Dear 100 Hour Board,
I was thinking about prenatal human development just a moment ago, inspired no doubt by Board Question #41148, and I got to wondering. Why a placenta? This question probably came up because of my interest with fish, and more specifically with my heavy involvement with live bearing fish. The majority of live bearing fishes basically have internal development of eggs, with no transfer of nutrients from mother to child. Not only that, but unlike birds, the fry (babies) are born able to take care of themselves.
As I was thinking about this, I got to wondering why exactly it was that humans didn't have a similar developmental process to these fish. I know that there are some snakes that also give live birth, in a manner similar to live bearing fishes, and that isn't really a surprise considering how similar reptiles and fish are. But I couldn't think of any other groups of animals that give birth to live, self sufficient young but don't have a placenta.
So, my first question. What other animals are there that give this kind of birth?
The other part of this question should be pretty obvious I think. And that has to do with why humans don't undergo a similar process. I have thought about this for a while and have a couple examples that I think are evidence for why. The first is the monotremes, the only egg laying mammals. It seems like they have to give some clues. The other is the marsupials. First of all, the monotremes don't exactly fit in, since they actually lay eggs. But they are mammals that have unsupported development of young. Anyway, to keep things shorter. It seems like size of the animal has something to do with this. What do you think?
Humans develop the way we do because of our genetic heritage. We have a longer gestation period and a longer life span, so it stands to reason that our young would require a longer time to mature. Plus, think about how much more a human child has to learn than other mammals. Other than that, the best we can do is tell you that animals bear after their own kind simply because that's the way they were designed by the Creator.
As for your other questions, it's apparent though your question that you know more about the subject at hand than any of us. Your hypotheses seem like they make sense. If you're aching to learn about alternate species that bear self-sufficient young, talk to a Biology professor.