This post is the fifth (and last) in this series. The first is here. In this post, we examine how the lineage leading to placental mammals may have shifted from laying eggs to live birth, with the subsequent development of a placenta.
In the first four posts in this series, we have seen that there is strong genetic evidence to support the hypothesis that placental mammals descend from egg-laying ancestors. One such line of evidence is the remains of three vitellogenin genes in the genomes of placental mammals exactly where common ancestry predicts them to be. Vitellogenins, as we have seen, are necessary for forming egg yolk. In placental mammals, yolk is not made, and embryos are connected to the mother to receive direct nourishment throughout development.
Often when I present on these lines of evidence, questions arise: how is it that an organism could suddenly stop laying eggs and switch to live birth? How could a placenta suddenly appear? Many of these questions are rooted in the misconception that evolution is a sudden process, perhaps requiring large mutations that dramatically alter one organism. Rather, evolution is a gradual process where average characteristics of a population shift over time. As such, evolution predicts that the transition to live birth and a placental connection was a gradual one. Even with these understandings in place, however, the gap from one form of reproduction to the other seems too far for evolution to bridge.
As Tompkins puts it:
The grand evolutionary story claims that egg-laying creatures share a common ancestry with placental mammals. Non-mammalian vertebrates, such as birds and reptiles, lay eggs with nutritional reserves in the egg yolk to nourish the growing embryo inside. In contrast, the embryos of placental mammals are nourished through the placenta, a specialized organ attached to the uterine wall of the mother. Placental mammals are born alive and do not hatch from eggs.
The supposed transition from an egg-laying reproductive system to a placenta-based system is notoriously difficult for evolutionists to explain.
So, is this transition “notoriously difficult” to explain, as Tompkins claims? No, actually, it’s not. Let’s take a look.
One thing to keep in mind in this discussion is that mammals, as a group, are nested within reptiles. In other words, mammals are modified reptiles. There are numerous transitional fossils linking reptiles to mammals, and the now out-dated names for these groups were once “mammal-like reptiles” and “reptile-like mammals” with no clear dividing line between them, illustrating the gradual shift from reptile to mammal.
So, present-day reptiles and present-day mammals are close relatives. Hold that thought.
Those who have read my Evolution Basics series will also be familiar with the idea of convergent evolution – the observation that evolution often “repeats itself” in lineages. We see fish and dolphins arrive at very similar body shapes, for example – shapes that were not inherited from their last common ancestral population.
One way to explore the possibility of an egg-laying to live-birth to live-birth-with-placental-development transition in the placental mammal lineage is to see if other lineages have also – convergently – arrived at something like placental development from an egg-laying starting point. The place to look for such transitions would be among mammalian relatives, such as reptiles.
When we do this, we find numerous examples of reptiles that have shifted from egg laying to live birth. We also find examples of live-bearing reptiles that have placentas. Moreover, there are even examples of reptiles (skinks, as it happens) of the same species that can either lay eggs, or bear live young, depending on the environmental conditions they find themselves in. In challenging conditions, they retain their embryos for live birth. In easier conditions, they lay eggs. All that is required for live birth in these “dual mode” species is an eggshell that progressively becomes thinner and thinner as development proceeds, such that it is merely a thin membrane at the time of birth.
Ironically, the young-earth creationist organization Answers in Genesis has commented on these observations, and concluded that the ability to switch between egg laying and bearing live young in reptiles (and the various types of placentas seen in reptiles) is consistent with their view of “created kinds”:
Extant live-bearing lizards and other reptiles display an array of placental morphologies. Furthermore, at least two species of skinks contain both egg-laying and live-bearing members. While touted as evidence for evolution, these species are actually good illustrations not of evolution but of genetic variation within created kinds. Those dual-mode skink species exist in different climates, each variety being well-adapted to the challenges of its environment. And there is no evidence that those skink populations are changing their reproductive habits at all or evolving into new kinds of creatures.
While we cannot know for certain how many created kinds of lizards God made in the beginning, we could postulate that the genetic ability for egg-laying and live-birth co-existed at least in skinks of the dual-mode types … the diversity of placental morphology among reptiles is consistent with the biblical concept that the created kinds of creatures diversified to fill the ecological niches of the world. Natural selection (and other mechanisms) would have allowed the best-adapted populations to survive in each habitat.
So, from a young-earth perspective, these sorts of shifts in reproductive strategy simply reflect the “genetic variation within created kinds”. Why such (repeated) transitions within reptiles is viewed as trivial, but such a transition within mammals would be “notoriously difficult” is not explained.
From egg to placenta
The evidence we see from present-day reptiles thus gives us ideas about how this transition may have occurred in the placental mammal lineage:
- An egg-laying species shifts towards being able to either lay eggs, or retain them for live birth with a thinned shell, depending on the conditions present. Vitellogenins would be needed and useful at this point.
- This lineage then becomes increasingly committed to live-bearing, still with intact vitellogenins. Eggs have yolk as per normal, but are not laid any longer.
- Once committed to live-bearing, embryos receiving nourishment from the uterine wall during the time they are being held becomes advantageous – such as uptake of calcium from the mother, as seen in present-day “dual mode” skinks.
- Over time, variation and selection allows for the connection between embryos and the uterine wall to become more elaborate – the beginning of a placenta (such as the varied “placentas” we observe in present-day live-bearing reptiles).
- As a stronger placental connection evolves, there is correspondingly less need for egg yolk to nourish the embryo. Eventually, the vitellogenins are lost, but without impact, since the placental connection has replaced their function.
Of course, these are ideas – hypotheses – about how this transition may have taken place. The point is not that we have determined exactly how it took place, but that it is a probable path based on what we see in other organisms. The point is that this transition seems readily accessible to present-day reptiles – and thus was likely possible for their close mammalian relatives as well. As such, the evidence for vitellogenin pseudogenes we see in placental mammal genomes should come as no surprise.