Submitted by fil- t3_1193rdl in askscience
Jason-_B t1_j9nb7vz wrote
So, the evolution of the placenta is actually a really fascinating topic. Basically, the placenta is an organ that develops in female mammals during pregnancy and allows for the exchange of nutrients, gases, and waste between the mother and the developing fetus.
The thing is, not all mammals have placentas. In fact, the evolution of the placenta is thought to be one of the key factors that allowed mammals to become as diverse and successful as they are today.
So how did it happen? Well, scientists believe that the earliest mammals were probably small, shrew-like creatures that laid eggs. Over time, some of these mammals evolved to give birth to live young, which provided certain advantages in terms of protecting the developing fetus and increasing its chances of survival.
Eventually, these live-birthing mammals began to develop specialized tissues and organs that allowed for more efficient exchange of nutrients and waste between the mother and fetus. These tissues eventually evolved into the placenta we see in modern-day mammals.
It's a really cool example of how evolution can lead to some pretty complex and amazing adaptations.
fil- OP t1_j9nc21d wrote
Thank you so much!
spacemonkeymafia42 t1_j9nfn1v wrote
There's evidence that a gene responsible for placental development in humans came from an ancient virus!
Syncytin-1, a protein coded by the ERV1 gene which is crucial for placental development. Syncytin-1 is a human endogenous retroviral element, viral genetic material that has incorportated into our genome. It is conserved among apes and old-world monkeys.
Syncytin-2, another placental development gene, is derived from a different retrovirus.
unconventionally_ t1_j9nleic wrote
Whoa. How do we adapt virus genes into our DNA? I’m so intrigued
joalheagney t1_j9nr2t2 wrote
Keeping it short? Viruses reproduce by inserting DNA into our cells' nuclei (Even the RNA viruses - there's just extra steps).
Several viruses have a dormant stage, where they infect a cell, but don't produce new viral particles unless triggered - usually by stress of the host. Herpes simplex is a good example, which is why cold sores come back.
Once a viral gene is in a stem cell, it's up for reuse and adaptation via evolution, just like any other gene.
[deleted] t1_j9pbykb wrote
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djublonskopf t1_j9oso0h wrote
Specifically, the syncytins are important because they keep the mother's immune cells from being able to reach "through" the placenta into the developing fetus.
There are certain immune cells that are able to slip between other epithelial cells. If the mother's immune cells were to slip past the cells of placenta, they would almost certainly attack the fetus. Placental mammals solve this by having a "boundary layer" between the placenta and the mother. The cells of the boundary layer use those viral syncytins to "fuse" together, becoming one giant solid mega-cell with lots of nuclei. Because there are no longer any individual cells to slip between, the mother's immune cells are unable to get past this boundary layer (the syncytiotrophoblast) and thus the rest of the placenta and fetus are protected from the mother's immune system.
[deleted] t1_j9npfvs wrote
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derioderio t1_j9p4tzj wrote
Do we have any fossil examples of any intermediates between egg-laying mammals and placental mammals?
djublonskopf t1_j9u5sbk wrote
Yes. Marsupial fossils would represent mammals that gave live birth, but aren’t placental mammals either.
Multituberculates are a lineage of mammal that is completely extinct today, but they were closer to therians (placentals+marsupials) than monotremes. Some of them had incredibly narrow pelvic openings too, hinting that they may also have been live-birthers (of tiny, underdeveloped offspring like the marsupials.)
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