A Quick Refresher:
Without going all the way back to my first post on this series 5 months ago, we have gone from the dawn of life to a mere 320 million years ago.
We have acquired multi-cellular-ism, a backbone, a face, bipedal-ism, a whole bunch more, and we just figured out how to crawl up onto land. The goal, as always, is to work our way to the Homo Sapiens - us. So what next?
Well, as we crawl onto land and our lungs develop in a specific way, we and amphibians part ways, with amphibians going into the class of anamniotes, and us into amniotes. Easy to remember, that. But herein lies a problem.
Going up on land and enjoying all the niche benefits is great... For a couple of years. But what happens when you want to settle down, start a family of your own, dominate the landscape? At this point, everybody is still laying their eggs in water - quite limiting. So what did the amniotes do to solve this?
Source: Petter Bøckman, CC BY-SA 2.0
Amniote is a greek term, of course, meaning 'membrane surrounding the fetus' which should give you a pretty good idea of what defines them - primarily the existence of a membrane surrounding the fetus.
Basically, we're talking eggs today.
Within this clade, you will find reptiles, birds and indeed mammals. Traditionally, the amniotes will have laid their eggs on the ground, eggs that have protection via membranes such as the amniotic sac. This now of course still applies to mammals, which carry their eggs inside the womb, with some weird freak exceptions. In fact early mammals may all have laid eggs, but more on that later.
The exact moment ground-laying eggers evolved is uncertain because the things that define them - reproductive organs - don't exactly fossilize well, or at all, but the earliest we know of is about 312 million years ago when the group Reptiliomorpha took to land and was pressured by this now inconvenient habit of needing water to lay eggs.
The process of adapting to land-egging is a complex one, but the most obvious is probably the hard, porous shell. Indeed modern day eggs are in fact porous, which is why an egg gone bad will float in water, having very slowly allowed air to seep inside.
There are three primary factors that define an amniote via the egg - and the image may prove useful here:
Allantois. In short, Oxygen that seeps through the hard shell will be absorbed into the fluid of the allantois where it can oxygenate the embryo. As the oxygenated fluid empties out, it gets replaced by waste produce.
Amnion. Literally translated as 'little lamb', it is essentially a gooey membrane that fills with amniotic fluid and develops into the amniotic sac where the fetus grows.
Chorion. Another membrane that forms from the yolk and forms another part of the amniotic sac and, among other things, allow the flow of nutrients from the blood of the mother to the child.
Interestingly these were not vital evolutionary steps, but did allow them to grow larger than 1 cm or so, a limitation occurring as a result of the ratio of surface area and volume affecting the rate of diffusion.
Indeed, amphibians lack these advanced features and are mostly limited to laying their eggs in water the old-fashioned way. At first, this was the case for every terrestrial creature, before moving to dark, moist terrestrial areas such as shallow ponds or moist under-log. But their gelatinous outer egg was slowly replaced with the hard shell which not only allowed things to get bigger, but also increase the rate of gas exchange. this is what was essential.
I came across a paper titled: 'Has the importance of the amniote egg been overstated?'*. It's an old paper from 1998 but let's hear it out. The author Joseph Skulan coins the term the Haeckelian framework, a series of apparently false assumptions that construct the argument that the amniote egg was the only way to lay eggs in the rough and tough conditions of land.
Skulan shows that the environment may have actually been easier to lay eggs than in water with surprisingly mild conditions bountiful, and that the view of amphibian eggs being more 'primitive' is simply false:
According to the Haeckelian framework, full liberation from standing water in vertebrates was made possible by the amniote egg. But the importance of the amniote egg in the history of vertebrate terrestrialization has been exaggerated. The amniote egg didn’t solve the problem of how to reproduce on land, because this wasn’t a problem in the first place.
So if animals didn't need to adapt in order to lay eggs on land, why did it even happen? Well, the above provides this answer too. By moving eggs onto land, animals can enjoy even greater freedom in evolution of their eggs:
If anything, the transition from water to land relaxed rather than tightened physical constraints on the vertebrate egg, and can have allowed terrestrial eggs to evolve in ways that would have been impossible in aquatic environments. Thick, calcareous shells, rapid development coupled with hatching at an advanced stage, and high metabolic rate are traits that could not easily have evolved in water.
Skulan's argument makes sense. Rather than evolving from a point of struggle, somehow desperately figuring out how to make things work their way, animals evolved from a point of opportunity, having already made it upon land and enjoyed freedom of adaptation. Not only that, but it was far away from those pesky fish and sharks. Perfect.
The allantois, amnion and chorion were simply adaptations that arose from this opportunity which now defines the amniotes; birds, reptiles and mammals.
Which came first, the mammal or the egg?
Obviously the Egg, I just felt it was a catchy subtitle. But things aren't as simple as they seem. As you probably guessed, we humans don't lay eggs. Most of us, anyway. Most of the internal workings of the amniote egg still exists in mammals, internally, with the exception of the hard, calcified shell.
The benefits are probably obvious; protection first and foremost, but the transition is quite a remarkable step, and we've managed to observe this in extant animals today - the skink.
Skinks are lizards, and lizards lay eggs, right? The yellow-bellied three-toed skink is no exception... with a few exceptions. There have been individuals observed of the species that happen to live higher up mountains that instead give birth to live young!
Another skink and a type of lizard are also known to practice both methods, but up to 20% of modern snakes and lizards give birth only to live young. This is amazing, but animals that do both? That's a whole other level and gives us a lovely glimpse into the evolutionary steps we mammals took.
The problem here is nutrients. In mammals, nutrients come from the placenta, but reptiles rely on the yolk. For calcium, they depend on the actual shell itself to make deposits. When eggs are kept inside the body like in fish, that wall of calcium is thinned back into a membrane that lacks this calcium - not good.
This skink demonstrates a process that allows calcium to pass from the uterus of the mother to the child inside her - an early form of a mammalian placenta in a reptile. For the yellow-bellied three-toed skink, the benefits of either method depend on its living conditions, but for us, live birth simply proved more successful on the whole.
But don't forget, Platypus and echidna are mammals that still lay eggs, presumably for the opposite reasons as the live-birthing skinks.
This is all to say that, when mammals separated from birds and reptiles 180 million years ago, we may have still been laying eggs - but as mentioned above, they don't tend to fossilize well so it's kinda hard to be sure.
We're finally breaking from the rest of the animal kingdom into our own class, mammalia! Specifically, 'Endothermic amniotes'. We're almost home! (I said that months ago, but whatever).
Thanks for reading =)
References: Allantois | Amnion | Amniote | Chorion | Has the importance of the amniote egg been overstated? | Reptiliomorpha | Evolution in Action: Lizard Moving From Eggs to Live Birth | The Evolution of Live-Bearing in Lizards and Snakes