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How and why did mammals go back to the oceans?

How and why did mammals go back to the oceans?



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If I understand evolutionary biology correctly, mammals first evolved on land as small, rodent-like creatures, in a time when reptiles were dominant on land. Eventually, they diversified into the species we know today. Some of these species, however, are aquatic - whales and dolphins readily spring to mind. Why did the ancestors of whales and dolphins leave the land to go to the oceans? How did they evolve from their original form to their superficially ichthyoid appearance today?


I'll focus on whales and dolphins (cetaceans) as you mention them by name and they are representative for other marine mammals such as seals or manatees. The evolution of cetaceans was one of the fascinating evolutionary mysteries. Clearly, they were mammals, but which mammals were their closest relatives? Clues to solve this mystery began to appear in the 1980s and 1990s.

During the 1980s and into the 1990s, paleontologists discovered several fossils that were clearly land-based mammals but shared many skeletal features with cetaceans. These fossils, including Pakicetus [(Gingerich et al. 1983)] and Rodhocetus (Gingerich et al. 1994), shared skeletal features that are unique to whales and to the Artiodactyla, commonly known as the even-toed mammals. Artiodactyls includes deer, sheep, pigs, bison, and hippopotamuses. Artiodactyls and the fossil whales share a unique skeletal feature of the ankle called an astrogalus, shown in the picture below. The astragalus is a "double pulley" structure that contributes to the ankle joint. This picture is taken from an excellent summary of whale evolution, provided by the Understanding Evolution site hosted by University of California at Berkeley. The astragalus is not present in other mammals.

The fossil whales and modern cetaceans also share unique features of the skull around the area of the ear, called a tympanic bulla. Although all mammals have bullae (plural of bulla), the bullae of cetaceans and the whale fossils is unique compared to other mammals. For more on the bulla, see this Talk Origins page. More on fossil whales can be found at the Berkeley Comparative Museum of Paleontology. The many fossils that have been found are transitional forms that link artiodactyls and modern cetaceans.

Source

Around the same time as the fossil discoveries, genetic analysis added additional evidence supporting a relationship between the artiodactyls and cetaceans (Milinkovitch et al. 1993). Although early genetic analytical techniques were not as robust as they are today, many genetic studies have since supported and refined the conclusions of Milinkovitch et al. Current evidence shows that the closest living relative of cetaceans are the hippos (see, for example Price et al. 2005 and Agnarsson and May-Collado 2008). The relationships are shown below in this figure from the Agnarsson and May-Collado paper. The dark lines are the artiodactyls and cetaceans, formally called the Cetartiodactyla.

The question remains: why? The most likely explanation is that cetaceans evolved to exploit an unfilled ecological niche or adapted to new niches that formed as a result of plate tectonics or other types of environmental changes that occurred 50-55 million years ago. The niche describes all of the living and non-living resources needed by an organism to survive. Although land-based mammals were increasing in diversity, few or none were present in the oceans. The basic hypothesis is that the early whale-like artiodactyls, like Indohyus and Pakicetus were land-based (terrestrial) mammals that spent most of their time near the water's edge. Over time, they adapted to the niches in the ocean. Fossils like Ambulcetus and Rodhocetus showed clear evidence of swimming ability, with flattened tails and the enlarged rear feet. In addition, the nostrils shifted from the front of the face to the top of the head, which we recognize as the blowhole.

The shift to the aquatic habitat allowed these species to exploit resources that were not available to land-based mammals, thereby reducing competition for the resources. Reduced competition allows more individuals to survive and reproduce.

Similar scenarios are very likely for other marine mammals, such as seals or manatees. They evolved to take advantage of ecological niches that were not filled by other organisms. This basic concept, evolving to fill available niches, is a common outcome of the evolutionary process.

The of adaptation of cetaceans and other mammals to the oceans may be similar to that of the hippopotamus. Hippos spend most of their time in the water, and they show many adaptations that allow them to live in the aquatic environment. The eyes and nostrils of the hippo are high on the head, which allows them to remain almost entirely submerged but still see and smell, as shown below.

(Hippo photo by Johannes Lunberg, Flickr Creative Commons.)

