Fish first emerged on to land some 375 million years ago.Those pioneering terrestrial vertebrates explored shallow waters, then soggy land, and finally dry land, with greater and greater confidence and dexterity.
Even as they mastered moving and breathing, eating and sensing on land, though, they were never fully clear of the aquatic lifestyle, as they had eggs that dried quickly unless immersed. These were early times, although those early times lasted for many millions of years. So even once they were able to spend their days—or more likely, nights—wandering the newly open landscapes, snapping up insects and making themselves at home, they needed reliable water close by if they were to reproduce.
These would become the first tetrapods—from the Greek: tetra, four; and pod, foot. (The name can be misleading, though, as some later tetrapods evolved out of four-footedness, while never losing the part of history that they came from.)
Tetrapods include not only the earliest amphibians, but also everything evolutionarily downstream of them: later, fancier amphibians like golden toads and giant salamanders, and also all the reptiles, including birds, and mammals. From dinosaurs to dugongs and bats to boas, from newts to narwhals and kangaroos to kookaburras, the diversity of tetrapods is remarkable.
Many (many) million years later, the amniotic egg evolved.The amniotic egg provides four new kinds of protection for the developing embryo, and all of the species who had an amniotic egg now enjoyed greater independence from water. These early amniotes—who would later diversify into, again, our modern reptiles, including birds, and mammals—could range farther afield. They no longer needed pools or streams of water into which to deposit their eggs. Places that had been off-limits were now open. Their explorations could now go deeper into deserts, and farther up mountains. Individuals still needed water, of course, as do we all, but they weren’t nearly so tied to it as their predecessors had been.
And yet, many years later yet—many millions of years, tens of millions of years, hundreds of millions of years even—one by one, after having gone through the grueling process of becoming fully terrestrial, several lineages of tetrapods returned to the sea.
Many of the tetrapods who returned to the sea are now extinct. Among these are the mosasaurs, sauropterygians and ichthyosaurs, all of whom made their way to the sea, and thrived for millions of years before disappearing into history.
Some other sea-returning tetrapods have not gone extinct, though, including sea snakes. Most sea snakes give birth to live young in the water, although a few species lay eggs in crevices on land. Sea snakes tend to be very docile in the water, but should you happen upon one, best to give it a wide berth—they are elapids, their closest relatives being cobras and coral snakes and the like. The neurotoxin that they will deliver if harassed is unlikely to provide a pleasant death.
These secondary returns to the sea were all independent of one another, each of the species responding to some shared ecological problem, to which the common solution was—back to the sea! These moves back to the water were all similar evolutionary solutions to one another in some ways, but not identical. Indeed, even sea-snakedness may have evolved twice, although that is still an open question.
And we haven’t even yet begun to speak of the mammals, who have had a long and successful history on land. Among us mammals, four different lineages have returned to the sea, to a greater or lesser extent.
There are of course the cetaceans, which includes baleen whales, such as humpbacks and blue whales, and also toothed whales, like orcas and dolphins. Their closest living terrestrial relatives are hippos. They have returned to the sea as fully as mammals can, living their lives entirely in the water.
There are also the sirenians, which includes manatees, dugongs, and sea cows. It has been suggested, at least as long ago as 1878,that these were the animals that formed the basis of the myths of mermaids and mermen. Their closest living terrestrial relatives are elephants and hyraxes. Manatees and their kin, while not mermaids, are, like the whales, fully aquatic.
A third lineage of mammals that have returned to the sea are the otters, a group of thirteen utterly charismatic and charming species. Their closest living terrestrial relatives are weasels (which in turn includes ferrets and stoats and mink). And despite some species bearing names that seem to suggest a full return to the water—sea otter, river otters—all otters are both obligately associated with water, but only semi-aquatic in their habit.
The final lineage of mammals that have returned to the sea are the pinnipeds, which include the seals, sea lions, and the singular walrus. With whom they are most closely related is an open question—maybe bears, or maybe, again, weasels, with bears a slightly more distant relative.And pinnipeds, like the otters, are obligately associated with water, but do not live in it full time. And unlike otters, they seem particularly ill-suited to life on land, galumphing and hauling themselves about, while otters can manage graceful, slinky gallops.
