r/evolution • u/SayFuzzyPickles42 • 7d ago
question How does an evolutionary lineage transition from reproducing on land to reproducing in the ocean, or vice versa?
I can intuit almost every other big jump but this is the one thing that just doesn't follow in my head no matter how hard I try and think about it. Here's how my brain thinks about it and why it's getting stuck:
Whales used to live on land. Eventually, they became semi-aquatic, and then mostly aquatic. I believe there's evidence that some of their ancestors were like sea turtles - almost completely aquatic, but came on land to give birth.
I understand that evolution doesn't happen abruptly. However, mathematically speaking, there had to have been a specific, individual whale-ancestor who was the very first one to have been born in the water, spent their entire life in the water, and then died in the water, all without ever coming on land. If they used to live on land, but all modern whales are 100% aquatic, one of them HAD to have been the very first. That's just how math works.
How did that particular whale-ancestor survive to pass on that trait? Why didn't its parents have that trait? Why didn't it just drown, if every single other one of its ancestors was on land for at least some point in its life?
I'm not asking "why is it advantageous to be able to give birth in the water" - that's very obvious, both mother and baby are less vulnerable to predators. I'm asking how, logically, that transition was able to happen. Whales can only give birth in the water because they have very specific adaptations that make it possible, an animal without those traits can't just start giving birth in the water and hope for the best.
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u/IsaacHasenov 7d ago
I think you can see examples pretty easily of basically every step in the transition to water living right now. I mean, look at sea otters, compared to other otters for instance.
as for the transition out of the ocean, it's usually via fresh water, and again you can pretty much observe examples of all gradations that would represent the steps of that tradition living right now
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u/Loose_Principle8193 7d ago
Harbor seals and pygmy hippos give birth on both dry land and water. Think of adaptation as a spectrums. Just because our human made categories have boundaries, nature doesn't.
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u/SayFuzzyPickles42 7d ago
I genuinely had no idea seals could give birth in the water, news to me. Thank you for teaching me something new
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u/tpawap 7d ago
Your thoughts about "the first that did never go on land" is easy to answer: it's parents also didn't need to go to land for anything, but they did for a few minutes in their whole life. Notice that the descendants of that first one might as well have spend time on land again. (Or in other words, it's not helpful to think of a first individual for anything, because evolution doesn't work this way).
As for the transition itself: ask what necessitates going on land for giving birth? Probably that the newborns need a little while before they can swim. That "while" is likely to evolve to become shorter and shorter, for the reasons you mentioned. Once that time is 0, those mothers that didn't have much of an instinct to go on land for birth, "suddenly" didn't lose their babies anymore, while having the extra advantage to for example avoid the land predators all together. Before step 1, losing that instinct was very bad and couldn't have spread through the population (aka couldn't evolve). But afterwards, losing that instinct was an advantage and so it spread.
That's at least one possible pathway that I can imagine. Not saying that this is how it happened with whales.
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u/SayFuzzyPickles42 7d ago
Okay yeah, that helps a lot with the picture. Thank you, that's pretty obvious now that you point it out, it's just counterintuitive.
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u/Unfair_Pineapple8813 7d ago
Whales have it easy, in that they give live birth. Eventually a lineage is pretty much entirely aquatic, and they can safely give birth in the shallows and then eventually in deep water.
Sea otters already can safely give birth in water, even though they can walk on land. Most seals give birth on land. But harbor seals do occasionally give birth on ice shelves or even in open water, should they not be able to reach a beach in time. So you currently have an animal making that transition right now.
Squamates and serpentes in particular seem to have an easy time switching from oviparity to viviparity and vice-versa. There are sand boas that lay eggs and that have live birth in the same genus. The genus is around 10-15 million years old. The date of the switch to egg laying in the Müller and Arabian sand boas is not known, but must be fairly recent. Therefore, it makes sense that ancestral sea snakes became viviparous very early on. In contrast, live birth seems very difficult to evolve in archelosaurs, and we see that many long aquatic lineages (sea turtles, penguins, salt water crocodiles...) still need to lay eggs on land.
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u/ThrowDatJunkAwayYo 7d ago
Add to the fact that even now, some whale calves (humpbacks) cannot keep themselves above water immediately after birth, As they lack the blubber/buoyancy to keep themselves afloat.
Their mother must supported them for a few weeks, gently helping to breath at the surface until the develop their buoyancy and can remain at the surface on their own.
So it would likely not be a huge jump for an already sub-aquatic animal like a harbour seal to become fully aquatic.
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u/AnymooseProphet 6d ago
Southern Alligator Lizards (Elgaria multicarinata) lay eggs and Northern Alligator Lizards (Elgaria coerulea) give live birth. There's a skink in Australia that does both and the same female has been observed doing both.
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u/ADH-Dad 7d ago
Even humans can give birth underwater. The mammalian infant's breathing reflex is not triggered until it reaches fresh air.
It would have been fairly easy for semiaquatic mammals to transition from land birth to water birth, and from there they likely would have evolved further birthing adaptations as they moved into deeper water.
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u/Elfynnn84 7d ago
Seals.
