The placenta is a transient organ cooperatively developed by the mother and the
fetus. Our mammalian line, termed the placental mammals, has developed a placenta
that allows the fetus to safely be transported internally by its mother rather than
risking the parents needing to guard eggs. Placental mammals have claimed mammalian dominance, leaving the monotremes and even the marsupials marginalized. It
certainly seems as though we, the placental mammals, thusly have gained an
evolutionary advantage by successful cheating!
The viral envelope gene which humans and other primates use when initiating
formation of a placenta comes from Human endogenous retrovirus W. The involved
gene product is called Syncytin-1. Unfortunately, the envelope genes of endogenous
retroviruses may contribute to the pathogenic inflammatory process associated with
multiple sclerosis.
The upper image of Fig. 26.35 shows the monotreme Tachyglossus aculeatus,
commonly known as either the Australian echidna or the Short-beaked echidna, and
it lays eggs. The lower image shows the placental Rangifer tarandus, commonly
known as reindeer. When I was a young child, I could not understand why this
animal species seemed to have been named for the form of precipitation known as
‘rain’. My understanding has improved and I now know that the term reindeer means
the deer can be harnessed. Placental mammals have in one sense harnessed our
endogenous retroviruses, although we lack full control of the reins.
26.11 Summary
The competition for evolutionary success has included discovering ways to do things
differently. Competitive advances sometimes can result from changes which seemingly are minor but somehow prove to be powerful, such as genomic rearrangements
which improve regulatory functions and changes in the efficiency of enzyme
kinetics.
Development of effective symbiotic interactions has been one of the factors that
drive evolutionary competition. Symbiotic interactions seem to have shaken up the
entire order of life on several occasions, including the development of mitochondria,
chloroplasts, and a placenta. All of these might be perceived as having represented
ways of cheating to produce biased outcomes.
Success is defined by the winners, history is written by the victorious, and we the
placental mammals . . . have won! We have either out competed or just simply eaten
everything that presented a siginificant obstacle in our pathway to evolutionary
success.
That declaration of victory by placental mammals will stand until microbes
decide to reclaim the title. We could not exist without cooperative agreements that
have been forged with the microbes. The microbes survived for a long time before
we evolved, and the microbes largely don’t need us.
588
C. J. Hurst
fetus. Our mammalian line, termed the placental mammals, has developed a placenta
that allows the fetus to safely be transported internally by its mother rather than
risking the parents needing to guard eggs. Placental mammals have claimed mammalian dominance, leaving the monotremes and even the marsupials marginalized. It
certainly seems as though we, the placental mammals, thusly have gained an
evolutionary advantage by successful cheating!
The viral envelope gene which humans and other primates use when initiating
formation of a placenta comes from Human endogenous retrovirus W. The involved
gene product is called Syncytin-1. Unfortunately, the envelope genes of endogenous
retroviruses may contribute to the pathogenic inflammatory process associated with
multiple sclerosis.
The upper image of Fig. 26.35 shows the monotreme Tachyglossus aculeatus,
commonly known as either the Australian echidna or the Short-beaked echidna, and
it lays eggs. The lower image shows the placental Rangifer tarandus, commonly
known as reindeer. When I was a young child, I could not understand why this
animal species seemed to have been named for the form of precipitation known as
‘rain’. My understanding has improved and I now know that the term reindeer means
the deer can be harnessed. Placental mammals have in one sense harnessed our
endogenous retroviruses, although we lack full control of the reins.
26.11 Summary
The competition for evolutionary success has included discovering ways to do things
differently. Competitive advances sometimes can result from changes which seemingly are minor but somehow prove to be powerful, such as genomic rearrangements
which improve regulatory functions and changes in the efficiency of enzyme
kinetics.
Development of effective symbiotic interactions has been one of the factors that
drive evolutionary competition. Symbiotic interactions seem to have shaken up the
entire order of life on several occasions, including the development of mitochondria,
chloroplasts, and a placenta. All of these might be perceived as having represented
ways of cheating to produce biased outcomes.
Success is defined by the winners, history is written by the victorious, and we the
placental mammals . . . have won! We have either out competed or just simply eaten
everything that presented a siginificant obstacle in our pathway to evolutionary
success.
That declaration of victory by placental mammals will stand until microbes
decide to reclaim the title. We could not exist without cooperative agreements that
have been forged with the microbes. The microbes survived for a long time before
we evolved, and the microbes largely don’t need us.
588
C. J. Hurst