The return to the sea required the re-retooling of myriad systems, systems which had already needed to be retooled to accommodate life on land. From skeletal structure to how to breathe to how to tunes one's senses,everything needs to be a little, or a lot, different, if you live life in the water.
One of the many things that takes some adjusting, if you are a tetrapod who has returned to the sea, is how to sleep. Sleep is a universal among tetrapods, it seems, and certainly we can predict that it should be. As Bret Weinstein and I wrote in the Sleep chapter of A Hunter-Gatherer’s Guide to the 21st Century:
For every animal about which scientists have asked, do you sleep? the answer has come back in the affirmative, which leads us to the question of why.
Sleep almost certainly came to be part of our lives as the result of a simple trade- off: it is impossible to build an eye that is optimized for both day and night. You could have two sets of eyes, but it would be impossible to build a visual cortex optimized for both without vastly increasing the brain’s size and energy requirements. That creates a predicament. The predicament is: Should you be a creature with a very compromised eye, one that is not especially attuned to either day or night? Or should you specialize in one condition and sacrifice the other? All solutions exist. Day specialists are diurnal; night specialists, nocturnal; and those who specialize in the in-between times, crepuscular. There are diurnal dik-diks, nocturnal nightingales, and crepuscular capybaras. All solutions come with trade-offs.
There are, of course, many other advantages of sleep—and of dreams—which we explore in the book, but for now suffice it to say that sleep is both universal, and apparently mandatory, among tetrapods.
However, no tetrapod who has returned to the sea has reverted to having gills and breathing underwater. This, it would appear, is unreachable by evolution, at least thus far. You cannot get there from here.Rather, if you are a tetrapod who has returned to the sea, you are reliant on your lungs to breathe. You have also likely evolved a great capacity to hold your breath for long periods of time, and you are definitely required to come up for air with some regularity.
How then, can the secondarily aquatic tetrapod solve the problem of both needing to sleep, and needing to breathe?
Let us focus just on the mammals who have made the return trip, who have embraced a salty, briny, watery life, with all of the attendant costs and freedoms that come with it.
Again, there are four known instances of mammalian returns to the sea: otters, whales, manatees+, and pinnipeds.
The otters haven’t gone all the way: they don’t sleep in the water, at least not mostly. (Sea otters do, mostly by wrapping themselves in kelp, which protects them from both drowning and from predators.) So the sleep conundrum of most otters isn’t much of one: they sleep on land, just like the vast majority of mammals. They can go all in, sleeping both hemispheres at a time, their entire brain devoted to the activity of rest and repair, of learning and planning. This is what humans do, and again, what most mammals do, and it works out well, except that one can’t remain vigilant while asleep so fully. Watching out for predators, and making sure that you don’t drown, is not easy if your brain sleeps all at once.
What, then, do the marine mammals who are fully aquatic do? Whales do not spend time on land,so have innovated a new way of sleeping: half a brain at a time. This unihemispheric sleep allows them to stay vigilant, while still getting the sleep that they need. And the other fully aquatic marine mammals, manatees and dugongs, appear to have evolved the same trick, although less is known about their sleep.
This leaves us with the pinnipeds. The walrus and sea lions haul out onto land to sleep, so do not appear to be doing anything special, sleepwise, at least, so far as we know.
Seals, however, are sleep generalists. They’ll sleep anywhere.
What do you do if you sometimes sleep on land, and sometimes sleep in the water? How do you sleep if you’re a seal?
If you’re a seal, it turns out, you retain the old kind of sleep for land, and evolve a new kind of sleep for water. And then you sleep according to where you are.
Seals sleep with all their brain at once when they’re on land. Mostly. And when they’re in the water, they keep one half of their brain awake even when the other half is asleep, rather like in whales and manatees. Various species also adopt a number of positions that help keep their nostrils out of the water while sleeping, including pointing their snouts directly up in a position called bottling. It’s both adorable and functional. Thus, seals sleep according to the particular conditions that they find themselves in.
