7 What's special about placental mammals?

As a pointer to the later units in this series, it's helpful to end this course (as LoM and the TV programme do) by stressing the differences between marsupials and the placental mammals - the mammals that provide the focus of the remainder of the units in this series. To do so, we return to the topic of mammalian reproduction.

In each mammalian type, pregnancy begins as the blastocyst becomes embedded in the tissues of the womb. In both marsupial and placental mammals, the developing tissues of the mother's womb and the embryo lie in very close contact, such that 'oxygen, carbon dioxide, nutrients and waste materials' [p. 36] can transfer between mother and offspring. In marsupials, this contact is loose and seldom any more than a very rudimentary 'placenta of sorts'. But in some marsupials, the contact between the two sets of tissues becomes more elaborate; in the koala, for example, the structure is considered by most experts to be a true placenta, albeit short-lived. The fact that a functional placenta evolved in at least a couple of marsupial species is a striking example of parallel evolution - the independent development of a structure in two distinct groups. As LoM explains [p. 37], marsupials seem to be descended from a different ancestral group than placental mammals; they can't be thought of as in any sense intermediate. It is indeed the case that there are 'different, independent solutions to the problem of rearing young' [p. 37] but sometimes, as with the marsupial 'placenta of sorts', the solutions have much in common.

The terms 'marsupial' and 'placental' were established in the late 18th century when mammals were first classified. 'Marsupial' is derived from the Latin word marsupium, meaning pocket. As you know, this feature is conspicuous in kangaroos and wallabies but is not present in all marsupials. Then the need for renaming was even more evident once marsupials were found to briefly form a simple placenta. So the terms Eutheria (= placental mammals), Metatheria (= marsupials) and Prototheria (= monotremes) were proposed instead. You may well come across these terms yourself in your own reading, but I'll be using the original and more familiar terminology in the units in this series.

What are the apparent benefits that the development of a more elaborate placenta can bring? In placental mammals, close and extensive contact of maternal and embryonic tissue reaches its peak; in the human placenta there is an estimated 40-50 kilometres of such fine-scale contact! And yet the bloods of mother and offspring never fully mix - to do so would risk the mother's rejection of the embryo as a 'foreign' object. The transfer between the two blood systems is so extensive that the needs of the growing embryo can be met for a prolonged period. The net benefit, as DA points out, is that the young of placental mammals are often born relatively mature after a prolonged pregnancy, with all the attendant benefits to their early wellbeing that size can bring. The efficiency of the mammalian placenta is another factor that helps explain the group's biological success - an evolutionary flowering that the following units illustrate to full effect.