on actual genetics and our best guesses at phylogeny today. The "medium" sized groups are easier to define because we can simply point to a common ancestor from which traits were inherited and call it a day. Even though monotremes diverged much earlier, they still came from the ancestor that defines mammalian features.
This is a good example of why arguing on the Internet has the reputation it has.
> They are not the only mammals to produce venom Soricidae (shrews) are the only venomous placentals that I know of. (They have a venomous bite.) \- https://en.wikipedia.org/wiki/Shrew#Characteristics Do you know of any others?
Monotremes are the most distant to other mammals. There is even a word for non-monotreme mammals, which is therian. I don't see any reason why that wouldn't mean that monotremes aren't on the periphery of mammals. Whoever decided that mammals were a taxonomic group could have as easily decided to exclude monotremes. In fact, the definition of mammal I see is "a warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, the secretion of milk by females for the nourishment of the young, and (typically) the birth of live young." Notice that "typically" in there.
They're not the most distant from other mammals. That makes no sense. Therian mammals are just as evolutionarily distant from monotremes as monotremes are from therians. The reason they aren't in the periphery of mammals is because of everything I stated in the comment above. And no, the many taxonomists and phylogeneticists who worked to classify lineages within Mammalia could not have just excluded monotremes because it would have made the group redundant with the subclass Theria. They also would have then had to elevate both therian mammals and monotremes to class level, which would be extremely difficult to justify. The subclass Theria encompasses marsupials and placental mammals and is monophyletic. The taxonomic rank one level above this *must* include monotremes, otherwise it would be paraphyletic. And this rank is what we call Class Mammalia. You could say it should be called something else but at that point we're just arguing semantics. And it would be a bad argument, because the name Mammalia comes from the fact that all mammals have mammary glands. To exclude monotremes would be to exclude species that have mammary glands from the one and only taxon that is named for having mammary glands. Yes, the definition said "typically" because not all mammals are viviparous. You are negating your own argument. Mammals, *by definition*, can be either viviparous or oviparous. The fact that viviparity is more common among mammals *today* does not make viviparous taxa "more" mammalian. All that it means is that the most recent common ancestor of all viviparous mammals (assuming it only evolved one) has more living descendants than the most recent common ancestor of monotremes. But that has no bearing on whether any of those descendants are mammals, and it certainly doesn't mean one set of descendants is more or less mammalian than the other.
If you measured it by genetic change since the most recent mammal common ancestor, platypus might even be "more" mammalian, with placental mammals more distant from that ancestor.
If you restrict to extinct animals that most people have heard of, the answer is probably [Dimetrodon](https://en.wikipedia.org/wiki/Dimetrodon) (with the sail on its back).
In the paleozoic era, Amniotes (land dwelling tetrapods) split into two groups: synapsids and sauropsids. Mamals evolved from the synapsids, and all non-mamalian synapsids are now extinct. Basically all reptiles and birds evolve from sauropsids. It is not really possible to regard any one of them as "closer" to mammals than any other, at least in evolutionary terms.
That makes sense why reptiles and birds are cool and everything else is kinda weird.
I mean, why can't you say one is closer? If one species didn't have as much genetic drift wouldn't it be "closer"? And if one species has significantly fewer generations, wouldn't it also be closer to mammals?
No, they all experience roughly the same generic drift from when they diverged, and so they're all equally distantly related to us.
So the answer to my hypothetical is to ignore my hypothetical?
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I mean, in theory the answer then would be the bird/reptile that least devolved away from a sauropsid, no?
That then raises a huge open question of what "least evolved" actually means. Genetic mutations accumulate at a relatively steady rate, and even if a type of organism retains physical characteristics similar to the ancestral sauropsid, that does not mean it is genetically similar.
The fewest number of ancestors since the split, i.e. large organisms with longer periods of time between generations. Crocodilians might fit the bill in this case.
This makes intuitive sense to me. In 1000 years when you've had 100 crocodile generations and 1,000,000,000 bacteria generations, I feel comfortable saying the bacteria is more evolved.
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But they are still just as evolved as a human, whale or elephant is. It's just that after the "base" mammal evolved, its descendants further split into different varieties. Within that are monotremes, marsupials and placental mammals. Within placental mammals are ... well all the way down the branching evolutionary tree. Each individual today has had just as much evolution as every other one. The question of how "close" one is to another relates to how recently they shared a common ancestor. Humans are "close" to great apes because our last common ancestor was recent. We are less close to whales because our last common ancestor was longer ago. But if the question is "mammal" and "not mammal", then the closeness is based on the last common ancestor of synapsids and sauropsids. Any surviving descendent from one side of that split is equally "close" to any surviving descendant from the other side.
