We know evolution happened because of a convergence of evidence. Consider the tale of the dog. With so many breeds of dogs popular for so many thousands of years, one would think there would be an abundance of transitional fossils providing paleontologists with copious data from which to reconstruct their evolutionary ancestry. In fact, according to Jennifer A. Leonard, an evolutionary biologist then at the Smithsonian Institution's National Museum of Natural History, "the fossil record from wolves to dogs is pretty sparse.
In the November 22, , Science , Leonard and her colleagues report that mitochondrial DNA mtDNA data from early dog remains "strongly support the hypothesis that ancient American and Eurasian domestic dogs share a common origin from Old World gray wolves. Finally, anthropologist Brian Hare of Harvard University and his colleagues describe in this same issue the results of a study showing that domestic dogs are more skillful than wolves at using human signals to indicate the location of hidden food.
Yet "dogs and wolves do not perform differently in a nonsocial memory task, ruling out the possibility that dogs outperform wolves in all human-guided tasks," they write. Therefore, "dogs' social-communicative skills with humans were acquired during the process of domestication. The tale of human evolution is divulged in a similar manner although here we do have an abundance of fossils , as it is for all concestors in the history of life.
We know evolution happened because innumerable bits of data from myriad fields of science conjoin to paint a rich portrait of life's pilgrimage. Newsletter Get smart.
We know that pakicetids were closely related to whales and dolphins based on a number of unique specializations of the ear. But pakicetids lived on land and had nostrils at the front of the skull, as modern cows and sheep do.
The ancestors of whales probably looked something like Pakicetus. How did evolution go from something like Pakicetus to modern whales below right , with nostrils aka, the blowhole at the top of the head? If a pakicetid-like ancestor gave rise to modern whales, we would expect the lineage to have passed through an intermediate form — one with nostrils in the middle of the skull.
And indeed we do find evidence of this transition in the fossils of Aetiocetus above middle , which had nostrils in the middle of its skull. The fossil record of horses provides other examples of transitional features.
Modern horses members of the genus Equus , which also includes donkeys and zebras have just one toe—the hoof. However, the ancestors of modern horses, which lived more than 50 million years ago, had four toes. We know this from fossils of the earliest horses, like those of Eohippus also known as Hyracotherium.
In addition, there are many more extinct families that are familiar only to paleontologists, including the primitive diacodexeids, the pig-like entelodonts, the ubiquitous oreodonts, the bizarrely horned protoceratids and dromomerycines, and many others. Nearly all domesticated animals that we eat cattle, pigs, sheep, goats or get milk from cattle, goats or use for leather or wool cattle, sheep are artiodactyls. As such, they are much more familiar to us, even though a lot remains to be learned about their evolution.
Artiodactyls have many other distinctive characteristics in their skulls and skeletons, especially in the unique crescent-shaped crests selenodonty that many groups independently evolved in their cheek teeth. The origin and early evolution of artiodactyls is just now becoming better known as new discoveries are made Prothero and Foss The sister taxon of artiodactyls is still controversial Prothero et al.
Various candidates have been proposed, ranging from archaic ungulates like the huge predatory mesonychids to the coatimundi-like arctocyonid ungulates such as Chriacus. In any case, the evidence suggests that artiodactyls are one of the first groups to branch off from the rest of the hoofed mammals. By the early Eocene, very primitive artiodactyls known as diacodexeids or dichobunids were widespread across Eurasia and North America.
To the casual viewer, these creatures about the size of a small dog would look like a small musk deer or even rabbit-like, since they had long slender hind limbs for leaping. However, a closer look at the teeth and ankles and feet shows that they have all the hallmarks of artiodactyls, especially in the double-pulley astragalus and paraxonic foot.
By the middle Eocene, these diacodexeids had been replaced by a huge radiation of archaic artiodactyl groups in North America and Asia Gazin ; Stucky ; chapters in Prothero and Foss , nearly all of which are now extinct. Each of these groups is only slightly more advanced than their primitive sister groups, yet there are already trends toward the low-crowned grinding teeth bunodonty in the lineages that led to pigs, peccaries, and hippos numerous genera from the middle—late Eocene of China and Thailand—Harris and Liu There were still others that were specialized in the direction of ruminants Archaeomeryx from the middle Eocene of Mongolia—Metais and Vislobokova and camels middle Eocene North American forms such as oromerycids and the camel Poebrodon.
Europe had its own unique endemic radiation of seven artiodactyl families that evolved in isolation when Europe was a flooded archipelago Erfurt and Metais There are so many of these excellent examples of evolution within these families that an entire book e.
For the purposes of this essay, however, we will examine two that are particularly striking: the camels and the giraffes. Most of us think of camels as humped creatures of the African and Asian deserts, but the two Old World camelids the one-humped dromedary and two-humped Bactrian camels are actually exceptions to the general trend.
