Summary
Viviparity and the placenta evolved independently many times across vertebrates.
In depth
Placenta evolved independently in mammals, some lizards, sharks and fish. Types: epitheliochorial, endotheliochorial and haemochorial (humans). Retroviral envelope genes (syncytin) were co-opted for placental fusion.
Placenta types differ in the degree of tissue contact. Evolutionary pressures include maternal-fetal conflict. Eutherian placentas are highly invasive.
Remember: viviparity evolved many times.
Viviparity (live birth) has evolved independently more than 150 times in vertebrates, including in sharks, fish, lizards, snakes and mammals. A placenta, an organ for exchange between mother and embryo, evolved separately in many of them, from different tissues.
In eutherian mammals, the chorioallantoic placenta supports a long pregnancy; marsupials have a short-lived yolk sac placenta and give birth to tiny young that develop in a pouch; monotremes lay eggs. Some skinks and sharks (such as hammerheads) have placental structures. Placentation also requires immune tolerance of the fetus, and endogenous retrovirus genes (syncytin) contributed to placenta fusion in several lineages.
Example: seahorse males brood young in a pouch with placental-like features.
Frequently asked questions
What is Evolution of Placentation?
Viviparity and the placenta evolved independently many times across vertebrates.
Which area of biology covers Evolution of Placentation, and how advanced is it?
Evolution of Placentation is one of the 54 topics in Reproduction & Endocrinology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Pregnancy, Genomic Imprinting in Placenta, Parthenogenesis.
Why does Evolution of Placentation matter?
It illustrates convergent evolution.
Can you give an example of Evolution of Placentation?
Kangaroos have a short-lived placenta then rely on lactation.
What should I learn before Evolution of Placentation?
Work through the Beginner topics in Reproduction & Endocrinology first, such as Reproduction, Asexual Reproduction, Sexual Reproduction, then return to Evolution of Placentation. Advanced topics assume the Intermediate vocabulary.
What topics are related to Evolution of Placentation?
The closest topics to Evolution of Placentation are Pregnancy, Genomic Imprinting in Placenta, Parthenogenesis, Sexual Reproduction. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Evolution of Placentation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Evolution of Placentation. To practise the whole of Reproduction & Endocrinology, use the Reproduction & Endocrinology quiz.
How is Evolution of Placentation different from Pregnancy?
Evolution of Placentation: Viviparity and the placenta evolved independently many times across vertebrates. Pregnancy: The development of a fetus in the uterus lasting about 40 weeks in humans. Read both side by side and note what each one explains.
How should I study Evolution of Placentation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Pregnancy and explain the difference aloud. Finally, take the Reproduction & Endocrinology quiz and come back to this page for any question you miss.