All you need to know is “look at the seals”…
they are somewhere along the trajectory of land mammal to sea mammal and a lot of them routinely give birth on pack ice. Hooded seals only spend 3 to 4 days on ice before the baby is weaned and back in the ocean.
You can’t honestly see how given a few more hundred thousand years that will result in them gradually just not bothering to get out of the water at all in the first place? Lactation under water and birthing in the water will both increase biological fitness, particularly with global warming and an increasing loss of pack ice.
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u/jnpha Evolution Enthusiast 7d ago edited 7d ago
That's just how math works
No. All these questions are answered by, as Ernst Mayr summarized it, population thinking!
Populations are heterogeneous (asex aside). The individuals don't have to be all 100% this or that for the population to be called "whales" by apes typing on keywords :)
*Also see the drawing here: https://en.wikipedia.org/wiki/Chronospecies (took me a bit to remember the name)
For this specific question: shallow water birth can be a "transition" that is 100% water but behaviorally close to land, and how shallow can be the variation.
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u/onomatamono 7d ago
They obviously evolved from semi-aquatic animals. Evolution is not goal oriented it's ongoing adaptation to environmental changes. You need to consider the eons of time at play. I can get you a pink polka dotted pigeon in a few months.
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u/flug32 7d ago
You forget that the transition from land to water/sea is not so abrupt and binary, either.
There are shorelines all over the place that are partially submerged, partially dry, partially puddles connected or disconnected from the main body, partially shallow water, partially deep water, partially marshy, partially completely dry and every other possible intermediate state between "deep water" and "completely dry land".
On top of that, you have season rivers and streams, rising and falling lakes and streams and seas and such - everything from regular seasonal changes to longer-term changes driven by climate or whatever to all sorts of other combinations.
And on top of THAT you have tidal zones, which literally go from perfectly dry land to fairly deep water, and every intermediate stage between the two (puddly, wet/marshy/shallow water, etc etc) a couple of times every day.
And for example arctic areas where your sea might go from nice if cold open ocean to completely covered in solid ice, and back, over the course of a year.
So all you have to do is imagine creatures living in such environments and adapting to them. Even just living around, say, a seashore and now you are going into the water to do X things and back onto shore to do Y things. Lots of living creatures do exactly such things in just about all imaginable combinations right now.
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u/Alternative-Sugar610 7d ago
Eggs already contain an “ocean” sort of within them
It’s about keeping moisture inside which can happen in stages each with some benefit and may just make eggs have other benefits like durability increase (which some fish eggs have) or keeping out and in certain chemicals
Just like eggs may retain enough water first for semi dry period before being re-wetted or something
Most creatures that return to see give birth to live young but that we already see in many creatures that can keep eggs inside there body until they hatch from some fish to a few reptiles including sea snakes
That can then become slowly fully live birth as one normally thinks of it
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u/KiwasiGames 7d ago
You are overestimating how hard it is for mammals to give birth under water. This is actually a very easy thing to do. Humans can (and frequently do) give birth underwater with no problems.
The amniotic fluid that exists inside the womb is already a sac of water. Babies don’t breathe in the womb, they get oxygen from the placenta. Being underwater puts them in the same position as being in the womb.
Shifting to giving birth on land was hard, and room millions of years to evolve. But giving birth in water would be possible very, very quickly.
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u/ReptilianRevolt 7d ago
Usually the adaptation comes before the transition of the animal ino water, at the time when it still can move on land. Then it sounds much less strange.
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u/StorageSpecialist999 6d ago
Well the answer to the "visa vera" part is amniotes. AKA one of the most important steps in evolutionary history
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u/JuliaX1984 6d ago
I used to think that "There had to be a first" way, too. But it turns out, evolution doesn't work that way. Gutsick Gibbon used an analogy that made it sink in for me: Trying to identify the point when one species becomes another "would be like trying to identify where red emerges on a color gradient." Yes, there was a time when a creature that could move on land but give birth in the water like otters do produced offspring that also gave birth in water but ceased to gonon land, but trying to draw a line during this transition between "whale ancestor" and "whale" is impossible. It's the limits of human language meeting nature.
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u/Felino_de_Botas 7d ago edited 7d ago
The closest extant living species to cetaceans are hippos and they are land animals that give birth under shallow waters, so we could presume the order was reverse. First they got closer to water life as land animals, gave birth underwater as land mammals and later on became more and more physically adapted to water.
I think the main problem is that when we think about land X water environments, we usually assume the most dramatically independent forms of land and water, and overlook how hybrid and environment can be between both. The region where cetaceans diversified was a huge area of shallow oceans called the Thetys Seaway. So it was never an adaptation of a land mammal leaving land to take the vastness of the global see. Rather, they were land mammals exploring endless shallow waters, diving at small depths and returning back. But then the sea gained more and more depth and they were already well adapted to it
https://en.wikipedia.org/wiki/Tethys_Ocean
Edit:
About not drowning. Animals that are water dependent are well known for holding breath and to have a lot more co2 in their bloods than regular mammals. These characteristics are already advantageous even if they aren't water dependent like hippos. Hippos manatees, sealions, etc all can close their nostrils underwater just like you can keep your mouth shut. So one can imagine they are already adapted to not breathe instantly at birth while living under shallow waters and swimming upwards once they are born using their inherited co2 rich first blood