Hippos feed underwaters, they are heavy enough to walk on the bottom of the river, and the mate and give birth underwater. The young can suckle underwater. Clearly, hippos seem to be another mammal that is "returning to water." Similar types of processes must have occurred in cetaceans for them to adapt to the marine habitat.

Citations

Agnarsson, I. and L.J. May-Collado. 2008. The phylogeny of Cetartiodactyla: The importance of dense taxon sampling, missing data, and the remarkable promise of cytochrome b to provide reliable species-level phylogenies. Molecular Phylogenetics and Evolution 48: 964-985.

Gingerich, P.D. et al. 1983. Origin of whales in epicontinental remnant seas: New evidence from the early Eocene of Pakistan. Science 220: 403-406.

Gingerich, P.D. et al. 1994. New whale from the Eocene of Pakistan and the origin of cetacean swimming. Nature 368: 844-847.

Milinkovitch, M.C. et al. 1993. Revised phylogeny of whales suggested by mitochondrial ribosomal DNA sequences. Nature 361: 346-348.

Price, S.A. 2005. A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla). Biological Reviews 80: 445-473.


How did they evolve from their original form to their superficially ichthyoid appearance today?

This is an example of convergent evolution. Fish appear as they do (streamlined body shape, wide tail, fins, etc.) since these are adaptations to the underwater environment they're living and evolving in. These features are only "ichthyoid" or "fishy" because the fish 'got there first'. Instead, we can think of such features as "ocean-favourable".

Reptiles evolved on land, and hence have a generally different appearance from fish, but those which went back into the oceans to become the ichthyosaurs adapted again to live in that environment. Since they faced similar problems as fish (drag from the water, the need to maneuver in 3D, etc.), natural selection would favour the same "ocean-favourable" features as the fish (but not exclusively; for example, turtles have a rather different appearance, which is "ocean-favourable" but not as "fishy").

When mammals ventured into the oceans, they also faced these same problems and the same selection pressures, so they also gained a more "ocean-favourable" appearance.


12.4: Evolution of Modern Mammals

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Why so many different types of large mammals?

Elephants, zebras, gazelles, giraffes, rhinoceros, lions, and hippopotamuses, to name just some of the animals of Africa. Each species must have its own niche, otherwise they could not coexist. So, there must be a role for each species within their ecosystem.


3. What is the purpose of the spines?

4. How did ancestral populations of ocean-dwelling fish come to live in freshwater lakes?

5. Watch the video about pelvic reduction in freshwater stickleback. The loss of the pelvic spines is similar to the loss of which body parts in other four-legged vertebrates?

6. Watch the video with evolutionary biology Dr. Michael Bell. Why is the threespine stickleback a model organism for studies in evolution. List at least two reasons.

7. Click on the link at the top of the page to go to the "overview," then click on the interactive fish. Describe the location of the stickleback spine.

8. Watch the video about stickleback fish armor. In addition to the spines, what is another component of the armor of a stickleback fish?


How the Whale Lost Its Legs And Returned To the Sea

AGES after some adventurous (or misadventurous) fish left the sea and planted the flag of vertebrate animal life on land, their descendants had it both ways as amphibians and then completed the epic transition, evolving into terrestrial reptiles, mammals and birds. But something about the water must have kept beckoning, until a few irredentists among the mammals did eventually reclaim a place in the sea.

Most prominent of these mammals are the whales. Although they may swim the oceans with power and grace, these leviathans are more closely related to the camel and cow than any fish in their wake. Their anatomies retain vestiges of the four-legged land animals in their ancestry, the ones that began the bold return to the sea more than 50 million years ago.

As early as Aristotle, people recognized that more than size set whales apart from other marine life. Aristotle noted that whales as well as dolphins, the other members of the cetacean group, bear their young live, unlike fish. This made them, by definition, mammals. (It is now known that some fish do bear live young.) But even Darwin much later could not fathom the evolutionary steps by which some land mammals had become whales.

"It was a big evolutionary plunge, one that has eluded documentation for many decades," said Dr. Michael J. Novacek, a paleontologist who is dean of science at the American Museum of Natural History in New York City.