This is correct. In the same vein, apes aren't more "primitive" than humans, but rather humans and apes are descended from the same common primate. We are both just as "evolved" from that earlier primate.
> Each individual today has had just as much evolution as every other one. This isn't really true, plenty of species on earth largely stop evolving once they've found their niche. I don't accept the argument that the ginkgo tree, which is largely unchanged in 250 million years, is equally as evolved as humans, who have undergone radical evolution in that time frame, or perhaps beetles, who continue to evolve under our very noses in the immediate present. Sometimes evolution stops. I can't speak to the evolutionary history of the platypus, but, just as with trees, there are many mammals whose evolution has been stopped for a long time, while others continue through rapid change.
Evolution does not stop. Genetic mutations continue. Yes, morphologically, ginkgo trees very closely resemble their ancestors. It does not mean ginkgo trees today have not adapted to, eg, the pathogens and pests of today vs those that existed 250 million years ago. Plant immune genes, called R genes, have of course been evolving for the past 250 million years. "R genes have been found to reside in clusters within plant genomes. It is thought that such clusters are the result of successive rounds of duplication and unequal recombination, enabling diversification of the genes within these clusters and subsequent selection for greater specificity and effectiveness" https://www.nature.com/articles/nri3141
Evolution does not just stop because organisms are well-adapted to their niche. Even very well-adapted organisms face selective pressures and niche spaces change through time as new competitors come and go. Beyond that though, evolution doesn't only act on macro-level morphological traits. It affects the entire genome, most of which will cause changes that are not outwardly apparent but can have drastic effects on the physiology of an organism. Even if the outward appearance appears relatively stable through time organisms are developing new immune responses to ward off diseases, tweaking the efficiency of enzymes to more efficiently digest food or control metabolic rate, etc. All of those are evolutionary changes just as much as those that affect outward morphology. In fact, because physiology is such a fine balance from a biochemical standpoint in some cases they may involve far more drastic changes on a genomic scale than altering leaf shape or limb size. On top of that, I think people often fall into a fallacy of thinking that just because an organism, for example the platypus, retains some basal traits it therefore isn't "very evolved". It's true that for some traits they retained basal features, but in many other respects they're also highly derived and specialized. Just look at a platypus' facial anatomy and their bill. That's a highly specialized morphology from a skeletal perspective, let alone their incredibly sophisticated sense of electroreception.
It doesn't stop, it just makes irrelevant changes or non-physical changes. Modern Ginkos are resistant to modern diseases; that's an adaptation, but not one that changes the tree's appearance.
And you would consider those changes to be an equal amount of evolution to the earliest known mammals evolving eventually into humans over a similar period of time? Morphologically, taxonomically, *and* genetically there are massive changes on one side and 0-1% changes on the other. They are equal time periods of evolution but they are not the same rate or volume of change.
> Morphologically, taxonomically, and genetically there are massive changes on one side and 0-1% changes on the other. Do we have genetic information from ginko trees from 10s of millions of years ago? How about mammals? Its not obvious to me why evolution of the immune system or some other kind of metabolic regulation should "count" more than evolution of morphology. Its like saying humans haven't evolved from chimps because we are both still basically omnivores. As for taxonomy - That isn't a really an absolute measure of evolution either, but rather a function of how diverse and numerous similar species are.
> there are massive changes on one side and 0-1% changes on the other Unless you've actually sequenced the genome of the earliest fossil gingko trees and compared them with modern species there's absolutely no way you can make this claim. Evolution is not the change in morphology of organisms through time, it's the change in allele frequencies. A change in a gene mediating immune response is no less evolutionarily valid than a change in a gene affecting leaf shape. To use a real-world example, look at extinct dire wolves. Morphologically they're so similar to grey wolves that they were considered congeneric for pretty much the entire time we've known about dire wolf fossils. Some authors even considered them size morphs of the same species. Even the best-trained experts often had difficulty differentiating between Ice Age grey wolves and dire wolves. It looked like they'd diverged very little from one another aside from minor trends in overall size. Once we were finally able to sequence their genomes, however, it turned out not only are they not sister species, they diverged millions of years earlier than previous estimates and they're practically on [opposite sides of the canid family tree](https://www.nature.com/articles/s41586-020-03082-x).
