14
$\begingroup$

The common evolutionary belief is that life originated in water, and that it developed to survive on land later on. Eventually, mammals evolved on land.

Cetaceans, which include dolphins, whales, and other marine mammals, then evolved from land-dwelling creatures, going back into the water.

From evolution.berkley.edu

Some believe this change has made it unlikely for cetaceans to revert back to life on land again. Notably:

  • Their legs atrophied over time and now they rely on one pair of fins and a tail. These are not as suited as the two pairs of fins and relatively smaller, lighter tails that the first amphibious animals had.
  • Some have twice as much hemoglobin as most mammals, so they don't have to come up for air as much. If they breathed air constantly, they would receive twice as much oxygen - which may cause fluid buildup in their lungs, pain, and eventual suffocation.
  • Many of them are enormous. The square cube law means that they would have to evolve to support their own weight out of water, which is simply not feasible for larger species.
  • Echolocation is suited for use underwater - some cetaceans cannot communicate on land.
  • Baleen designed to filter small food is useless out of the water, so there is no feasible way for some species to eat out of the water.

Of course, if mammals completely unsuited to life underwater can evolve into whales and dolphins, it must be feasible that cetaceans can eventually evolve to walk on land once again. Just as these changes can be done, they can be undone.


What natural evolutionary pressures would cause these problems (and others) to vanish, allowing both whales and dolphins to become land mammals?

Assume there is no human intervention, and the world is normal. General-purpose answers work - you do not need to address specific concerns for every subspecies, just "whales" and "dolphins" in general. Assume the whales in question are species with baleen.

$\endgroup$
  • 1
    $\begingroup$ No way whales could retain their mass. $\endgroup$ – Mołot Dec 11 '16 at 15:41
  • $\begingroup$ @Mołot I agree. You could answer and include the evolutionary pressures that would reduce their masses $\endgroup$ – Zxyrra Dec 11 '16 at 16:18
  • $\begingroup$ Seems John already did it, so no point for me to answer. I have nothing to add, really. $\endgroup$ – Mołot Dec 11 '16 at 16:37
  • $\begingroup$ @Mołot surely more elaboration on his ideas and more specifics / less generalization of what has to happen would make a strong answer. I would appreciate that $\endgroup$ – Zxyrra Dec 11 '16 at 17:05
  • 2
    $\begingroup$ Gee whiz, poor cetaceans barely settled into a peaceful oceanic life, full of frolicking in the foam, and an OP comes along wants to drive them back onto land. Life can be cruel. $\endgroup$ – a4android Dec 12 '16 at 3:54
8
$\begingroup$

Many marine creatures temporarily "beach" themselves for a variety of reasons, including food. If climate or geological events left ocean food supplies decimated, critters that could hunt on shore would have the distinct advantage of not starving. Once the "low hanging fruit" close to the water was consumed, differentiation in gene lines would let some individuals spend more and more time out of the water. Mutated traits for heavier limbs, lower body weight and rebalanced metabolism would be selected for. Given time they will shed features that were advantageous in water but a literal drag on land. If the ocean's life and nutrient cycles aren't restored, aquatic life would become a distant genetic memory.

Nature would re-use biological mechanisms. Echolocation hardware can be repurposed for hearing sound through air to escape predators, communicate danger and find mates. Each of these is an evolutionary advantage. Half-working ears that accidentally trigger flight response means you pass your mutated ability on to a new generation while your neighbor gets eaten. At some point the aversion to smoke against what used to be a gill will keep a bloodline from walking into a forest fire. All these little mutations, responses and abilities will eventually result in a variety of land-viable species as they create new niches in the rebalancing ecosystem.

Hemoglobin may not be that big an issue. A newborn human's normal levels vary from 10 to 18 g/dl and a dolphin's levels may reflect its environment more than its genes. We have multiple overlapping systems to preserve homeostasis. Dolphins deal with a greater range of atmospheric pressure and their hemoglobin balancing systems might be fine tuned for this.