New fossil discoveries have now revealed several of the critical evolutionary steps in the earliest history of whales. Scientists have identified some intermediate species as land mammals steadily changed physical form while adapting to swimming, diving, feeding and otherwise thriving in their new habitat. One surprise is that the transformation of four-legged land mammals into an animal completely adapted to marine life took only 10 million years -- hardly any time at all in evolutionary terms.

"I am absolutely delighted to report that our usually recalcitrant fossil record has come through in exemplary fashion," Dr. Stephen Jay Gould, a Harvard University paleontologist and essayist, wrote in this month's issue of Natural History magazine. "The embarrassment of past absence has been replaced by a bounty of new evidence -- and by the sweetest series of transitional fossils an evolutionist could ever hope to find."

One discovery, which Dr. Gould called "a remarkable smoking gun," introduced scientists to an animal that lived 50 million years ago. It was about the size of a modern sea lion, weighing 600 to 700 pounds and measuring about 10 feet from snout to tail. Judging by fossil remains, the animal was an amphibious species clearly intermediate between a terrestrial ancestor of whales and aquatic modern whales.

This fossil animal, named Ambulocetus natans, which means swimming walking-whale, was excavated from sediments of an ancient seabed in Pakistan. Dr. J. G. M. Thewissen, an anatomist and paleobiologist at Northeastern Ohio Universities College of Medicine in Rootstown, Ohio, reported the discovery in January.

Ambulocetus was just what scientists would have expected to find. The animal still had four limbs for walking on land, though probably with diminished agility. It could also hunt in the sea, probably swimming by kicking its big feet. Find in Pakistan

Another discovery in Pakistan has now advanced the transition story three or four million years and out into deeper waters. Scientists reported last week that they had found fossils of an animal that had obviously made a more resolute commitment to a life at sea.

Writing in the current issue of the journal Nature, Dr. Philip D. Gingerich, a paleontologist at the University of Michigan at Ann Arbor, said the new species, Rodhocetus kasrani, was the earliest known transitional whale with an anatomy adapted for swimming like a whale. It had a more streamlined body and a fully flexible rear spinal column, which could have produced the motions for the powerful beat of a horizontal tail fluke that propels modern whales.

Whether Rodhocetus had indeed made this important advance cannot be determined until more complete tail fossils are uncovered. But the evidence, Dr. Gingerich and colleagues concluded, "shows that tail swimming evolved early in the history of cetaceans."

In a commentary accompanying the Nature report, Dr. Novacek said the Ambulocetus and Rhodocetus findings provided a "fascinating mosaic of early adaptive experiments in the transition from land to water."

Besides whales and dolphins, two other major types of mammals have made this dramatic transition: Sirenia, which includes dugongs and manatees, and the group composed of seals, sea lions and walruses.

The migration of whales to the sea is a classic example of transcendental evolutionary change, along with the first fishlike animals establishing their beachhead 370 million years ago, birds becoming flying animals and early human ancestors developing upright walking. But why would animals like whales, with all the apparatus for successful living on the land and ages of experience doing it, seek such a different habitat? Food as Lure Back to Sea

"The motive for any such transition is always opportunity," Dr. Gingerich said. "Look at what changed first, the teeth. What was initially important in the transition was food. These land animals were exploiting an opportunity to feed on fish in the sea."

Other change over time involved improvements in the animals' maneuverability in their new environment. These included hearing systems evolved for underwater communications and navigation and completely new ways of locomotion, first as amphibians and then as fully developed marine animals.

For a long time, scientist had almost no direct evidence about the nature of the whale transition. The best they could do was make inferences based on a comparison of modern whales and fossils of archaic whales with the remains of their putative terrestrial relatives.

From this scientists concluded that whales were distantly related to ungulate mammals, a group whose modern members include camels, cows, pigs and deer. The link between other ungulates and whales is thought to be mesonychids, extinct four-legged mammals that sometimes feasted on fish at river edges.

Beginning in 1983, paleontologists have collected more revealing evidence of the transition. In that year, Dr. Gingerich reported the discovery of the oldest whale, which lived about 52 million years ago. A fossil skull was found in Pakistan in river sediments near an ancient sea. The animal was given the name Pakicetus.