Most of camelid evolution took place in North America, with only later migrations to Eurasia about 7 million years ago and to South America about 3 million years ago. Based on their sister-group relationships, there is no reason to think that extinct camels had humps; it is likely that it is a unique feature of the desert-dwelling Old World camelids. In their North American homeland, camels evolved from the tiny but hypsodont Poebrodon of the middle Eocene to the larger sheep-sized late Eocene—Oligocene Poebrotherium Prothero Camels then diversified into many different families Fig.
Some were even bigger than giraffes, such as Titanotylopus and Gigantocamelus. Many camels apparently performed the roles of gazelles or antelopes or giraffes in the North American savannas during the Miocene, since those African groups never reached North America at that time. Thus, the evolution of the camels is just as amazingly branching and bushy as the example of horse or rhino evolution.
Unfortunately, the basic systematics of the group has not yet been fully documented yet although Jim Honey and I are currently working on this as a long-term project ; the latest summary can be found in Honey et al.
Only the living African dromedary and two-humped Asian Bactrian camels have humps after Prothero Although camel phylogeny was bushy and branching, we can still observe some overall trends in their evolution Fig. From their sister group, the oromerycids like Protylopus , to the earliest camelids like Poebrotherium , to the larger more advanced camels like Procamelus , we can document a striking change in size, crown height of the teeth, reduction in side toes, and eventually fusion of the metacarpals and metatarsals into a cannon bone.
There was also an elongation of the snout and development of gaps, or diastemata, between the anterior teeth. This example shows that the earliest camels look nothing like modern camels and that we have all the transitional fossils that link the earliest camels with their living descendants and from the camelids back to the oromerycids and hence back to more primitive sister taxa among the artiodactyls. Evolutionary trends within the camels, from the tiny oromerycid Protylopus through the Oligocene camel Poebrotherium through more advanced Procamelus.
Although their history is not a straight line of evolution but a bushy branched pattern, there are trends toward larger body size, loss of the front teeth, longer snouts and larger eyes, longer legs and toes reducing to just two toes fused together , and higher-crowned cheek teeth after Scott Our second example from the artiodactyls is the giraffids.
Creationists often scoff at the notion that there are fossils that show how the giraffes evolved, but they could not be more mistaken. In fact, the giraffids have an excellent fossil record, although nearly all giraffes both extinct and living are short-necked, much like the modern okapi.
Only the living genus Giraffa has the long neck that we consider typical of the group. All the rest of the giraffids Fig. Some, like Sivatherium , were stocky moose-like creatures with broad palmate horns somewhat like those of a moose.
Others, like Climacoceras , looked more like deer or antelopes. Despite these superficial convergences, they all show the characteristic hallmarks of giraffids in their teeth, skulls, and skeletons.
Evolution of the giraffe family. Only the lineage of the modern giraffe evolved a long neck after Prothero Most of these taxa are known from skulls and jaws and a few from skeletons, but the neck vertebrae are not often preserved. However, Nikos Solounias , personal communication is currently publishing a description of a new fossil of the giraffid Bohlinia that preserves a neck that is intermediate in length between Giraffa and the okapi Fig.
Thus, we do know how the giraffe got its long neck, and we have the transitional fossils to show how and when it occurred! Once again, the fossil record has provided a specimen whose very existence the creationists have long denied. Neck vertebrae of a recently discovered fossil giraffid Bohlinia that is intermediate in length between those of primitive giraffids Okapia , bottom and the modern long-necked species Giraffa , top.
Both molecular and paleontological evidence agree that artiodactyls and perissodactyls are a natural group of ungulates. However, when it comes to a third major hoofed mammal clade, the tethytheres elephants, sirenians, and their kin , there is a conflict between molecular evidence which places them in the Afrotheria Springer et al.
We will not discuss this issue further here, because numerous laboratories and paleontologists are working to resolve the conflict. Many of primitive tethytheres had hooves, so we will treat them as hoofed mammals in an ecological sense, even if it is not clear that they are part of the Ungulata.
The Proboscidea, or the order of elephants and their extinct relatives, have an outstanding fossil record Shoshani and Tassy , since they are large-bodied heavy-boned animals that fossilize well.
The details of their systematics are still not fully worked out Lambert and Shoshani , but many of the broader trends are well documented. The earliest proboscidean in the fossil record is known as Phosphatherium ; it comes from the late Paleocene of Morocco Gheerbrant et al.
Although it consists of a partial skull, the teeth already have the classic mastodont pattern. By the early Eocene, we have Numidotherium from Algeria Mahboubi et al. By the late Eocene and Oligocene, we have the well-known Moeritherium , which looked more like a large tapir or pygmy hippo than an elephant Fig.
