The basic theme of whale evolution is the development of large animals from much smaller ancestors, and nowhere is this more evident than in the case of multi-ton sperm and gray whales, whose ultimate forebears were small, dog-sized prehistoric mammals that prowled the riverbeds of central Asia 50 million years ago.
Perhaps more intriguingly, whales are also a case study in the gradual evolution of mammals from fully terrestrial to fully marine lifestyles, with corresponding adaptations elongated bodies, webbed feet, blowholes, etc. Until the turn of the 21st century, the ultimate origin of whales was shrouded in mystery, with scarce remains of early species. That all changed with the discovery of a huge trove of fossils in central Asia specifically, the country of Pakistansome of which are still being analyzed and described.
These fossils, which date from only 15 to 20 million years after the demise of the dinosaurs 65 million years ago, prove that the ultimate ancestors of whales were closely related to artiodactyls, the even-toed, hooved mammals represented today by pigs and sheep.
In most ways, Pakicetus Greek for "Pakistan whale" was indistinguishable from other small mammals of the early Eocene epoch: about 50 pounds or so, with long, dog-like legs, a long tail, and a narrow snout. Crucially, though, the anatomy of this mammal's inner ears closely matches that of modern whales, the main "diagnostic" feature that places Pakicetus at the root of whale evolution.
One of Pakicetus' closest relatives was Indohyus "Indian pig"an ancient artiodactyl with some intriguing marine adaptations, such as a thick, hippopotamus-like hide. Ambulocetusaka the "walking whale," flourished a few million years after Pakicetus and already displayed some distinctly whale-like characteristics.
Whereas Pakicetus led a mostly terrestrial lifestyle, occasionally dipping into lakes or rivers to find food, Ambulocetus possessed a long, slender, otter-like body, with webbed, padded feet and a narrow, crocodile-like snout.
Ambulocetus was much bigger than Pakicetus and probably spent a significant amount of time in the water. Named after the region of Pakistan where its bones were discovered, Rodhocetus shows even more striking adaptations to an aquatic lifestyle.
This prehistoric whale was genuinely amphibious, crawling up onto dry land only to forage for food and possibly give birth. In evolutionary terms, though, the most telling feature of Rodhocetus was the structure of its hip bones, which weren't fused to its backbone and thus provided it greater flexibility when swimming.
The remains of Rodhocetus and its predecessors have been found mostly in central Asia, but the larger prehistoric whales of the late Eocene epoch which were able to swim faster and farther have been unearthed in more diverse locations. The deceptively named Protocetus it wasn't really the "first whale" had a long, seal-like body, powerful legs for propelling itself through the water, and nostrils that had already begun to migrate halfway up it forehead, a development foreshadowing the blowholes of modern whales.
Protocetus shared one important characteristic with two roughly contemporary prehistoric whales, Maiacetus, and Zygorhiza. The front limbs of Zygorhiza were hinged at the elbows, a strong clue that it crawled onto land to give birth, and a specimen of Maiacetus meaning "good mother whale" has been found with a fossilized embryo inside, positioned in the birth canal for terrestrial delivery.
Clearly, the prehistoric whales of the Eocene epoch had a lot in common with modern giant tortoises! By about 35 million years ago, some prehistoric whales had attained gigantic sizes, bigger even than modern blue or sperm whales. The largest genus yet known is Basilosaurusthe bones of which discovered in the midth century were once thought to belong to a dinosaur, hence its deceptive name, meaning "king lizard. Even more important from an evolutionary perspective, Basilosaurus led a fully aquatic lifestyle, birthing as well as swimming and feeding in the ocean.
Contemporaries of Basilosaurus were much less fearsome, perhaps because there was only room for one giant mammalian predator in the undersea food chain.
Dorudon was once thought to be a baby Basilosaurus; only later was it realized that this small whale only about 16 feet long and half a ton merited its own genus. And the much later Aetiocetus which lived about 25 million years agothough it weighed only a few tons, shows the first primitive adaptation to plankton feeding; small plates of baleen alongside its ordinary teeth.