Although they had nothing to ponder but this skull, scientists could see that Pakicetus had teeth resembling those of mesonychids, but it was well adapted to feeding on fish in surface waters of shallow seas. Other parts of the skull were becoming whalelike, but it lacked the auditory equipment for a fully marine existence. Another fossil specimen from about the same time, Indocetus ramani, probably led the same kind of life, entering the sea to feed on fish, but returning to land to rest and raise its young. Skeleton in Egypt

As Dr. Gould noted in his review of recent whale research, the next important discovery was the first complete hind limbs of a fossil whale, Basilosaurus isis, which lived 5 million to 10 million years after Pakicetus. The skeleton, found in Egypt, was described in 1990 by Dr. Gingerich and a team of Michigan and Duke University scientists.

Since the hind limbs were a mere 2 feet long and the whole body was 50 feet long, the discoverers concluded that the legs could not have supported the body on land or assisted in swimming. This fossil whale had passed the point of intermediacy, and so the search continued.

If the matter of locomotion is the functional test of intermediacy, as Dr. Gould wrote, Dr. Thewissen's Ambulocetus came closest to the long-sought breakthrough in early whale evolution. Its toes are even terminated by hooves as in mesonychids and other ungulates, those earlier ancestors. The skeleton also revealed that the animal had stubby forelimbs with hands that splayed outward like the flippers on a sea lion, while the rear legs were still large and powerful, with possibly webbed feet. It had not yet evolved a tail fluke, but its spine seemed to be flexible enough to allow the undulations associated with whale propulsion.

In this intermediate phase, the up-and-down undulations probably operated in concert with the vigorous paddling of Ambulocetus's large feet. In modern whales, they contribute mightily to the propulsive beat of the tail fluke.

On the basis of this evidence, Dr. Thewissen concluded, "Ambulocetus represents a critical intermediate between land mammals and marine cetaceans." Earliest Whales

Whales in their present form began appearing about 30 million years ago. Their flippers are what remains of the forelimbs of their terrestrial past. The only hints of the former hind limbs are the vestiges of a pelvis and femur, the upper leg bone, embedded in the body wall.

Reflecting on the recent succession of discoveries, Dr. Novacek said, "This expanding fossil casebook on the origins of whales is one of the triumphs of modern vertebrate paleontology."


Major Events in Mammalian Evolution

Scientists do generally agree on the major events in the evolution of mammals. These are summarized in Table below. Refer back to the table as you read about the events in this concept. *mya = millions of years ago

Era Period Epoch Major Events Start (mya)*
Cenozoic Neogene Holocene Rise of human civilization spread and dominance of modern humans 0.01
- - Pleistocene Spread and then extinction of many large mammals appearance of modern humans 1.8
- - Pliocene Appearance of many existing genera of mammals, including the genus Homo 5.3
- - Miocene Appearance of remaining modern mammal families diversification of horses and mastodons first apes 23.0
- Paleogene Oligocene Rapid evolution and diversification of placental mammals 33.9
- - Eocene Appearance of several modern mammal families diversification of primitive whales 55.8
- - Paleocene Appearance of the first large mammals 65.5
Mesozoic Cretaceous - Emergence of monotreme, marsupial, andplacental mammals possible first appearance of four clades (superorders) of placental mammals (Afrotheria, Xenarthra, Laurasiatheria, Supraprimates) 145.5
- Jurassic - Spread of mammals, which remain small in size 199.6
- Triassic - Evolution of cynodonts to become smaller and more mammal-like appearance of the first mammals 251.0
Paleozoic Permian - Evolution and spread of synapsids (pelycosaurs and therapsids) 299.0
- Carboniferous - Appearance of amniotes, the first fully terrestrial vertebrates 359.0


NCPR provides this essential service.

MF: So they have a pelvis, anything else?

CS: Well, there&rsquos the pelvis and the blowhole and all that, and if you look at their front flippers, you&rsquod say they look like fins, and there shouldn&rsquot really be much in there. They actually have bones in there too, which are the same, basically the same kinds of bones that we have in our forearms that other mammals will have too. They&rsquove got a single upper arm bone, you&rsquod call it a humerus, and there&rsquos two forearm bones, your radius and ulna, and then the wrist bones and the finger bones are all in the front limbs of whales as well, which again would suggest they&rsquore developed or modified from something that had four limbs, just like we do, or the land walking critters that you can now see in the fossil record.