Nevertheless, the skull shows evidence of a short proboscis, short tusks in the upper and lower jaws, teeth typical of primitive mastodonts, and many details in the rest of the skull that unquestionably link it with the Proboscidea.
Details of the evolution of the skull, tusks, and trunk of proboscideans, from the pygmy hippo-like Moeritherium through mastodonts with longer tusks and trunks to mammoths after Scheele From Moeritherium , there was a tremendous radiation of mastodonts and mammoths in the Oligocene and Miocene Fig.
All of these can be traced back to primitive gomphotheres of the Oligocene and Miocene, which had short trunks and tusks, but were otherwise unspecialized. Once again, the phylogeny is bushy and branching, although it can be summarized in terms of its general trends Fig. Evolutionary history of the elephants and their kin Proboscidea , starting with pygmy hippo-like forms like Moeritherium with no trunk or tusks, through mastodonts with short trunks and tusks, and concluding with the huge mammoths and the two living species.
Early in their history, the other tethytheres branched off from the Proboscidea. These include the manatees, order Sirenia, the extinct desmostylians, and the extinct horned arsinotheres from Prothero Today, these peaceful aquatic creatures float in shallow tropical waters and graze on sea grass. They are fully aquatic, with two front flippers and no visible hind legs. For decades, their relationships to the rest of the mammals were unknown until McKenna showed that they are the sister taxon of the Proboscidea and part of a group he called the Tethytheria.
Subsequent work has found a huge number of highly specialized features that confirm this hunch, so it is now a well-established notion. In the past decade, the monophyly of the Tethytheria was also confirmed by later molecular analysis. However, most sirenian fossils are incomplete, usually consisting of the distinctive extremely dense bone of their ribs used for ballast or occasional skulls and teeth. Yet this creature had four perfectly good legs complete with terrestrial hands and feet, not flippers as seen in the living sirenians Fig.
The mounted skeleton of Pezosiren portelli , the sirenian with feet rather than flippers, next to Daryl Domning, who described and named it photo courtesy of Dr. Raymond L. One could not ask for a better example of a transitional fossil! It closely parallels the intermediate pattern of locomotion seen in walking whales such as Ambulocetus Thewissen, this volume. When creationists have addressed this discovery at all on their websites; none of their books mention it yet , they show their complete ignorance of the basics of anatomy and paleontology.
In short, they do not understand the basic notion of homology and analogy. This kind of mental straitjacket and getting out of a dilemma by defining it away might make them feel better, but it is no excuse for knowing their anatomy or fossils or getting the facts straight.
In short, the fossil record of hoofed mammals is full of transitional fossils and even longer transitional sequences that demonstrate the origins of nearly all the living ungulates and tethytheres from ancestors that looked almost completely unlike their descendants.
We now have the fossils that show where the perissodactyls came from phenacodonts, Radinskya and that document the radiation of the earliest horses, tapirs, rhinos, and brontotheres when they were almost indistinguishable to the untrained eye Fig. We have the fossils that demonstrate the evolution of the horse family, the rhinoceroses, the tapirs, and the brontotheres, along with other examples not covered in this article.
Their phylogenies are now much more bushy and branching, but otherwise, the general trends are the same that were observed over a century ago. Likewise, we now have the fossils to document the early stages of the radiation of the artiodactyls and especially the bushy branching history of camels and giraffes, both of which lacked humps or long necks in their respective early histories.
One of the best transitional fossils of all is Pezosiren portelli , a perfect intermediate form that shows how the aquatic manatees evolved from walking ancestors. All of these examples are largely ignored by creationists, or when they do mention them, they use completely outdated arguments, quotes out of context, or simple lies and distortions that demonstrate the fact that creationists have no training in anatomy or paleontology and cannot tell one bone from another.
In the most extreme cases, the creationists resort to semantic gyrations that define the problem away, so that if a fossil has terrestrial legs and feet, it cannot be a sirenian or a whale, even if every other aspect of the anatomy clearly indicates its phylogenetic affinities.
Arguments such as this reveal the dogmatism and complete intellectual and scientific bankruptcy of creationists.
Since they do not even bother to do this, their arguments are worthless. Domning DP. The earliest known fully quadrupedal sirenian. Erfurt J, Metais G. The evolution of artiodactyls. Google Scholar. Froehlich DJ. Quo vadis Eohippus? The systematics and taxonomy of the early Eocene equids Perissodactyla. Zool J Linn Soc Lond. Article Google Scholar. Gazin CL. A review of the upper Eocene Artiodactyla of North America. Smithson Misc Contrib. A Palaeocene proboscidean from Morocco.
Paenungulata Sirenia, Proboscidea, Hyracoidea, and relatives. The rise of placental mammals. Gish D. Evolution, the fossils still say NO!. Superfamily Suoidea.
Evolution of tertiary mammals of North America.
0コメント