In summary: we are fish who walk on land. Some of us fish took a good long look around and went back into the water. We still need our sleep, though, and swimming while sleeping can be rough going, so all of us fish who came onto land and then went back to the sea have solved the problem one way or another, over and over and over again. We swim. We sleep. Evolution finds a way.
Natural Selections will surprise you: From adjectives to evolution, sex to seals, there’s something for everyone. Coming soon: reflections on a life that was well lived.
This number is inherently imprecise, but is in the right ballpark. See e.g., Daeschler, Shubin, and Jenkins 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, 440(7085): 757-763.
Tetrapods are named for the number of feet that their ancestors had, but all their descendants remain tetrapods even if their number of feet has changed. We cannot change what part of history we are from, even if we change our form and our function. Humans and birds, both having become bipedal, are still tetrapods. Snakes, have lost all their legs entirely, are still tetrapods.
If that seems vague, you’re right: it is. The origin of amniotes—when, from whom, all of it—is a live question in biology. Here’s one cool paper that will help the reader resolve the question of when amniotes showed up not at all: Starck, Stewart, and Blackburn 2021. Phylogeny and evolutionary history of the amniote egg. Journal of Morphology, 282(7): 1080-1122.
Slowinski, Knight and Rooney 1997. Inferring species trees from gene trees: a phylogenetic analysis of the Elapidae (Serpentes) based on the amino acid sequences of venom proteins. Molecular phylogenetics and evolution, 8(3): 349-362.
Brown 1878. The Sirenia. The American Naturalist, 12(5): 291-298.
The sirenians, elephants, and hyraxes seem, at first (and second, and third) glance to be rather completely different, but phylogenetic analyses consistently show them not only as very closely related, but as having diverged so rapidly there it is difficult to determine who among this group of three diverged first. See e.g., Nishihara, Satta, Nikaido, Thewissen, Stanhope, and Okada, 2005. A retroposon analysis of Afrotherian phylogeny. Molecular biology and evolution, 22(9): 1823-1833.
Koepfli and Wayne 1998. Phylogenetic relationships of otters (Carnivora: Mustelidae) based on mitochondrial cytochrome b sequences. Journal of Zoology, 246(4): 401-416.
Fulton and Strobeck 2006. Molecular phylogeny of the Arctoidea (Carnivora): effect of missing data on supertree and supermatrix analyses of multiple gene data sets. Molecular phylogenetics and evolution, 41(1): 165-181.
This is a fantastic book for those interested in the sensory constraints and adaptations of tetrapods that have returned to the sea: Thewissen and Nummela, eds., 2008. Sensory evolution on the threshold: adaptations in secondarily aquatic vertebrates. Univ of California Press.
The technical term for this is phylogenetic constraint.
Except: An occasional hunting tactic of orcas seems to be intentionally beaching themselves in order to capture elephant seal pups,then rolling back out into deeper water as the tide comes in. This requires intelligence, cross-generational learning and even apprenticeship, precise awareness of tides and landscapes, and some amount of luck. See e.g., Guinet and Bouvier 1995. Development of intentional stranding hunting techniques in killer whale (Orcinus orca) calves at Crozet Archipelago. Canadian Journal of Zoology, 73(1):27-33.
The story of how whales sleep is of course rather more complex. For a solid overview see Allada and Siegel 2008. Unearthing the phylogenetic roots of sleep. Current biology, 18(15): R670-R679.
It’s even more complex than that. For a bit more on the variety of ways in which seals solve their sleep problem—and for some revelations about the unihemispheric sleep of some birds (!), try: Mascetti 2016. Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives. Nature and Science of Sleep, pp.221-238.
Heather, Well this was off-the-wall but fascinating. Never even thought to think about this. Many thanks for broadening my horizons in unexpected directions this morning.
Fascinating article, making your work well worth the price of admission. A question for you, is there a plan to go back even further to elaborate on a pre oxygen planet and what the transition from carbon dioxide would have looked like? I’d be particularly interested in knowing what degree of mobility a co2 life form could be capable of.