Evolution of Fish

The evolution of fish from hagfish to finned fish is a long and involved process. One step in this evolution involves the change in function of gills. Invertebrate chordates use their gills to filter food out of water, not to absorb oxygen. In the early evolution of fish, there was a switch to using gills to absorb oxygen instead of to filter food. Gills consist of many thin, folded tissues that provide a large surface area for oxygen uptake. With more oxygen absorbed by the gills, fish could become much larger and more active.

Fossilized fish shown in different sizes.

Timing of Fish Evolution

Ancestors of hagfish are thought to have been the earliest vertebrates. Their fossils date back to about 550 million years ago. Fossils of cartilaginous fish with jaws, resembling living sharks, first appeared in the fossil record about 450 million years ago. They were followed about 50 million years later by the bony fish.

The Bony Fish

At first, the lobe-finned bony fish were much more common than the ray-finned bony fishthat dominate today. Lobe-finned fish were also ancestral to amphibians. Their stump-like appendages and lung-like organs evolved into amphibian legs and lungs. Ray-finned bony fish may have been the first fish to evolve in freshwater. They eventually became the most diverse and dominant class of fish.


Evolution: Out Of The Sea

Thursday 26th July saw the launch of SciLogs.com, a new English language science blog network. SciLogs.com, the brand-new home for Nature Network bloggers, forms part of the SciLogs international collection of blogs which already exist in German, Spanish and Dutch. To celebrate this addition to the NPG science blogging family, some of the NPG blogs are publishing posts focusing on “Beginnings”.

Participating in this cross-network blogging festival is nature.com’s Soapbox Science blog, Scitable’s Student Voices blog and bloggers from SciLogs.com, SciLogs.de, Scitable and Scientific American’s Blog Network. Join us as we explore the diverse interpretations of beginnings – from scientific examples such as stem cells to first time experiences such as publishing your first paper. You can also follow and contribute to the conversations on social media by using the #BeginScights hashtag. – Bora

In the beginning, the earth was without form, and void and darkness was upon the face of the deep, as a giant cloud of gas and dust collapsed to form our solar system. The planets were forged as the nebula spun, jolted into motion by a nearby supernova, and in the center, the most rapid compression of particles ignited to become our sun. Around 4.5 billion years ago, a molten earth began to cool. Violent collisions with comets and asteroids brought the fluid of life - water - and the clouds and oceans began to take shape. It wasn't until a billion years later that the first life was brought forth, filling the atmosphere with oxygen.

Over the next few billion years, single-celled organisms fused and became multicellular body plans diversified and radiated, exploding into an array of invertebrates. Yet all this abundance and life was restricted to the seas, and a vast and bountiful land sat unused. Around 530 million years ago, there is evidence that centipede-like animals began to explore the world above water. Somewhere around 430 million years ago, plants and colonized the bare earth, creating a land rich in food and resources, while fish evolved from ancestral vertebrates in the sea. It was another 30 million years before those prehistoric fish crawled out of the water and began the evolutionary lineage we sit atop today. To understand life as we know it, we have to look back at where we came from, and understand how our ancestors braved a brand new world above the waves.

It was a small step for fish, but a giant leap for animalkind. Though, looking at modern fish species, it's not so hard to envision the slow adaptation to life out of the sea. Just the other day, I was feeding my pet scorpionfish Stumpy, and he surprised me with this slow, deliberate crawl towards his food:

A number of fish exhibit traits which are not unlike those of the first tetrapods: the four-limbed vertebrates that first braved life on land, direct descendants of ancient fish. Many of Stumpy's relatives, including the gurnards, are known for their "walking" behaviors. Similarly, mudskippers have adapted anatomically and behaviorally to survive on land. Not only can they use their fins to skip from place to place, they can breathe through their skin like amphibians do, allowing them to survive when they leave their shallow pools. Walking catfishes have modified their respiratory system so much that they can survive days out of water. But all of these are only glimpses at how the first tetrapods began, as none of these animals has fully adapted to life on land. To understand how tetrapods achieved such a feat, we must first understand the barriers that lay between their life under the sea and the land above that awaited them.

