Evolutionary Novelty: Mammalian Placenta

For me one of the most visceral confirmations of the common descent of humans and other mammals came while witnessing the birth of my children. Having grown up on a small farm, I have vivid memories of the birth of kittens, lambs, and goats; and after the births of my children, I was struck by the similarity of human placenta and umbilical cord to those of other mammals.  Given common descent, how did something as complex as the mammalian placenta originate in the first place? The answer, according to research published last summer in Genome Research, involves the evolutionary mechanisms of co-option and gene duplication.

Fig 1.  For me, witnessing the birth of goats, humans, kittens, and sheep - and especially umbilical cords and placentas, was a visceral re-enforcement of the scientific fact of the common ancestry of eutherian mammals.

Mammalian placentas can be considered an evolutionary novelty, like light sensitivity, hair and animal photosynthesis.  Like these other traits, a placenta is not one thing, but a collection of many structural and functional components.  We know that placentas are present in "Eutherian" mammals, but absent in marsupials, monotremes, and most non-mammals.  Therefore, based on simple parsimony, placentas originated prior to the common ancestor of eutherians.  This presents us with a null hypothesis, that components of placentas also originated at the same time.

Figure 2 - Mammal phylogeny.  Only Eutherians have placentas.  Monotremes lay eggs, and marsupials carry babies in a pouch.  Did all the components of placentas also originate with eutherians?

Research published last summer by Knox and Baker investigated the timing of the evolutionary origins of genes expressed in mouse placentas.  Since scientists have determined the sequence of all genes of the mouse genome, placental expression of all those genes could be investigated simultaneously using microarray technology.  The genes expressed in early development of mouse placentas have ancient origins.  In contrast, the genes expressed later during the development of mouse placentas have much more recent evolutionary origins.  Here, the authors define the origins of genes to be related to when the last time was they were duplicated.

When the genes expressed in a structure originated before the structure itself, we can consider this a co-option event: Genes used for other purposes are incorporated (co-opted) into the new structure.  Given common ancestry of all genes and organisms, it should not be surprising when we demonstrate co-option.  Nevertheless, there are not many cases where comprehensive gene expression within a structure has been studied in the context of evolution.  In the case of placentas, early development involves rapid growth of tissue, and deploys many genes involved in cell proliferation.  Cell proliferation and the genetic machinery for accomplishing this is conserved in evolution.  In the case of placentas, instead of re-inventing a new way of proliferating cells, or instead of duplicating cell proliferation genes especially for use in the placenta, existing genes were deployed in a new context.  This can be thought of as co-option.  An observation further consistent with co-option is that egg-laying relatives of eutherians use a membrane in eggs for oxygenation that may be similar to placentas.

Unlike the genes in early development of placentas, genes expressed later in placental development tend to be recently duplicated.  To test whether or not recent duplication of late expressed placental genes was unique to mice, the authors also examined genes expressed in human placentas.  Here again, many human placental genes were recently duplicated.  The authors suggest that the diversity of placental forms may in part be due the expression of recently duplicated genes. 

The visceral re-enforcement of common ancestry I felt when seeing a human placenta and umbilical cord extends to the genes used in developing placentas, which themselves have ancient origins, and are shared across many organisms.

Reference
K. Knox, J. C. Baker (2008). Genomic evolution of the placenta using co-option and duplication and divergence Genome Research, 18 (5), 695-705 DOI: 10.1101/gr.071407.107