"According to the theory of common descent, all true mammals are supposed to have descended from a common ancestor with a placenta. This is a trait common to all mammals. However, it has been a puzzle for some time that placentas differ in the form they take among different mammalian clades.
In the year 2000, French researcher Thierry Heidmann and coworkers found that genes derived from
endogenous retroviruses (ERVs) appear to have been coopted to perform an essential role in placental formation.
These genes, which resemble the ERV envelope gene env, make a protein that originally promoted fusion of the virus with its host cell's membrane, but now acts to promote fusion of membranes between the embryo and the lining of the uterus. These "repurposed" proteins are called syncytins.
They are essential for placental formation, yet are of independent origin in different kinds of mammals -- primates have one kind, mice another, rabbits, cows, and carnivores yet others. They are clade-specific. In fact, in 2015 a functional syncytin was found in several marsupials, extending the presence and essential function of the protein to all placental mammals examined.
All syncytins are lineage-specific, meaning that each mammalian clade has its own syncytin, with a unique sequence and location in the genome. They must have inserted themselves (or been placed there) after the separation of the mammals into different clades! This means there must have been multiple independent acquisitions of these syncytins to participate in an essential process that is common to all mammals. Why should there be unique syncytins in each clade?
I repeat: distinct syncytins for cows, carnivores, rodents, primates, rabbits, and even tenrecs (a species thought to retain features of primitive mammals). We all recognize these clades based on their traits -- cows are different from carnivores, which are different from rodents, which are different from chimpanzees. Even nursery school children can tell the difference. And they are all thought to have descended from a common ancestor, the proto-mammal. Well, apparently, these well-defined clades of mammals make their placentas using analogous but distinct proteins. This perhaps explains why the placentas of each clade differ in their structures. But it flies in the face of the idea that all mammals are descended from a single kind of ancestor with a single kind of placenta." EN&V
In the year 2000, French researcher Thierry Heidmann and coworkers found that genes derived from
I repeat: distinct syncytins for cows, carnivores, rodents, primates, rabbits, and even tenrecs (a species thought to retain features of primitive mammals). We all recognize these clades based on their traits -- cows are different from carnivores, which are different from rodents, which are different from chimpanzees. Even nursery school children can tell the difference. And they are all thought to have descended from a common ancestor, the proto-mammal. Well, apparently, these well-defined clades of mammals make their placentas using analogous but distinct proteins. This perhaps explains why the placentas of each clade differ in their structures. But it flies in the face of the idea that all mammals are descended from a single kind of ancestor with a single kind of placenta." EN&V
And God said, Let the earth bring forth the living creature after his kind,
cattle, and creeping thing, and beast of the earth after his kind:
Genesis 1:24