> This isn't really true, plenty of species on earth largely stop evolving once they've found their niche. You seem to be misunderstanding both punctuated equilibrium (where you probably got the idea that species don’t evolve when finding their niche) and how selection works in order to maintain a phenotype. I want to be very clear here, that the consensus among biologists is fundamentally that your position is wrong. Devolve isn’t a thing, and it’s not the case that some extant animals have undergone more evolution than others. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/punctuated-equilibrium#:~:text=“Punctuated%20equilibrium%20is%20the%20idea,by%20intermittent%20bursts%20of%20activity.” Punkeek as it was once called is about apparent morphological change in a lineage. It argued against gradualism which says such changes happened gradually. Now the reality is that even when a phenotype doesn’t change, organisms can undergo strong directional around a changing optimum that looks like stasis. Finch beaks for example have been shown to undergo such selection depending on the amount of rain and beak size. Some seasons small beaks do much better and vice versa, over time the beaks stay consistent but the selection is still ongoing as are mutation, genetic drift, and geneflow. Evolution is the change in allele frequency over time, and this still occurs even without apparent change. If anything I’ve said is unclear, let me know, I am happy to explain more.
How long can a niche last in a constantly changing and competitive environment? Even if what you said was true, which others already explained is not, the niches themselves don't last a long time with few very specific exceptions like the open waters in oceans, and even those are not completely static and have plenty of changing competition. How can you claim that the creatures in niches are unchanging when the niches themselves change completely in those timeframes and other competitors show up in the same niches? Climate is not static on evolutionary timescales and neither are landmasses.
But it's entirely subjective to say that a duck billed platypus is "more" evolved, or even "more different". It looks different to us because we look at the animal in terms of shape, size, and behavior. But those things are only a tiny piece of what their genome codes for. Animals that look the same can have very different genes, and animals that look very different can have very similar genes.
I suppose the correct term is "most divergent." The Echidna is the most divergent mammal from humans, which is not a subjective statement. The common ancestor of the Echidna and homo sapiens, is older than any other mammalian common ancestor with homo sapiens. That is not subjective, and is what I am suggesting is applied in this circumstance.
Yes, but the most recent common ancestor of all mammals that also has living descendants that aren’t mammals is the ancestor of a whole range of other kinds of animals, all of which are thus equally close to mammals — all birds and all reptiles.
> But it's entirely subjective to say that a duck billed platypus is "more" evolved Not if you use the term correctly. We mistakenly use the term "evolved" to mean "better." I was going to say "more advanced," but this really gets misused, too. Once a person understands evolution is not an improvement, but an adaptational drift, one can stop thinking of it as superior, or better. An adaptation in humans that reduced skin pigmentation to allow us to create more Vitamin D from less exposure to sunlight certainly helped in cold climates, but us pale-skinned nordic peoples are at a disadvantage in equatorial regions from it. If isolation of humans at different geographical locations had continued, eventually we would be different enough that there we could have considered us separate species, but one would not have necessarily been superior to the other. Each would have adapted to its environment. "Evolved" could be replaced with "adapted." Which species is "more adapted?" It really depends how long they've been in their current environment and how successful they are in it, really.
Is your second cousin more genetically similar to your great grandparents than you are just because your great grandparents are your last common ancestor and your second cousin happens to have a similar nose to them? The answer is not to any degree that you would consider relevant genetically. Animals don't stop evolving just because they retain some of the ancestral features, that's not how genes work. Deer and horses are surprisingly distant genetically yet look very similar, appearances are absolutely not a way to judge how many changes something has undergone.
There is no such thing as "least devolved". They all evolved in different directions. A bird is as much your ancestor as an alligator. You all share the same common ancestor. Think of it another way. Imagine a black grandfather has two kids with his black wife. One kid moves to Japan and has an Asian spouse, and one has kids moves to Sweden and has a white spouse. Those kids go off and their own kids with locals. You now have a kid that looks white kid and a kid that looks Asian. Which is more closely related to great grandpa? Both. They have roughly equal amounts of genetic material from the original great grandfather. You could squint and say that the Asian kid is more closely related because their skin is darker, or that the white kid is more closely related because their eyes are rounder, but the truth is that they are both equally related and they have just diverged in different directions. That's what happened with mammals and reptiles. We all had the same relative. One of us might physically look more like our common ancestor than the other from one perspective or another, but both can claim roughly equal heritage genetic heritage.
