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

I was a teenaged nerd boy

I guess I've always made nerdy hypotheses about the world. Over at Observations of a Nerd, there is an interesting post that is in line with an hypothesis I remember making long ago, when I was a teenager or so.

Here is the gist of the post:

An ingenious study published in Personality and Social Psychology Bulletin (PDF) looked at how well people knew their own face. They took neutral photos of people and morphed their images to different degrees to either be more "attractive" (like a composite face) or "less attractive" (people with craniofacial syndrome). They then asked the participants to choose their actual, unmorphed photo from the variety of choices.
But instead, more than half the participants picked the morphs that were more attractive (even by their own opinions) than their real photos. In other words, they thought they were hotter than they really were....
Participants in the study were also asked to do the same lineup-picking for close friends, and their bias towards attractiveness also extended to them.

I actually remember making this hypothesis long ago, and wanting to test it in a similar way. My "data" were that when I saw people I knew well in a mirror, like family, I was surprised at how asymmetrical they looked to me. Of course symmetry is linked with attractiveness, thus the link with the work above. Back then, I hypothesized that I was somehow correcting in my mind for people's asymmetries. Similarly, when I saw a double-reflection of myself, I was struck by how asymmetrical (=ugly) I actually am, compared to what I "see" when I view my single reflection. Again, I was somehow correcting for my own asymmetries when viewing my reflection, and that correction didn't work the same when seeing a double-reflected image.

I haven't had a chance to read the original paper, but here is an extension based on my nerdy observations of the world: I think this symmetry-enhancement extends to pets. I remember seeing my cat PeeWee in the mirror, and being struck in the same way at how asymmetrical her reflection appeared to me.

I remember trying to explain this to some friends once. They thought I was crazy. So I thought maybe I am the only one who experiences this reflection asymmetry shock. The work described at OoaN makes me feel like I might not be the only one!

Losing meme virginity

I vowed I don't actually have time for memes, but since I got tagged twice, once by Pleion, and once by Observations of a Nerd, I suppose I should oblige. I normally don't like to do things without thinking a lot about them. I mean a lot. So, I think it would take me a long time to feel satisfied I could really do this meme right. I mean a long time. Like more than 1 life time. But, since I can't now indulge in thinking of great science books for a few life times, I will name some off the top of my head:

The meme:
Imagine: YOU are asked to assign a half-dozen-or-so books as required reading for ALL science majors at a college as part of their 4-year degree; NOT technical or text books, but other works, old or new, touching upon the nature of science, philosophy, thought, or methodology in a way that a practicing scientist might gain from.

1. Darwin, C. R. 1881. The formation of vegetable mould, through the action of worms, with observations on their habits.

2. Thompson, D. 1917. On growth and form.

3. Capra, F. 1975. The Tao of Physics: An Exploration of the Parallels Between Modern Physics and Eastern Mysticism

4. Gould S.J. 1989. Wonderful Life. paired with Conway-Morris. Life's Solution: Ineviteble Humans in a lonely universe.

5. Kuhn. T. The Structure of Scientific Revolutions

6. Tufte. E. The visual display of scientific information



I won't tag other blogs though. I don't like to promote pyramid schemes, in general.