Living in air instead of water is fraught with difficulties. Locomotion is one problem, though as evolution in a number of lineages has shown, not as big a problem as you might think. Still, while mudskippers and catfish seem to walk with ease, the same cannot be said of our ancestors. Some of the earliest tetrapods, like Ichthyostega were quite cumbersome on land, and likely spent most of their time in the comfort of water. These first tetrapods came from an ancient lineages of fishes called the Sarcopterygii or Lobe-Finned Fish, of which only a few survive today. As the name implies, these animals have meaty, paddle-like fins instead of the flimsy rays of most modern day fish species. These lobe fins, covered with flesh, were ripe for adapting into limbs.

But these early tetrapods had to develop more than a new way to walk - their entire skeletons had to change to support more weight, as water supports mass in a way that air simply doesn't. Each vertebrae had to become stronger for support. Ribs and vertebrae changed shape and evolved for extra support and to better distribute weight. Skulls disconnected, and necks evolved to allow better mobility of the head and to absorb the shock of walking. Bones were lost and shifted, streamlining the limbs and creating the five-digit pattern that is still reflected in our own hands and feet. Joints articulated for movement, and rotated forward to allow four-legged crawling. Overall, it took a long 30 million years or so to develop a body plan fit for walking on land.

At the same time, these cumbersome wanna-be land dwellers faced another obstacle: the air itself. With gills adept at drawing oxygen from water, early tetrapods were ill-equipped to breathing air. While many think that early tetrapods transformed their gills into lungs, this actually isn't true - instead, it was the fish's digestive system that adapted to form lungs. The first tetrapods to leave the water breathed by swallowing air and absorbing oxygen in their gut. Over time, a special pocket formed, allowing for better gas exchange. In many fish, a similar structure - called a swim bladder - exists which allows them to adjust buoyancy in the water, and thus many have hypothesized that tetrapod lungs are co-opted swim bladders. In fact, exactly when tetrapods developed lungs is unclear. While the only surviving relatives to early tetrapods - the lungfishes - also possess lungs (if their name didn't give that away), many fossil tetrapods don't seem to have them, suggesting that lungfish independently evolved their ability to breathe air. What we do know is that it wasn't until around 360 million years ago that tetrapods truly breathed like their modern descendants.

The other trouble with air is that it tends to make things dry. You may have heard the statistic that our bodies are 98% water, but, as well-evolved land organisms, we have highly evolved structures which ensure that all that water doesn't simply evaporate. The early tetrapods needed to develop these on their own. At first, like the amphibians that would arise from them, many tetrapods likely stuck to moist habitats to avoid water loss. But eventually, to conquer dry lands and deserts, animals had to find another way to keep themselves from drying out. It's likely that many of the early tetrapods began experimenting with ways to waterproof their skin. Even more important was the issue of dry eggs. Amphibians solve the dryness issue by laying their eggs in water, but the tetrapods which conquered land didn't have that luxury.

The solution to land's dry nature was to encase eggs in a number of membrane layers, in what is now known as an amniote egg. Even our own children reflect this, as human babies still grow in an amniotic sac that surrounds the fetus, even though we no longer lay eggs. This crucial adaptation allowed animals to cut ties with watery habitats, and distinguishes the major lineage of tetrapods, including reptiles, birds and mammals, from amphibians.

These crucial adaptations to tetrapod skeletons and anatomy allowed them to conquer the world above the waves. Without their evolutionary ingenuity, a diverse set of animals, including all mammals, would not be where they are today. Yet still we barely understand the ecological settings that drove these early animals out of the sea. Did dry land offer an endless bounty of food not to be passed up? Perhaps, but there is evidence that our ancestors braved the dry world very early on, even before most terrestrial plants or insects, so it's possible earth was barren. Were they escaping competition and predation in the deep? Or was land important for some yet undetermined reason? We may never know. But as we reflect upon our beginnings, we have to give credit to the daring animals that began the diverse evolutionary lineage we are a part of. While we may never understand why they left the water, we are thankful that they did.

Other Posts in the Evolution Series:

Photo: A model of Tiktaalik rosea, one of the earliest tetrapod ancestors. Photo courtesy of Tyler Keillor.


Was our common ancestor with ocean based mammals like dolphins and whales, a water or land dwelling creature?