Baleen whales do face a new gross physical challenge, but even the krill eaters eat fish by accident. Maybe whalebone would be useful in sifting nutrients from beach sand. If not, and if nature doesn't find another use for those organic mechanisms, they're going to be under heavy evolutionary pressure and be one of the categories that get wiped out.

Now I have images of poisonous tree-climbing squid swooping in unsuspecting prey like a flying squirrel. Nature is metal.

$\endgroup$
  • 5
    $\begingroup$ Nature is metal. ?? $\endgroup$ – Jan Doggen Dec 11 '16 at 22:13
  • 4
    $\begingroup$ @JanDoggen As in the music genre. Brutal, hardcore, awesome. $\endgroup$ – Lu22 Dec 12 '16 at 10:38
  • 1
    $\begingroup$ Small nitpick: aquatic mammals, such as dolphins or whales, don't have gills. $\endgroup$ – TVann Oct 18 '17 at 14:45
6
$\begingroup$

You need to eliminate all vertebrate life on land so they have no competition. This is going to take a long time. Whales and dolphins are extremely specialized, so you need to remove as many obstacles as possible. Any terrestrial vertebrate will be able to fill open niches much faster than whales and dolphins. You need nothing in the way so the the highly disadvantageous specialized adaptations don't matter.

You need to get them into swamps to have them start redeveloping feet and hind limbs. It will be dolphins, whales just won't happen due to their size, you need small animals to make such a transition. So if you absolutely need whales you need to get rid of dolphins so whales will expand into their niche with smaller size.

Hemoglobin is an insignificant problem easily changed, and echolocation works on land (see bats). Baleen again requires the elimination of toothed whales and dolphins to encourage them to re-evolve teeth or an analog.

If you just need one or two species of toothed whale (or dolphins) to do it, it is not to bad. You use killer whale beach hunting behavior to encourage more terrestrial adaptations, but that won't work for baleen whales.

$\endgroup$
  • $\begingroup$ I disagree with drastic measures such as "eliminate all vertebrates" or "get rid of dolphins" - the first land amphibians came after arthropods, and surely there are places without dolphins already $\endgroup$ – Zxyrra Dec 11 '16 at 17:02
  • $\begingroup$ arthropods are size limited, early vertebrates were larger than most insects can hope to achieve. Plus vertebrates have other advantages over insects. the big thing is anything that lives on land vertebrate is going to outcompete any attempt whales make to move onto land. so you have to not have them there to compete with. You need an unprecedented opportunity for whales to speciate in order to get baleen whales on land. You need to remove anything that could possibly evolve to fill the niche faster. $\endgroup$ – John Dec 11 '16 at 18:36
  • $\begingroup$ @zxyrra, yes there are plenty of places without dolphins they have seals, otters, hippo, crocodiles, turtles, aquatic rodents, penguins, ect. It is far easier to go from lant to sea than the reverse. that is why land=>water has happened dozens of times, while the reverse has only happened a few and always when there was nothing or nearly nothing to compete with. $\endgroup$ – John Dec 11 '16 at 18:40
  • $\begingroup$ While "all vertebrate life" is rather extreme, it is theorized that a primary motivation for the evolution of cetaceans was the sudden disappearance of nearly all marine reptiles at the K/T mass extinction. The abundance of smaller remaining sea life and fewer predators to compete with was a gold mine for whatever taxa could evolve for sea life. A reversal of this situation could likewise 'motivate' modern cetaceans to move back to land if there was an abundant, relatively uncontested food supply! $\endgroup$ – MSet Aug 16 '17 at 18:53
2
$\begingroup$

I'm not sure what the etiquette on this site is regarding reposting answers to past questions, but I remember answering a similar question on dolphins readapting for life on land, that might be useful here. Whether the reason I remember this answer is due to my fondness for the goofy MSPaint image I made to accompany it is neither here nor there...