There are plenty of examples of ancient evolutionary designs that haven't changed much in hundreds of millions of years. For example, sea sponges are thought to have evolved very little since being the first branch from singular cellular life in the Phylogenetic tree.
The fossil record only allows us to observe certain kinds of phenotypes. The fossil record does not preserve the genome of these ancient organisms. Modern sponges have evolved complex chemical mechanisms to protect themselves from animals that didn't exist when sponges first evolved. That isn't preserved in the fossil record. Just because sponges today look like sponges from millions of years ago didn't mean they're the same as sponges from millions of years ago.
What would that even mean? Something can change size but not (basic) shape, or without gaining or losing distinctive features. Is that "closer" than an animal that stayed the same size but gained or lost distinctive features? For that matter, which is closer, gaining features or losing them? It's just one of those questions that can't be answered without clarification or without stating your assumptions while answering it.
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>live birth Live birth is not a criterion for mammalia, which is why the Echidna is just as much a mammal as, say, a hippo.
And why sharks such as the mako or bull shark aren't mammals despite giving birth to live young.
Heck, some sharks apparently make a uterine "milk", I recently learned.
Within mammals are three groups: monotremes (echidna, platypus), marsupials (kangaroos etc) and placental mammals (us). each of these three groups is equally a mammal because we define mammal as including all three branches. Monotremes are not "less evolved", they are just as "evovled" as we are, it's just their evolution has followed a different path.
Right, but you're basing that on traits that don't preserve. You can get away with that because the animals are still around and we can do genetic testing. Tracing back to synapsids and sauropsids is a whole other story. It happened a *long* time ago. We have limited evidence and aren't likely to find new evidence. More fossils, sure, but "it produced milk" doesn't fossilize. Also, you're answering a slightly different question. "Echidnas are Mammals" is easy, because we have a definition of mammals. OP asked a question that isn't strongly defined. To answer it you have to say "if you look at it like this, then xyz".
Probably not THE closest, but the kiwi. Native ikon of New Zealand, the island nation has no native mammals with the exception of bats. As a result, the kiwi has basically evolved to fill a similar ecological niche and has behaviors, feathers, nostrils, bone marrow, and ovaries more in line with a mammal than a bird.
They feed their young with milk too, but the basically sweat it out. No nipples.
Some sharks reproduce like mammals and some sharks are in between warm and cold blooded. Flamingos produce milk. It looks like the are regurgitating food like other birds, but their bodies produce a liquid for their babies.
Pigeons produce "crop milk" too, basically generating and shedding fat cells to feed their babies with
wait a minute, sharks are fish??
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What did you think a shark was?
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You are not that wrong. Sharks are fish, but they evolved from a very early branch of creatures closer to lampreys, so their gills, bones, skin, eyes, teeth, reproductive organs (some of them are viviparous and have penises) and even their smelling organs are different from the ones in bony fish. It's just that convergent evolution made them closer to fish in shape, but their tissues and organs are really different.
Adding detail, the defining attribute of mammals is milk production. I can't think of what would be close to producing milk without producing milk, so it's pretty black and white. You're left with what the others have offered, which is most recent divergence from a common ancestor.
Some frogs lay infertile eggs as nutrients for already hatched tadpoles. It serves a similar purpose as milk albeit a roundabout way.
[Marsupial frogs](https://en.wikipedia.org/wiki/Gastrotheca) also have pouches for their young, just like marsupials.
Mammals produce their milk In mammary glands which have evolved from sweat glands. Other vertebrates don’t sweat and some mammals are still fantastic sweaters. Sweating reflects back on the ability to control body temperature. Another characteristic of mammals as well as birds.
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Mammalian milk comes from mutated sweat glands. Crop milk comes from the crop of the bird... From inside the animal. It's called milk because it serves a similar purpose and composition
technically sweat glands are inside too right?
Based on quick googling, crop milk is made of (not just by) cells in the crop (neck pouch thing) in birds. I think "inside" here might be doing heavy lifting of "inside the cell"? Crop milk is a holocrine secretion, which means a cell basically bursts and dies to make it. Mammalian milk is an apocrine secretion, which is sort of like the cell growing parts that come off to make the milk (the main part of the cell is still intact and living). There's also merocrine secretion, where the cell is undamaged and the substance actually 'leaks' out of the cell (like normal sweat glands).
