Submitted by fil- t3_1193rdl in askscience
I recently read & listened to a book by Steve Brusatte about the evolution and rise of mammals. Sadly he fails to get into one of the most fascinating aspects: not laying eggs anymore but giving live birth or putting fetuses in a pouch, in the case of marsupials. Can someone explain, what led to this evolution, where it came from and how it worked?
Thank you so much!
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.
Whoa. How do we adapt virus genes into our DNA? I’m so intrigued
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.
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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.
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Do we have any fossil examples of any intermediates between egg-laying mammals and placental mammals?
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.)
It's worth noting, in addition to u/Jason-_B's excellent comment, that the placenta is not unique to mammals - it's seen in fish, lizards, and snakes as well. More importantly, unlike mammals, the intermediate states are still around, and plentifully represented.
In species with internal fertilization, the egg has to spend at least some time in the female regardless, just to add yolk and a shell. But more time in the female also lets her more precisely control the egg's environment, especially temperature, so keeping them interally has advantages (as well as the disadvantage of not being able to ditch them to escape a predator, and being "weighed down"). So a lot of species have variable time before laying, all the way up to laying right before hatching. Oxygen, CO2 and water can transfer, but it helps to ditch the shell in that case. However, no nutrient transfer occurs. At the very highest extreme, this is ovoviviparity - where the eggs entirely lack calcified shells, and the mom "lays" them immediately before or as the offspring are "hatching". From an outside perspective, this looks just like viviparity, but the key is the lack of nutrients - they need a yolk.
But if you've got eggs interally for a while, why not transfer some nutreints? There are lots of ways to do this, with the most bizarre probably being some species of caecilians (long, worm-like, burrowing amphibians) in which the mother grows nutritive lining in her uterus, which the young scrape from the walls and eat. However, a common way is to vascularize the yolk sac, squish it up against the uterus, let them fuse, and transfer nutrients across - bingo, you've got a placenta. Some of these are every bit as complex and specialized as mammal placentas.
The most useful thing is we have numerous independent evolutions of the placenta outside of mammals (who only evolved it once, as far as we know), as well as living examples of every intermediate you could ask for. There are even species (three-toed skinks) where some populations give live birth and others lay eggs.
Even crazier is the exception - Archosaurs (crocodiles, birds, dinosaurs, and their relatives) cannot ever evolve live birth. Unlike other species, the Archosaur embryo uses the calcium in the eggshell for bone calcification and, if the shell is removed, the hatchling is basically a gummy-bird or gummy-gator (obviously non-viable). This means they can never ditch the shell, and never take those first steps. And not a single Archosaur has ever evolved live birth, despite hundreds of millions of years of opportunities, and literally ruling the planet for most of that time.
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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.