$\endgroup$
  • 1
    $\begingroup$ Etiiquette, as I’ve found successfully, is to link to the answer and specify if it’s a portion of interest or certain explaination is useful here or whatnot; then to both prevent a link-only answer and point out the specific area, blockquote the main statement, possibly cutting it down to the bare essentials. $\endgroup$ – JDługosz Dec 13 '16 at 8:33
1
$\begingroup$

While it is unlikely that cetaceans would regain their back legs, it is possible for tails to be re-purposed for terrestrial locomotion. Sea lions have done just that, and they are quite agile on land for a primarily aquatic species.

It seems unlikely that baleen whales would return to land, as the main motivation for returning to land would probably be to find food and baleen is useless out of the water. Baleen whales could probably return to having teeth, though, given the right circumstances.

The process of returning to land could begin with a carnivorous whale or dolphin species that "beaches" itself sometimes in order to acquire food. Orcas are known to do this to hunt seals, for instance. Individuals that can propel themselves on land somewhat with their flippers have an advantage over those that cannot. Eventually, they might evolve an amphibious shape similar to a seal, which can then branch out into the more terrestrial sea lion shape. If a fully terrestrial niche opened up, they might leave the water completely. This could lead to a whole new branch of various kinds of mammal that uses their tails for back legs.

$\endgroup$
1
$\begingroup$

One of the key factors here is to realize the shift from land animal to whale was over 55 million years of evolutionary pressures...it's not a short shift by any means that will include many staging points along the way. It's also worth noting that there were several failed steps along the way that eventually went extinct as they were not able to adapt.

First and foremost, the pressure to get out of the sea must exist...and to do this I would go one of two routes.

-1 massive predator...with some warmer waters, it is possible to reintroduce the Megalodon, an absolutely massive shark. This shark relies on deep water to hide itself, locating a prey (whale) near the surface, and swimming quickly for a surprise attack from the depths. This makes deep waters exceedingly dangerous to the whale and dolphin populations and forces them to shallow water habitats. This would likely bring about the extinction of larger whale species as they die off to this new competitor. Dolphins and smaller whale species are only capable of surviving because they can function in shallow water.

-2 toxin or 'dead' zones become prevalent. Algae blooms and other natural phenomenon can become rampant. These blooms consume the oxygen from the water making for large dead zones. Since ocean mammals still breathe air, they are not directly impacted, but their food source sure is. This forces them to where the prey is...shallow water. Once again, larger whale species won't be able to transition to shallow or they would be unable to find the scale of food they require in shallow habitats. Small whale species become dominant.

In either case above, you now have the whale population either extinct or in shallow waters. In deep ocean, speed has the advantage...in shallow waters, agility dominates. Over the next million or so years, the whale population slowly adapts to become more suited to quick movements instead of fast speeds.

Out of this comes the ability to 'push off', using the tail to fling itself off of the ground or nearby rocks to reach top speed at moments notice. At this stage (potentially 3-4 million years in the future) the whale now has the shape and predatory tactics closer to that of a crocodile (kutchicetus or potentially ambulocetus) where it lies in wait, in shallow water, until a prey species comes by for them to quickly strike at. This creature would have descended from the dolphin populations, but no longer resemble much of a dolphin. Legs for pushing off could begin development at this stage (potentially 5 million years in the future, maybe longer). Sonar and the sort will have gradually faded away, allowing more of the brain to be used for sight and smell.

And at this phase you need something to push them fully on land...a shifting landscape (like the Americas seperate allowing the ocean current to flow between north and south america) changes currents bringing cold water into the formerly warm shallows these creatures lived in...forcing them onto land to sun themselves for warmth. Or a change in the prey available forcing the former-dolphin-like creatures further onto land for prey (kinda crocodile like). Over the next few million years, these traits slowly become like the Pakicetus until it's once again a land creature.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.