No discussion of prehistoric whales would be complete without a mention of a fairly new genus, the aptly named Leviathanwhich was announced to the world in the summer of This foot-long sperm whale weighed "only" about 25 tons, but it seems to have preyed on its fellow whales along with prehistoric fish and squids, and it may have been preyed on in turn by the largest prehistoric shark of all time, the Basilosaurus-sized Megalodon.
Share Flipboard Email. Bob Strauss. Science Writer. Updated January 04, They may run, swim or fly. They may weigh less than a penny or more than a dozen school buses. From humans to whales to bats, the placental mammals—so named for the placenta that nourishes the fetus during development—are mind-bogglingly diverse.
The placental mammals are one of three major groups of mammals; the other two are the egg-laying monotremes and the pouched marsupials. Now a major new analysis of thousands of anatomical features of modern and extinct mammals, as well as molecular sequences from living species, is helping them to do just that. The study also hints at what the ancestral placental mammal—the one that ultimately gave rise to creatures as disparate as tree sloths and sea lions--looked like. Previous attempts to reconstruct the evolutionary history of mammals yielded conflicting scenarios.
Fossil evidence suggested that the placentals burst onto the scene shortly after a massive asteroid slammed into the earth around 65 million years ago and snuffed out the dinosaurs. Studies that instead used molecular sequence data to get at the question of when placentals originated and diversified painted a different picture, indicating that the group appeared as early as million years ago, when dinosaurs were still thriving.
This date is far older than the oldest known placental fossils, but it helped to explain the diversification of the group: the supercontinent Gondwana was fragmenting at that time and the breakup would have provided opportunities for populations to become separated and evolve in isolation into new forms.
The new study, which looked at more than 4, traits in 86 fossil and living mammal species, controverts the early origin model, concluding instead that the placentals originated after the mass extinction event, with the first members of modern placental orders evolving some two million to three million years later--after the continental breakup.
Perhaps the coolest part of the paper is the bit where the authors reconstruct the characteristics of the hypothetical placental ancestor—a tree-climbing, insect-eating beastie that weighed between 6 and grams and gave birth to one hairless baby at a time, among other fascinating details. I'd love to see what other hypothetical ancestors look like--last common ancestor of chimps and humans, anyone?
The views expressed are those of the author s and are not necessarily those of Scientific American. Kate Wong is a senior editor for evolution and ecology at Scientific American. You have free article s left. Already a subscriber? Sign in.
See Subscription Options. Get smart. Sign up for our email newsletter. Sign Up. Read More Previous. See Subscription Options Already a subscriber? Sign In See Subscription Options.All rights reserved.
The blue whale— tonnes in weight and beautifully adapted for swimming—is a placental mammal. The bumblebee bat—1.
So are you. So is a bear, an anteater, a giraffe and a squirrel. Also: armadillos, rhinos, rabbits, manatees, and pangolins. All of these creatures, in their wondrous array of shapes and sizes, evolved from a small, unassuming, scurrying insect-eater that lived a few hundred thousand years after the apocalypse that finished off most of the dinosaurs.
A team of US scientists have now reconstructed what this ancestral placental was like, to an extraordinary level of detail. They have predicted how much it would have weighed, the number of molars in its jaws, the shape of its sperm, and the path that its carotid artery took up its neck.
None of this comes from a fossil of the creature itself. Instead, the predictions are based on 80 of its descendants, including some that are still alive and others that joined it in extinction. To find out more details about the results and what they mean about when placentals evolved have a look at my coverage for Nature News.
Meanwhile, I wanted to publish the explanation that one of my sources—Olaf Bininda-Emonds from Oldenburg University—sent me, which explains just how much work went into this. To do their analysis, the team had to score the skeleton of 86 different species according to more than 4, anatomical traits. Think of an enormous table that they had to fill in. I find the study to be absolutely stunning.
Incredibly arduous and not really that exciting. Kinda dusty too. Now scale that up by a factor of over 20! That seems hard to believe, but you have to remember that they had to come up with a list of characters and character definitions that fits everything from a bumblebee bat to a blue whale and all the weird and wonderful forms in between e.
