So, I have thought about the evolution of my aliens and how they could evolve things like lactation but still retain their reptilian characteristics.

Here is my best thought about it so far:


500 million years before

A lizard like animal crossbreeds with a kangaroo-like animal. This diverges on 2 paths, 1 where reptilian characteristics are lost and another where they are retained but a pouch is added. The mammalian branch of this crossbreed dies out because in their environment, being cold blooded is advantageous.

400 million years before

This still lizard-like animal that has a pouch crossbreeds with an intelligent biped. Now being bipedal is retained along with the rest of the characteristics(the pouch and reptilian characteristics).

300 million years before

This bipedal reptilian crossbreeds with this time a full blown mammal, not just an animal with mamallian characteristics that doesn't lactate. Now lactation evolves. Around the same time milk proteins specific to sleep schedule evolve. They start to lose their scales and have bare skin in place of it and even a little bit of hair.

200 million years before

This almost half-mammal half-reptile creature crossbreeds with another reptile. They lose all but the lactation and previously retained traits. Now have fully scaly bodies again. Also at this point in time, kidney-like organs have evolved. These organs only allow m1 proteins in the blood vessels supplying the higher nipples, allow both m1 and m2 proteins in the blood vessels supplying the middle nipples, and only allow m2 proteins in the blood vessels supplying the lower nipples in a similar way to how kidneys selectively secrete and absord different substances in different areas of the nephron. So what nipple an alien suckles from determines their sleep schedule after they hatch.

100 million years before

All that is left to evolve is sixth digits. This proto-alien crossbreeds with a bipedal lizard-like animal that has 6 digits on every hand and foot and has the outer 2 digits on the hands more like thumbs.

So after all that you have the alien species.

Impossible? I don't think so

Now I have been told that this is impossible because mammals are too far diverged from reptiles to crossbreed. But there was a point in time when there were mammal-like reptiles and a point in time when there were reptile-like mammals. So clearly, millions of years ago, the difference between reptiles and mammals was slim. This slim difference could mean potential crossbreeding.

Here is what I think happens when the aliens evolve:

First crossbreed:

Reptile x Mammal-like Reptile(warm blooded, scaleless)

Second crossbreed:

Reptile x Reptile-like mammal

Third crossbreed:

Mammal-like Reptile x Mammal

Fourth crossbreed:

Reptile-like Mammal x Reptile

Fifth crossbreed:

Mammal like Reptile x Reptile


Mostly reptilian with just a few mammalian characteristics

Is the evolution of my aliens plausible the way I wrote it down?

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    $\begingroup$ (1) This is most definitely not how natural evolution works. You may want to revisit grade-school biology. (2) Mammals don't have a monopoly in making a nourishing substance to feed their young. Some birds make crop milk; and birds are dinosaurs... $\endgroup$
    – AlexP
    Commented Aug 11, 2018 at 17:06
  • $\begingroup$ You should absolutely read en.wikipedia.org/wiki/Evolution_of_mammals because it is such a great example for what you are trying to accomplish here $\endgroup$
    – Raditz_35
    Commented Aug 12, 2018 at 9:45
  • 1
    $\begingroup$ Not really understanding why all the downvotes on this. The question is clear and well formed, Not everyone has a degree in BioScience, nor do you need to to ask a really interesting question, which I consider this to be. $\endgroup$
    – Mr_road
    Commented Jun 14, 2019 at 16:48
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    $\begingroup$ Cross-breeding a "lizard-like" animal with a "kangaroo-like" animal is unlikely to work on any world - for there to be that much difference between them, it is likely that they are separate species, which by definition means that they cannot crossbreed. That would pretty much preclude the whole proposed 'evolution '. $\endgroup$
    – Monty Wild
    Commented Jun 15, 2019 at 6:18

5 Answers 5


Not really, but it doesn't matter

First of all, 500 million years of evolution would change the appearance of your creatures so drastically that they would almost certainly look nothing like a reptile. In 500 million years, we got from this:

enter image description here

To this:

enter image description here

Secondly, crossbreeding doesn't work like that. In the wild, no two animals of different species, let alone classes, would ever breed.

For this to be plausible, your organisms would have to have some extremely exotic form of passing on genes. But it doesn't matter, because:

These are aliens, mate

Mammalia is a clade. And, by definition, all the members of a clade are descended from a common ancestor. But, on an alien world, there can be no common ancestry between clades of both planets. Therefore, there are neither mammals nor reptiles on your world.

With that, it's fine to have lactating organisms with superficially reptilian characteristics. It's not as if milk-giving fauna has to be furry, live-bearing and endothermic.

