User:Acather96/Drafts/Evolution of placentas

Placentas are organs formed by the bringing-together (apposition) or fusion of fetal membranes and parental tissue for the exchange of nutrients, hormones, water and gases between a mother and a developing fetus. The study of the evolution of placentas encompasses the biological events and processes involved in the origin, elaboration and diversification of placentas across the animal kingdom. Unlike many other vertebrate organs which arose both only once and early in vertebrate evolution, placentas have evolved more recently and originated independently at least 137 times in vertebrates. The multiple separate origins of placentas, their functional and morphological diversity (including the profusion of so-called 'transitional forms') and their recent origins provides evolutionary biologists the potential to conduct wide-ranging comparative, experimental and theoretical research to understand both how and why placentas evolve and to use them as a model system to understand organ evolution more generally.

Origins
By scoring the presence or absence of a placenta for different species/clades on phylogenetic trees and applying ancestral reconstruction algortihms, the number of separate evolutionary origins of placentas can be calculated. For vertebrates, sufficent comparative data has been collected from placental and aplacental (lacking a placenta) species of mammals, squamates (lizards, snakes and worm lizards), amphibians and ray-finned fishes (actinopterygians) to deduce that placentas have arisen at least more than 137 times (as data from both placental and aplacental species has not yet been collected in chondricthyans, these methods cannot be applied ). Different vertebrate groups differ markedly in the number of placental origins. Placentas have evolved at least 115 times in squamates, and these placental lineages differing considerably in placental complexity and the age of their evolutionary shift from being an aplacental lineage to a placental one (indeed four extant species have both placental and aplacental populations). In contrast placentation in therian mammals (the clade containing marsupials and so-called placental mammals) evolved only once in the therian ancestor 130 million years ago (MYA). Whilst placental evolution in mammals and squamates (both belonging to the amniote clade) has involved the repeated (but independent) elaboration of the same pre-existing extra-embryonic tissue and uterus, the 14 instances of placental evolution in actinopterygian fishes (which are not amniotes) have been much more varied in the parental structures that are recruited to build the placenta;8 origins are estimated amongst amphibians.

Understanding the evolutionary origins of placentas in any given species requires answering two distinct but related questions. Firstly, how did the placenta evolve? What was the mechanistic pathway by which the lineage transitioned from being aplacental to placental? What mutations occured, and in what regions of the genome? What pre-exisitng proteins, pathways and structures were co-opted and re-deployed during placental evolution, and which arose de novo? Secondly, why did the placenta evolve? What adaptive functions does the palcenta serve? Why has natural selection favoured placentas in some lineages but not in others?

How placentas evolve
The evolution of a new organ involves two distinct but necessarily interrelated evolutionary processes: a lineage must acquire both new biological structures (structural innovation) and these structures must perform new biological functions (functional innovation). Structural innovation occurs when mutations change the sequence and/or expression of genes whose protein products are involved in tissue patterning, and the concomitant changes in tissue patterning lead to novel biological structures. Functional innovation occurs when mutations affect the sequence and/or expression of genes in such a way that their protein product now leads to a change in the processes/activites of a cell type and/or tissue.

One common theme in placental evolution is that pre-exisitng biological components are re-used and re-deployed by natural selection to build placentas; this applies to both structural and functional innovation. At a gross structural level, it is clear that placentas evolve via the modification of existing parental and fetal tissues. For example, in amniotes (the clade containing