Cetartiodactyla is the
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
to which
whaleWhale is the common name for marine mammals of the order Cetacea. The term whale is sometimes used to refer to all cetaceans, but in more common English usage it generally excludes the members of the Delphinoidea superfamily, such as dolphins and porpoises...
s (including
dolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly...
s) and
even-toed ungulateThe even-toed ungulates form the mammal order Artiodactyla, the group that contains the pigs, peccaries, hippopotamuses, camels, chevrotains , deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle...
s have currently been placed. The term was coined by merging the name for the two orders,
CetaceaThe order Cetacea includes whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea monster"...
and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the classification that whales evolved from within the artiodactyls. Under this definition their closest living land relative is thought to be the
hippopotamusThe hippopotamus or hippo is a large, mostly plant-eating mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae...
.
Cetartiodactyla is the
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
to which
whaleWhale is the common name for marine mammals of the order Cetacea. The term whale is sometimes used to refer to all cetaceans, but in more common English usage it generally excludes the members of the Delphinoidea superfamily, such as dolphins and porpoises...
s (including
dolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly...
s) and
even-toed ungulateThe even-toed ungulates form the mammal order Artiodactyla, the group that contains the pigs, peccaries, hippopotamuses, camels, chevrotains , deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle...
s have currently been placed. The term was coined by merging the name for the two orders,
CetaceaThe order Cetacea includes whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea monster"...
and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the classification that whales evolved from within the artiodactyls. Under this definition their closest living land relative is thought to be the
hippopotamusThe hippopotamus or hippo is a large, mostly plant-eating mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae...
. The clade formed by uniting whales and hippos is called
WhippomorphaWhippomorpha is a whimsical name for the clade containing the Cetacea and their closest living relatives, the hippopotamuses. This would be a sub-grouping of the Cetartiodactyla, which also includes pigs and ruminants...
or Cetancodonta. Alternatively, the term Cetartiodactyla is used to denote a clade where Cetacea and Artiodactyla are sister groups, but where Cetacea did not actually evolve from within the Artiodactyla. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.
Whales evolved from land
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s and appear to form a monophyletic group. The notion that all whales evolved from a single ancestor is not in dispute. The most widely accepted hypothesis before the 1990s was that the closest relatives to whales was the fossil group Mesonychia. These were
hoovedA hoof is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole and the edge of the hoof wall....
, predominantly carnivorous, mammals that are known only from
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
s. Few modern authorities still consider mesonychids to be more closely related to whales than artiodactyls. Instead, they are usually considered to be the closest relative of the Cetartiodactyla as a whole.
Kin to hippos
The idea that whales evolved from within the Artiodactyla was based on analysis of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
sequences. In the initial molecular analyses, whales were shown to be more closely related to
ruminantPhysiologically, a ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first stomach, known as the rumen, then regurgitating the semi-digested mass, now known as cud, and chewing it again. The process of rechewing the cud to...
s (such as
cattleCattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
and
deerDeer are the ruminant mammals forming the family Cervidae. They include for example Moose, Red Deer, Reindeer, Roe and Chital. Animals from related families within the order Artiodactyla are often also considered to be deer – these include muntjac and water deer...
) than ruminants are to
pigPigs are a genus of even-toed ungulates within the family Suidae. The name hog most commonly refers to the domestic pig in everyday parlance, but technically encompasses several distinct species, including the wild boar...
s. In order for the order name to reflect a real evolutionary unit, the term Cetartiodactyla was coined.
Later molecular analyses included a wider sampling of artiodactyls and produced a more complete tale. Hippos were determined to be the closest relative of whales, ruminants were related to a whale/hippo clade, and pigs were more distant. In addition to producing the controversial whale/hippo clade, these analyses debunked the idea that hippos and pigs are closely related. This had been a popular taxonomic hypothesis (Suiformes) based on similarities in
morphologicalIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
(physical) characteristics.
In addition to DNA and
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
sequences, researchers tracked the movement of
transposonTransposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome. Transposons were also once called "jumping genes", and are examples...
s called
SINEMaurice Sinet, known as Siné is a French cartoonist.As a young man he studied drawing and graphic arts, while earning a living as a cabaret singer. After his military service he started publishing his drawings and also worked as a photo-retoucher for porn magazines. His first published drawing...
s in the
genomeIn modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
(see the method at
retrotransposon markerRetrotransposon markers are retrotransposons that are used as cladistic markers.The analysis of SINEs – Short INterspersed Elements – LINEs – Long INterspersed Elements – or truncated LTRs – Long Terminal Repeats – as molecular cladistic markers represents a particularly interesting complement to...
). A transposon is a DNA sequence that will occasionally make a copy of itself and insert that copy into another part of the genome. It is considered highly unlikely that SINEs will insert themselves into the exact same part of a genome by chance. The data indicate that several transposons inserted themselves at the same point in the genomes of whales, ruminants and hippos (sometimes referred to as "pseudoruminants" because although they have four-chambered stomachs like true ruminants, they do not chew the cud). This insertion point is not shared with
camelCamels are even-toed ungulates within the genus Camelus. The dromedary or Arabian camel has a single hump, and the Bactrian camel has two humps. They are native to the dry desert areas of western Asia, and central and east Asia, respectively...
s and pigs.