  • 1
    $\begingroup$ We evolved a lot over 500 million years because our environment drastically changed. Comparatively, ferns have not changed much over the ~360 million years they've been around, or crocodiles for the ~200 million years they've existed. It's perfectly possible for OP's aliens to live in an environment that hardly changed over those 500 million years, which would explain why OP's aliens have not evolved as much as we did. $\endgroup$
    – Flater
    Commented Mar 14, 2019 at 14:23
  • $\begingroup$ @Flater I mean, to be fair, crocodiles went through the exact same drastic changes in environment, they just so happened to be, like sharks and dragonflies, so absurdly well adapted to what they did that there was little to be changed, and at no point their lifestyle was so disturbed that the went extinct as a whole. $\endgroup$ Commented Dec 17, 2022 at 2:03
  • $\begingroup$ There seem to be lactating organisms on Earth with superficially reptile-like characteristics en.wikipedia.org/wiki/Pangolin , so there doesn't seem much reason why there couldn't be on an alien planet :-D $\endgroup$
    – A. B.
    Commented Dec 17, 2022 at 5:17
  • $\begingroup$ Actually, some "species" do interbreed - the exact dividing line between "the same species" and "not the same species" is a bit vague. Those are generally very closely related species, though. As the OP's posting implies, it seems like there must have been a time when the ancestors of mammals and the ancestors of reptiles were very closely related - but in that case, if you already have reptiles that are almost mammals or vice versa you might not really need to cross them with something to get a milk-producing lizard, just go on from where you are. $\endgroup$
    – A. B.
    Commented Dec 17, 2022 at 5:31
  • $\begingroup$ @Flater to be fair some ferns changed drastically, we call them gymnosperms and angiosperms now. $\endgroup$
    – John
    Commented Dec 18, 2022 at 0:14

Unless your mammals and reptiles are just subspecies of the same species, no it's not realistic.

You have to know that most inter-species reproduction only occurs in fiction.

Inter-subspecies however, is quite interesting, especially with a vast library of morphogenetic traits.

For your desired creatures to be conceived, your mammal-like and reptile-like can just be the same species, but with different gene expressions. What you want is a mechanism for passing down the exact states of these expressions.

Gene expression is how genes dictate cells how to function.

Your mammal-like aliens can be described as such only because the genes that encode for mammal-like traits (like fur, pouches, sweat glands, warm-bloodedness, lactation, etc.) are the only ones being expressed while all other genes are suppressed. The same goes for your reptiles.

"Crossbreeding" then (which sounded more like a eugenics program as you have described them), can lead to the results you want to because the growth manual encoded in the mammal-reptile children's DNA will have mixed instructions. Genes for mammal-like and reptile-like traits can be expressed and/or suppressed in various ways, while still belonging to the same species.

However, this method calls for a drastic shortening of your timeline since any number of mammal-reptile "hybrids" with different traits can be born every generation.


"A lizard-like animal crossbreeds with a kangaroo-like animal."

Unless these two were of the same species or at least very closely related, then this didn't happen. When the ancient ancestors of lizards and mammals were close enough to breed together, they were not lizards, nor mammals (they were probably looked more lizard-like, but that would be deceiving)

Cross-breeding is not a major driving force of evolution. If you want "lizard-like" aliens to have mammary glands, then just declare it by fiat. Mammary glands in ancient reptilians could have evolved (eg from modified sweat glands, or even from modified brood-pouches, as Darwin hypothesised)

There is no reason for a complex and biologically unrealistic sequence of "cross-breeds"

  • 1
    $\begingroup$ Now I'm curious whether you could work through a reasonable species where one gender looks like a lizard and the other looks like a kangaroo. That'd be one heck of a genome! $\endgroup$
    – Cort Ammon
    Commented Aug 12, 2018 at 7:16
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    $\begingroup$ Evolution has done stranger things (look for "parasitic male angler fish") $\endgroup$
    – James K
    Commented Aug 12, 2018 at 7:47
  • $\begingroup$ Kiwis, apparently, have what looks like fur on their bodies (they're technically feathers but they've evolved into just fluff, since kiwis don't need normal feathers since they can't fly) and what look like scales on their legs, so apparently that part can happen within the same genome. The shape, I'm not sure! $\endgroup$
    – A. B.
    Commented Dec 17, 2022 at 5:26

That is not really how multicellular evolution works on earth... but... who is to say that cross breeding/genetic sharing between vastly different groups is not a thing on your world. If it is then yes this is totally plausable, you just have to change how genetics work on your world.

Bacteria indecriminatley swap genetic material on plasmids, allowig advantageous genes to spead amoung diverse populations due to genetic drift and evolutionary pressures, so let's use that as a starting point.

May be your cross breeding is not, A of Reptile had offspring with B of Kangaroo, but Reptiles and Kanagroo spent a few thousand years in similar habitats and during this time a hybrid species formed call the Reptaroos, and one we will call KangaZiards. Some survive some die, rinse and repeate through millions of years and hundreds f envirinments and disasters and you could make it work, just a little tweaking of how genetics works in your world.

Also, 500 million years is a hell of a long time. As other posters have noted.


I know I'm extremely late but you’re making it way more complicated than it has to be.

As you said, this is a completely different planet, so there are no reptiles or mammals. You could create a species that literally just evolved those traits, without hybridizing and attaching earthly labels to it.


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