I've always been under the impression that mammals evolved on land. If this is indeed the case, then did the ancestor of mammals like dolphins and whales then go back to the water? Or did mammals in fact start off in the oceans? Or is my entire conception that all mammals must have a common ancestor invalid itself?

Land-Based. While all terrestrial mammals do have ancestros which crawled out of the oceans, Ceteceans (whales) and other sea-mammals appear to have later evolved adaptations to allow them back into an ocean environment.

Most notably, Whales still retain a vestigial hip-bone at the base of their tail where there legs would have previously attached

If it's not whales, then what is our closest sea-dwelling relative where the common ancestor lived in the oceans? Would it be the coelacanth, or something?

Not peer reviewed source, on a mobile also why I am not top level replying. Pakicetus, along with Indohyus and Ambulocetus represent the earliest recognized ancestors of cetaceans. http://www.amnh.org/explore/news-blogs/on-exhibit-posts/the-first-whale-pakicetus not sure this is up to snuff for a source, but I hadn't seen Pakicetus mentioned yet.

But what about the Aquatic Ape theory?

All of the mammals that live in water or spend a lot of their time in the water evolved from land dwelling animals. The transition creatures were probably like seals who spend a lot of their time in the water, but come up into land to have offspring and rest. Basically, marine mammals evolved from fish over a giaaaant time span then evolved back into the water.

Also things like beavers, which are semi-aquatic. Who knows, they might evolve into fully aquatic animals in 10-20 million years.

All of the mammals that live in water or spend a lot of their time in the water evolved from land dwelling mammals.

There IS a theory within paleoanthropology called the "Aquatic Ape" Theory (AAT). A fairly marginalized theory which has for the most part been rejected the larger anthropological community, but it does still have some veracious proponents. It basically states that at one point in human history, we became semi-aquatic adapted. The theory tries to account for many of the similar physical characteristics that humans share with many aquatic mammals (lack of body hair, salt sweat/tears, adipose fat layer). A quick Google will get you numerous links on the topic.

Cetaceans (dolphins, whales and porpoises) and modern hippos evolved from a common ancestor. It was a hoofed mammal that spent a lot of time wallowing in water and eventually adapted to live there, it would have been quite similar to a modern hippo. Cetaceans have the remnants of the bone structure of hind legs attached to their pelvises but it is tiny and internal so it can only be seen by xray.

it would have been quite similar to a modern hippo.

Not likely. The oldest member of the whale lineage we know of is Indohyus, a small, flee footed ungulate that nevertheless spent a lot of its time underwater.

We don't know hippopotamus ancestor fossils from this time, but just like whales have evolved a lot in the mean time we can assume that the hippopotamus has done as much. In fact, it is even possible that the hippopotamus ancestors weren't semiaquatic at all and they only turned to the water recently. Of two living species of hippopotami, the pygmy hippo lives mostly by water, but doesn't spend as much time in the water as the common hippo.

Our common ancestor with the cetaceans would have been in Boreoeutheria a land based animal. This group would give rise to diverse groups of mammals including rodents, primates, and hooved animals as well. Dolphins and whales are much more closely related to the hooved animals and large herbivores like cows, deer, camels, and horses. Even the carnivore orders including cats and dogs, and bats are closer. Primates and Rodents are along another branch of the mammal tree, so we're more closely related to lemurs and rabbits than whales.

All mammals have a terrestrial ancestor. That said, all terrestrial animals have reptilian, and before that, aquatic, ancestors.


Primates

Matteo De Stefano/MUSE / Wikimedia Commons / CC BY-SA 3.0

Technically speaking, there's no good reason to separate prehistoric primates from the other mammalian megafauna that succeeded the dinosaurs, but it's natural (if a bit egotistic) to want to distinguish our human ancestors from the mainstream of vertebrate evolution. The first primates appear in the fossil record as far back as the late Cretaceous period and diversified in the course of the Cenozoic Era into a bewildering array of lemurs, monkeys, apes, and anthropoids (the last the direct ancestors of modern humans). Paleontologists are still trying to sort out the evolutionary relationships of these fossil primates because new "missing link" species are constantly being discovered.


Watch the video: Why Do Animals Go Extinct? COLOSSAL QUESTIONS (August 2022).