Recently, a new creationist member of a Facebook group resorted to the familiar tactic of presenting the placenta as a complex organ that could not possibly have evolved. As so often with creationist arguments, this was little more than an argument from personal incredulity dressed up as a challenge. In place of scientific evidence, he relied on a god-of-the-gaps argument and a false dichotomy, implying that if he could not imagine a natural explanation, the only alternative must be magic performed by his preferred deity.
The new member appears to have left the group soon after replies began to appear in the comments, complete with links to articles and papers explaining exactly how the problem can be approached scientifically.
This paper in Nature by Oliver W. Griffith and Günter P. Wagner of Yale University provides precisely the sort of answer that exposes the weakness of this common creationist tactic on social media. Their argument amounts to little more than: “I do not know how this could have evolved, therefore God did it.” That is not an explanation; it is simply ignorance masquerading as evidence, and tells us more of the parochial ignorance of the creationist than they probably intended. The paper uses the evolution of the placenta to explain some basic principles of how complex organs evolved. Needless to say, no magic is involved anywhere in the process.
One of the authors, Oliver W. Griffith, has also written an article in The Conversation explaining their research and what it tells us about the evolution of complex organs in vertebrates. His article is reproduced here under a Creative Commons licence, reformatted for stylistic consistency.
First, a short explanation of the role of the placental in placental mammals:
The role of the placenta in pregnancy. The placenta is a temporary organ of pregnancy that forms in the uterus and links the mother’s circulation with that of the developing fetus through the umbilical cord. It acts as the fetus’s life-support system, supplying oxygen and nutrients while removing carbon dioxide and other waste products. Importantly, the mother’s blood and the fetus’s blood normally come into very close contact in the placenta without directly mixing. [1]
Its role is not just passive transport. The placenta is also an active physiological interface, controlling which substances cross from mother to fetus by processes such as diffusion and active transport. In effect, it substitutes for organs that the fetus cannot yet use fully on its own, especially the lungs for gas exchange and, in part, the digestive system for nutrient supply. [2]
The placenta is also an endocrine organ. It produces hormones that help maintain pregnancy and regulate changes in the mother’s body needed to support fetal development. Among these are hormones such as hCG and others involved in sustaining the uterine environment and coordinating maternal physiology during pregnancy. [3]
In addition, the placenta has important immune and protective functions. It helps mediate the complex relationship between mother and fetus, allowing the fetus to develop despite carrying genetic material from the father, while also acting as a selective barrier rather than a simple open conduit. It is therefore a multifunctional organ: respiratory, nutritive, excretory, hormonal and immunological all at once. [3]
In Summary, the placenta:
Using the placenta to understand how complex organs evolve
Developing lizard embryo beneath placental tissues.
Oliver Griffith, CC BY-ND
Considering how different they look from the outside, it might be surprising that all vertebrates – animals with a backbone – share the same, conserved set of organs. Chickens, fish, human beings – all have hearts, livers, brains, kidneys and so on. Each of these organs performs a specialized set of functions.
On the inside, most vertebrates have a similar set of organs, inherited from a common ancestor.
Thomas Wallace. Image of display at the National Zoo., CC BY-ND
To gain insight into how new organs come to be, my colleague and I decided to focus on one that’s more recently evolved – the placenta. It’s a complex organ that has evolved many times independently. In modern animals we see species with no placenta, species with a complex placenta and myriad species between these extremes. By investigating the evolution of the placenta, my colleague Günter Wagner and I identified several processes that appear fundamental to the evolution of all new organs.
What’s the placenta’s job?
In live-bearing animals, the placenta is the organ in the pregnant mother’s body that gives the unborn offspring the raw materials it needs to grow and develop. Made of both parental and embryonic tissue, it supports the exchange of nutrients and gasses between parent and developing embryo.
The first vertebrates, including early mammals, laid eggs and did not have placentas. But in the ancestor of marsupials and eutherian (formerly called placental) mammals, females evolved to hold their eggs inside the uterus until embryonic development was complete. For an embryo to be maintained inside the mother for the duration of pregnancy, it needs a placenta to supply it with oxygen and nutrients, and take away carbon dioxide and other waste products.
Placental structures have evolved to support pregnancy in most organisms that give birth to live young, totaling more than 100 independent origins across the animal kingdom. It didn’t happen out of the blue – but what are the steps that result in a new organ? Complex biological structures can evolve via simple changes that build up over time. The process depends on the fact that animals can acquire new body parts and body parts can acquire different functions over multiple generations through successive changes to a species’ DNA.