This hypothesis has been tested with DNA sequences from a host of genes: the complete mitochondrial genome (as well as several of its genes independently), beta-casein, kappa-casein,
von Willebrand factorVon Willebrand factor is a blood glycoprotein involved in hemostasis. It is deficient or defective in von Willebrand disease and is involved in a large number of other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic-uremic syndrome.- Synthesis :vWF...
, breast cancer 1,
recombination activating geneThe recombination activating genes encode enzymes that play an important role in the rearrangement and recombination of the genes of immunoglobulin and T cell receptor molecules during the process of VDJ recombination. There are two recombination activating gene products known as RAG-1 and RAG-2,...
s 1 and 2,
cannabinoid receptorThe cannabinoid receptors are a class of cell membrane receptors under the G-protein coupled receptor superfamily. Cannabinoid receptors are activated by ligands, which are lipid compounds known collectively as cannabinoids...
1, and several others. These sequence data and the transposons converge on the same conclusion that hippos and whales are more closely related to one another than either is to other artiodactyls.
Sequences analyzed in combined analyses with morphological characters have also produced the same results as sequences alone. Some have argued that the sheer number of characters (one for each
nucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides play central roles in metabolism...
) in sequences swamps out the effects of morphology. There have been a few morphology-based studies that have suggested (weakly) the same results as the molecular results, but overall most morphological studies have conflicted with the whale/hippo hypothesis of Cetartiodactyla.
An important exception is a recent conducted by Boisserie et al. (2005). They examined 80 hard morphological characters of fossil and extant cetartiodactylan taxa. Their results suggest that hippopotamids evolved from within a clade of anthracotheres. That anthracothere/hippopotamid clade appears to be sister to the Cetacea and supports the molecular results.
A clade apart
The morphology and
paleontologyPaleontology
[from Greek: παλαιός "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought"] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments...
-based opinion of the relationship of whales to other mammals has also undergone considerable changes since the early 1990s. Fossils such as
RodhocetusRodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.-Description:...
, have been discovered that refute the notion that whales are derived from or are closely related to the
mesonychidMesonychia are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls , and to cetaceans...
s. Many morphologists and paleontologists support the notion of a clade called Cetartiodactyla that unites Cetacea + Artiodactyla. Many are not, however, in support of the hypothesis that Cetacea evolved from within the Artiodactyla. Under this definition, Artiodactyla remains a valid clade since artiodactyls evolved from a common ancestor separate from whales. Cetartiodactyla would represent a grandorder or superorder uniting the two orders.
The vast majority of phylogenetic analyses based on morphological characters have not uncovered a whale/hippo clade, but show Cetacea and Artiodactyla as distinct from one another. Features of the bones of the
astragalusThe talus bone or astragalus is a bone in the tarsus of the foot that forms the lower part of the ankle joint through its articulations with the lateral and medial malleoli of the two bones of the lower leg, the tibia and fibula. Within the tarsus, it articulates with the calcaneus below and...
in the ankle region are cited as particular evidence for a monophyletic Artiodactyla. In any case, an origin of the Cetacea from within the Ungulata was proposed as early as 1900 by
Frank Evers BeddardFrank Evers Beddard was a British zoologist. He won the Linnean Medal in 1916 for his book on oligochaetes.Beddard was born in Dudley, educated at Harrow and studied at New College, Oxford. He was naturalist to the Challenger Expedition Commission from 1882 to 1884...
's
A book of whales. Finding the anatomical and fossil record at that time insufficient to rule out other possibilities – such as the notion that whales are a basal lineage among placental mammals, or the popular theory relating them to terestrial carnivores –, Beddard remained not fully convinced himself, and thus his bold proposal was largely forgotten.
Hippo fossils are not observed until the
MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
, but whale ancestors have been dated to the
EoceneThe Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma , is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the...
. The whale/hippo hypothesis imposes a gap of nearly 30 million years where no hippo ancestors are present. Certain anthracotheres have been proposed as hippo ancestors, but this concept has not garnered wide support among paleontologists. However, the most recent theory into the origins of hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other Artiodactyls around . This hypothesized ancestral group likely split into two branches around . One branch would
evolve into cetaceansThe cetaceans are descendants of land-living mammals. Their terrestrial origins are specifically indicated by:* Their need to breathe air from the surface;...
, possibly beginning with the proto-whale
PakicetusPakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae which was endemic to Asia from the Eocene .Pakicetus existed for approximately .-Taxonomy:...
from with other early whale ancestors collectively known as
ArchaeocetiArchaeocetes, or "ancient whales", are a paraphyletic group of cetaceans that gave rise to the modern cetaceans.The archaeocetes were once thought to have evolved from the mesonychids, based on dental characteristics...
, which eventually underwent
aquatic adaptationSeveral animal groups have undergone aquatic adaptation, going from being purely terrestrial animals to living at least part of the time in water. The adaptations in early speciation tend to develop as the animal ventures into water in order to find available food. As successive generations spend...
into the completely aquatic
cetaceaThe order Cetacea includes whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea monster"...
ns. Thus, Cetartiodactyla is supported as a monophyletic group.