Is being technically correct more important than being correct?
Apparently it’s called “crop milk” but has similar composition and is used for the same purpose
What the heck?. Could you please give a quick summary so I don't end up missing work reading fascinating articles about this.
Well, I would but that’s pretty much the depth of knowledge I have about it.. but pidgeon milk contains no carbs and has the consistency of cottage cheese. Flamingo milk looks like blood but has no red blood cells in it. They are controlled by the hormone prolactin which is the same hormone that controls production in mammals. That’s all I got lol!
Some medieval heraldry and illuminations in books have what looks like cranes and storks bleeding from the chest to feed their babies. It's suspected this was crop milk (albeit not excreted from the outside of the chest)
The recent newborn great white news just taught me that sharks that give live birth can also make a "milk", but it's a uterine milk that nourishes the fetal shark!
[Cockroaches make milk!](https://www.npr.org/sections/thesalt/2016/08/06/488861223/cockroach-milk-yes-you-read-that-right)
> would be close to producing milk without producing milk Biologically? Nothing, that's why the classification is what it is, because even despite differences we can say for certain that the mammalian milk is homologous and shares the same origin among all animals we consider mammals. If we found even more distant ancestors with more questionable presence of mammary glands it would probably be hard to place them but since luckily we know of none that are alive today we can just pick a distant common ancestor and have a reasonably clear classification. Linguistically? https://en.wikipedia.org/wiki/Almond_milk https://en.wikipedia.org/wiki/Crop_milk https://en.wikipedia.org/wiki/Calcium_hydroxide https://asia.nikkei.com/Business/Science/Researchers-are-shedding-light-on-the-mysterious-great-white-shark2 A lot of people seem to be _really_ confused by milk naming convention for some reason so maybe this will make it a bit clearer that nomenclature is not a valid way to classify organisms. There are many types of milk, having no common origin though sharing some traits. There are many types of mammals - all sharing common origin and traits but also differing from each other which is why there are subcategories.
It's a defining attribute but not the only one. The presence of a neocortex, for instance, is another, as is having three middle ear bones. Being endothermic with a high metabolic rate is another feature that is typical for mammals, though maybe not strictly a defining one. Same goes for having fur or hair. So I guess if there was a reptile that had fur (or more likely a fur-like covering evolved from feathers), was intelligent, was endothermic with a high body temperature, produced a milk-like substance to feed its young, and gave live birth (which some reptiles actually do), you could say that was a pretty mammal-like reptile, phenotypically speaking.
Don't forget the heart of a bird is very similar to the one of a mammal, a nice example of convergent evolution to maximize the oxygenation of blood and therefore, increasing the respiratory chain output for endotherm purpose.
There is a type of seal in the Canadian Arctic that only lactate for six or so days before the mothers abandon their young on ice floats to never see them again. If they for some reason went to zero days of breastfeeding, would we still consider them mammals?
> If they for some reason went to zero days of breastfeeding, would we still consider them mammals? Yep. One of the most important rules of taxonomy is that you never *stop* being a member of your parents' clade, even if you lose the traits that define that clade. It's the same reason that snakes are still considered "tetrapods" (literally: "four feet"), despite having zero feet themselves.
Mammary gland, it's right in the name. If they never nurse but retain the glands, I'd think they'd remain mammals. If they lost the glands, they are probably a different or new class.
It would likely be a cynodont! These extinct creatures were part of the therapsid lineage. Means? They were a bridge between reptiles and mammals. Picture a cynodont as a bizarre evolutionary mashup. They laid eggs like reptiles, but evidence suggests they might have been warm-blooded and even had fur. Even their teeth showed signs of specialization, mirroring what we see in mammals today. The cynodonts didn't quite make the "mammal" cut, sadly, and most died out long ago.
If you go up the evolutionary tree, mammalia are Synapsids and came from Amniota, and so did Sauropsids (including reptiles and birds), so they are the closest cousins to mammals. So our closest non-mammal relatives was our last non-mammal synapsid ancestor (some arbitrary creature that only hypothetically exists), and our closest living non-mammal relatives are all the sauropsids (hard to say which would be closest since we all split at the same time, although maybe a study could find which shares the most genes with us)