New uses and structures for old tissues Placentas have evolved across animals in various ways, but always by repurposing existing tissues.
Lizards and snakes provide one example. Most of them lay eggs, but live birth has evolved in this group more than 100 times. In all of these cases, the mother retains the eggs in utero until the offspring are fully developed. In these reptiles, placentas form from the uterus and embryonic membranes that ancestrally lined the internal surface of the eggshell. That’s how it works in mammals (like us), too.
Pregnant male seahorse.
In each of these cases, placentas form when embryonic tissues come into contact with a parental tissue during development.
While placentas have evolved by repurposing existing body parts, sometimes we also see the evolution of entirely new biological structures within these old body parts. Seahorses’ broodpouches are one example, originating evolutionarily after eggs in an ancestor species attached to the underside of the parent’s belly. New specialized cell types can evolve, too, to perform new functions.
Placentas have evolved many times independently in various animals. They form following the interaction of parental (red) and embryonic (blue) tissues.
Oliver Griffith, CC BY-SA
So in the case of the placenta, the evolution of a new organ involved repurposing existing tissues. These tissues had functions in the ancestor of the placental animal, some of which have been recruited to support the new organ functions.
One example of this is hormone production in embryonic membranes. In egg-laying terrestrial vertebrates, eggs are lined with a series of membranes. These embryonic membranes produce a diversity of hormones, which are likely important for the growth and development of the fetus. Following the evolution of live birth, the hormones were able to interact with maternal tissues, resulting in the evolution of fetal-maternal communication.
Prior research has shown that signaling between distinct tissues is typically how organ development is initiated in animals. We suspect that this signaling was important not just for the organ’s development, but for how it originated.
If a mutation results in tissues developing alongside each other in a new way, then the signaling dynamics inside these tissues are likely to affect each other’s development. This new signaling can then be an initiator for the development of a new organ.
We think the placenta is just one example of this phenomenon in action. This path may be a general way in which new organs arise in animals.
Pregnancy in eutherian mammals, which include humans, has required the evolution of a new cell type in the uterus.
Our research suggests that new organs evolved by repurposing existing body parts. Over evolutionary time, organs can evolve new structures and new functions that increase the fitness or reproductive success of the animal. These changes result from genetic alterations that we can identify by comparing living animals.
An individual animal doesn’t all of a sudden have an organ that’s never been seen before. But small genetic changes happen all the time. As they accumulate, eventually, complex biological structures can evolve. Now we’re starting to identify specific types of genetic changes that allow for new complex organs to evolve inside animals.
Oliver Griffith, Postdoctoral Associate in Ecology and Evolutionary Biology, Yale University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
What makes the placenta such a problem for creationism is that, far from appearing as a fully formed, all-or-nothing invention, it bears all the hallmarks of an organ shaped by evolution through modification of what already existed. Comparative biology shows a graded series across mammals, from egg-laying monotremes with no true placenta, through marsupials with simpler and shorter-lived placental structures, to the complex chorioallantoic placenta of eutherian mammals. That is exactly what evolution predicts: not sudden magical appearance, but successive adaptation, elaboration and co-option of existing tissues and genes.
The same is true at the molecular level. The placenta did not require the conjuring of some wholly novel biological principle; it evolved by repurposing ancient genetic pathways, including mechanisms involved in immunity, tissue invasion, nutrient transfer and hormonal signalling. Even elements derived from ancient viral insertions have been recruited into its function. In other words, the placenta is not evidence against evolution at all; it is one more example of evolution’s characteristic way of working with whatever materials happen to be available.
For creationists, this leaves only the familiar refuge of argument from ignorance: to declare that because they do not know how something evolved, it therefore could not have evolved. But ignorance is not evidence, and incredulity is not an argument. The history of science is full of phenomena once declared impossible by the scientifically illiterate, only to be explained in detail once the evidence was examined properly. The placenta belongs firmly in that category.
So, yet again, the choice is not between “evolution or nothing”, but between evidence-based explanation and a superstition inserted into the gaps of understanding. The more we learn about organs such as the placenta, the clearer it becomes that complexity is not a barrier to evolution but one of its most compelling products. Far from pointing to a designer, the placenta points instead to deep ancestry, gradual modification and the untidy, opportunistic ingenuity of natural selection.
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