If you think that someone having a PhD means they’re an expert in everything; I just had to tell my partner, who is about a year from earning his PhD in biophysics, that amphibians and reptiles are not the same thing.

Him: you know, newts and other lizards
Me: I’m sorry... what did you just say?
Him: you know, like newts and iguanas and-
Me: I’m sorry... do you think newts are lizards?
Him: ...they aren’t?
Me: oh my god babe they’re amphibians
Him: yeah...?

Me: you do know that amphibians and reptiles are different, right?
Him: wait... really? I’m gonna need a minute
Me: BABE I AM A FRESHWATER ECOLOGIST

Okay okay quick evolutionary biology lesson for y’all:
In the way way back when all life lived in the oceans, reproduction also occurred in the oceans. At this point, animals generally reproduced sexually through eggs surrounded by membranes, in saline water.

This encompasses a few classes of animals, but for simplicity let’s say that those are primarily fish. Most people know that fish eventually evolved sturdier fins to be able to support themselves on land, but what many people don’t know is that these animals had a different issue

Their eggs were evolved to survive in water only! The membrane-eggs would dry out on land. This means that even though the adults could be terrestrial, they needed to return to water to reproduce. This group can be generally thought of as amphibians.

Amphibians can walk on land to new places, but can only reproduce with water nearby. This is limiting! So the next evolutionary advancement came by enclosing water-membrane-eggs in SHELLS! If your eggs need water... just put them in water capsules!

So with new eggs with shells, this new group (reptiles, and by extension, birds... yes birds are reptiles, taxonomically), could live away from major water sources for their whole lives. But unfortunately shelled eggs are fragile, heavy, and must be kept in one place once laid.

Different solutions have been found for this problem among reptiles, but the next evolutionary step in reproduction mobility were the monotremes! A few species of monotremes still exist (echidnas, platypuses), and are famous for being egg-laying mammals.

Monotremes lay shelled eggs, but those eggs then hatch quickly and the underdeveloped young are cared for and nursed by their mother, allowing more mobility for the babies earlier in life, and more mobility for the species overall.

From monotremes, the next advantage was marsupials! These animals gestate their young for a very short time (functionally having the mothers become the eggshell to keep the embryo moist) and then the young are born and nursed in a pouch.

This allows much more mobility for the species (mothers can simply carry their babies with them), but the young are born very fragile and underdeveloped. Fully-placental mammals followed from here, in which an embryo is kept moist by the mother’s body itself.

It’s important also to notate that among placental mammals, humans are NOT typical. Human pregnancies are an extreme version of gestation that has much more to do with our quirks as a species than with placental mammals in general.

Most placental mammals produce babies that are relatively independent and highly mobile from birth (think of horses that can stand in 30 mins!). This allows maximum mobility for both parents and offspring, and is why the majority of nomadic animals are placental mammals.