So, there was a lot of science going on here as well in trying to decide if a particular flange on the top of the femur in something like a walrus was evolutionarily the same structure as a ridge in a similar position on the femur of an aardvark!
And, if so, then what should they call it? Read Caption. A reconstruction of the ancestral placental mammal. By Ed Yong. Continue Reading.A tiny, furry-tailed creature is the most complete picture yet as to what the ancestor of mice, elephants, lions, tigers, bears, whales, bats and humans once looked like, researchers say.
These new findings also suggest this forerunner of most mammals appeared shortly after the catastrophe that ended the age of dinosaursscientists added. The study was so thorough that the team, made up of 23 scientists from around the world, was able tospeculate on the appearance of this hypothetical ancestor inside and out, from its brain and inner ear bones to its ovaries and even what its sperm may have looked like it sported a head and tail like modern-day sperm cells do.
The largest living branch of the mammalian family tree holds the placentals — mammals that keep fetuses alive with placentas, as opposed to marsupials such as kangarooswhich raise offspring in pouches, or monotremes such as platypuses, which develop fetuses in eggs. Much remains controversial about the origins of placentals, such as when they arose and how they diversified. Fossil evidence suggests they evolved after the end-Cretaceous mass extinction event about 65 million years ago that ended the age of dinosaurs; and the "explosive model" based off this data proposes that placental lineages emerged and diversified to fill niches left vacant after this catastrophe.
However, genetics research suggests placental lineages were actually far older, hinting their diversification was linked to the breakup of the continents before the end of the Cretaceous period. To uncover the roots of the placental family tree and help resolve the decades-old debate as to when placentals evolved, an international team of researchers took part in a six-year research collaboration called Assembling the Tree of Life.
The project adopted two distinct approaches to evolutionary studies — molecular data, which examines DNA, and morphological data, which looks at anatomical features such as bone length, types of teeth and the presence of stripes in the fur.
The molecular team gathered DNA sequences of living animals while the morphology team analyzed the anatomy of both living and extinct mammals. The molecular team was limited largely to living mammals, because researchers currently cannot extract genetic material from fossils more than 30, years old, so morphological data was key when it came to exploring more ancient branches of the mammal family tree.
Combining all the evidence produces the most informed reconstruction of a past event. When it comes to studying morphology, a dataset of anatomical characteristics, or "characters," is often considered large. However, for this new project, the researchers generated a groundbreaking 4, characters using a cloud-based and publicly accessible database called MorphoBank.
Combining both DNA and morphological datasets led to an unprecedented amount of information for each of the 83 mammals they investigated. From all this data from living and extinct mammals, the scientists extrapolated the appearance of the most recent common ancestor of all placental mammals.
The scientists then worked with an artist to illustrate this ancestor. In addition to a furry tail, the researchers suggest the four-legged creature likely ate insects, weighed from 6 grams about the weight of some shrews up to grams — less than half a pound — and was more adapted for general scampering than built for more specialized forms of movement, such as swinging from trees. Also, its cerebral cortex — the part of the brain linked to higher mental processes — was probably convoluted, folds linked with greater brain activity, the researchers found.
Using the new family tree of mammals in tandem with this anatomical data, we were able to reconstruct what this common placental ancestor may have looked like.
Their research also suggested placental mammals appeared after the end of the age of dinosaurs, with the original ancestor developing abouttoyears after the event.
This supports the notion that the mass extinction was a critical event in mammalian evolutionary history.NEW YORK — By moving into the water full-time, the ancestors of whales paved the way for their descendants to become behemoths, largely free from gravity's constraints.
Today, the blue whale is the largest animal ever to live. But even before the move, this lineage was setting size records. One ancient cousin to modern whales and hippos, called Andrewsarchus mongoliensisranks as the largest mammal known to have stalked the land as a predator. A skull from this creature — the only fossil found so far from this beast — greets visitors on their way into a new exhibit on whales here at the American Museum of Natural History.
In an artist's rendering, the million-year-old Andrewsarchus has a profile not unlike a giant feral pig with a more streamlined snout. This 6-foot-tall 1. The "first whale," a creature whose lifestyle living on land but eating fish from the nearby sea represented the early stage of this transition into the water, was a wolf-size fish eater that lived about 50 million years ago on the edges of the ancient Tethys Seaaccording to the exhibit.
Whereas this creature had a body clearly adapted for land, its relatives began acquiring features better suited to life in the water, such as webbed feet and a more streamlined, hairless shape. The basilosaurids, which lived about 34 million to 40 million years ago, had a more familiar shape than their ancestors. Basilosaurids had nostrils situated toward the top of their heads, an ear structure that suggested they could hear well underwater, and forelimbs that took the shape of paddlelike flippers.
Their hips and legs were on the way out. A basilosaurid on display, Dorudon atroxdisplays a tiny pelvis and legs detached from its spinal column. These leftovers from land are still visible in some modern whales. For instance, the skeleton of a pygmy right whale hanging from the ceiling displayed two tiny bones, the remnant of the pelvis, Flynn pointed out. Nowadays, there are two varieties of whale. These are the baleen whales, such as the blue whale, which use plates of baleen, made from fingernail-like material, to filter food from the water, and toothed whales, such as dolphins, killer whales and narwhals, which kept their teeth.
In the case of narwhals, one tooth becomes a modified tusk. Around 30 million years ago, these lineages split and evolved into the more than 80 species living today.
The exhibit also explores whale biology, and includes a life-size replica of a blue whale heart.
Meet the Last Common Ancestor of Bats, Whales, Sloths and Humans
Whales' relationships with humans are also a focus. The exhibit addresses the whaling industry, modern dangers, such as ship collisions, as well as coastal peoples' interactions with them. It was modified by the American Museum of Natural History. In traditional Maori culture, whales were the source of important resources, such as oil, protein, bones and teeth, and the inspiration for stories of whale riders, ancestors transported to New Zealand atop a whale.
A preview of the exhibit opened with a Maori blessing intended to invoke the gods, the spirits of ancestors and spirits of the whales on display. The exhibit is on display until Jan. Original article on LiveScience. Live Science. Please deactivate your ad blocker in order to see our subscription offer. The male sperm whale Physeter macrocephalus is the largest living toothed predator on Earth.What are evograms?
The first thing to notice on this evogram is that hippos are the closest living relatives of whales, but they are not the ancestors of whales. In fact, none of the individual animals on the evogram is the direct ancestor of any other, as far as we know. That's why each of them gets its own branch on the family tree. Hippos are large and aquatic, like whales, but the two groups evolved those features separately from each other.
We know this because the ancient relatives of hippos called anthracotheres not shown here were not large or aquatic. Hippos likely evolved from a group of anthracotheres about 15 million years ago, the first whales evolved over 50 million years ago, and the ancestor of both these groups was terrestrial.
These first whales, such as Pakicetuswere typical land animals. They had long skulls and large carnivorous teeth. From the outside, they don't look much like whales at all. Often, seemingly minor features provide critical evidence to link animals that are highly specialized for their lifestyles such as whales with their less extreme-looking relatives.
Skeletons of two early whales. Compared to other early whales, like Indohyus and PakicetusAmbulocetus looks like it lived a more aquatic lifestyle. Its legs are shorter, and its hands and feet are enlarged like paddles. Its tail is longer and more muscular, too. Animals are what they eat and drink, and saltwater and freshwater have different ratios of oxygen isotopes. This means that we can learn about what sort of water an animal drank by studying the isotopes that were incorporated into its bones and teeth as it grew.
The isotopes show that Ambulocetus likely drank both saltwater and freshwater, which fits perfectly with the idea that these animals lived in estuaries or bays between freshwater and the open ocean. Isotopic analyses help us figure out the likely habitats of extinct whales like Ambulocetus.
Whales that evolved after Ambulocetus Kutchicetusetc. These animals evolved nostrils positioned further and further back along the snout. This trend has continued into living whales, which have a "blowhole" nostrils located on top of the head above the eyes.
As whales evolved increasingly aquatic lifestyles, they also evolved nostrils located further and further back on their skulls. These more aquatic whales showed other changes that also suggest they are closely related to today's whales. For example, the pelvis had evolved to be much reduced in size and separate from the backbone. This may reflect the increased use of the whole vertebral column, including the back and tail, in locomotion. If you watch films of dolphins and other whales swimming, you'll notice that their tailfins aren't vertical like those of fishes, but horizontal.
To swim, they move their tails up and down, rather than back and forth as fishes do.
How Whales' Ancestors Left Land Behind
This is because whales evolved from walking land mammals whose backbones did not naturally bend side to side, but up and down. You can easily see this if you watch a dog running. Its vertebral column undulates up and down in waves as it moves forward. Whales do the same thing as they swim, showing their ancient terrestrial heritage. As whales began to swim by undulating the whole body, other changes in the skeleton allowed their limbs to be used more for steering than for paddling.The evolution of cetaceans is thought to have begun in the Indian subcontinentfrom even-toed ungulates 50 million years ago, over a period of at least 15 million years.
Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses.
Meet Your Mama: First Ancestor of All Placental Mammals Revealed
Being mammals, they surface to breathe air; they have 5 finger bones even-toed in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Research conducted in the late s in Pakistan revealed several stages in the transition of cetaceans from land to sea. The two modern parvorders of cetaceans — Mysticeti baleen whales and Odontoceti toothed whales — are thought to have separated from each other around million years ago in a second cetacean radiationthe first occurring with the archaeocetes.
The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence and subsequent increase in filter feeding. The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years, but a jawbone discovered in Antarctica may reduce this to 5 million years.
The traditional hypothesis of cetacean evolution, first proposed by Van Valen in was that whales were related to the mesonychidsan extinct order of carnivorous ungulates hoofed animals that resembled wolves with hooves and were a sister group of the artiodactyls even-toed ungulates. This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychids and those of early whales.
However, molecular phylogeny data indicates that whales are very closely related to the artiodactyls, with hippopotamuses as their closest living relative. Because of this observation, cetaceans and hippopotamuses are placed in the same suborderWhippomorpha.
However, the earliest anthracotheresthe ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after Pakicetuswhereas the first known whale ancestor appeared during the Early Eocene; this difference in timing implies that the two groups diverged well before the Eocene.
Molecular analysis identifies artiodactyls as being very closely related to cetaceans, so mesonychids are probably an offshoot from Artiodactyla, and cetaceans did not derive directly from mesonychids, but the two groups may share a common ancestor. The molecular data are supported by the discovery of Pakicetusthe earliest archaeocete.
The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids.
Archaeocetes retained aspects of their mesonychid ancestry such as the triangular teeth which modern artiodactyls, and modern whales, have lost. The earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution.
Whales, however, retained their carnivorous diet because prey was more available and they needed higher caloric content in order to live as marine endotherms warm-blooded. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was uncommon. This may be why they were out-competed by better-adapted animals like the hyaenodontids and later Carnivora. Indohyus was a small chevrotain -like animal that lived about 48 million years ago in what is now Kashmir.
Indohyus is identified as an artiodactyl because it has two trochlea hingesa trait unique to artiodactyls. The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known cetaceans, with Indohyus being the closest sister group. Their fossils were first discovered in North Pakistan inlocated at a river not far from the shores of the former Tethys Sea. Based on this discovery, pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago.
Pakicetids are classified as cetaceans mainly due to the structure of the auditory bulla ear bonewhich is formed only from the ectotympanic bone.
The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. The jawbone of pakicetids also lacks the enlarged space mandibular foramen that is filled with fat or oil, which is used in receiving underwater sound in modern cetaceans. This eye placement helps submerged predators observe potential prey above the water. This method of hearing did not give directional hearing underwater.