■ Two hypotheses about the origin of cetaceans
Scientists believe that modern cetaceans originated from carnivorous quadruped mammals. The ancestors of these cetaceans are a bit like big wolves. They once roamed the continent and tracked their prey. Until the beginning of Eocene, 57 million years ago, these carnivores resolutely jumped into the sea to open up new ecological fields under the pressure of the dry living environment at that time, so their bodies experienced profound evolution. Gradually, their limbs and pelvis degenerated, and their tails became stronger and stronger, turning into paddles to beat the sea water, thus pushing these "sea behemoths" to swim in the ocean.
However, how did whales, which played an important role in the evolutionary history of mammals, evolve? Who is the ancestor of cetaceans? Because there is no exact evidence of terrestrial biochemical stones, there has been no accurate answer and there is controversy. Deng Tao said that according to the morphological characteristics of teeth and ear areas, paleontologists advocated that cetaceans were closest to intermediate mammals (an extinct carnivorous ungulate in the early Tertiary); However, molecular biologists believe that cetaceans belong to Artiodactyla (such as sheep, cattle, pigs, camels and deer) and are sister groups of hippos (a family of Artiodactyla).
How did the earliest terrestrial mammals evolve into modern aquatic whales? The Middle Eocene cetaceans 45 million years ago were discovered a century ago, but for a long time no limb bone materials were found, only the skulls of these animals were described. Since then, many fossils found in North America, Pakistan and Egypt show that these early cetaceans had movable elbows and articulated hind legs outside the body. However, except for amphibious whales (very similar to sea lions), they are all completely aquatic. Therefore, detailed anatomical information about the terrestrial ancestors of cetaceans is not available.
■ New hypotheses brought by new evidence
On September 20th this year, Nature magazine reported that Tyson of Ohio State University School of Medicine discovered a mammal fossil that lived about 50 million years ago in northeast Pakistan. Its ankle bones are similar to cloven-hoofed animals, and its head is very similar to a whale. They belong to the earliest known cetaceans. The newly discovered cetacean fossils also show that the early cetacean ancestors were completely land animals and even "effective runners". They also revealed that cetaceans are more closely related to the earliest known cloven-hoofed animals than any other mammal. These conclusions are based on reliable comparative anatomical data, which are different from the two hypotheses of paleontologists and molecular biologists.
Deng Tao explained that all mammals that lived in the early Tertiary (about 65 million to 50 million years ago) were terrestrial animals. Therefore, aquatic cetaceans must have evolved from terrestrial mammals and returned to the water. The forelimbs of cetaceans still have the same structure as terrestrial mammals. However, modern cetaceans are highly specialized, and many adaptive characteristics enable them to swim, dive and forage.
Living cetaceans are very different from other mammals, so another old question in this field is which mammal group contains close relatives of cetaceans, that is, who is their "sister group"? Compared with terrestrial mammals, the anatomical characteristics of cetaceans' skulls and bones have changed greatly. The fossils of early cetaceans are very rare and always incomplete, so it is difficult to establish the genetic relationship of this group.
New fossils show that the morphological type of ankle bone is adapted to running, which was once thought to be unique to cloven-hoofed, but now it is obviously found in whales. Therefore, morphological evidence shows for the first time that cloven-hoofed animals are close relatives of cetaceans.
This means that cloven-hoofed animals and cetaceans form two branches of a larger group (that is, cloven-hoofed cetaceans). The researchers also excluded China animals from this larger group. They also pointed out that hippopotamus is not a living sister group of cetaceans, and the closest fossil relatives of cetaceans may be the earliest known cloven-hoofed animals, such as the ancient cloven-hoofed animals (about 50 million years ago).
Just one day later, on September 2 1, Science magazine reported that a research team led by gingerich, a professor of geology and paleontology at the University of Michigan, found another animal skeleton fossil about 47 million years ago in southwestern Pakistan. These fossils also provide strong morphological evidence that whales are related to cloven-hoofed animals, but they are descendants of cloven-hoofed animals. This undoubtedly provides more evidence for solving the previous disputes, because this result makes the morphological evidence consistent with the molecular data at least at the naked eye level.
Deng Tao told reporters that primitive ancient whales have been reported before, but these new specimens are directly connected with whale bones for the first time, and they are well preserved. The specialized ankle bone is enough to provide important clues for the genetic relationship between whales and cloven-hoofed animals. Moreover, these new fossil evidences show that cetaceans differentiated from cloven-hoofed animals long ago and originated from an unknown primitive cloven-hoofed animal, which is more extinct than any known primitive cloven-hoofed animals, including ancient cloven-hoofed animals and Carboniferous animals (an animal similar in form to pigs, but unrelated to pigs) and closely related to hippopotamus's system. Carboniferous mammals flourished in the Middle Tertiary, about 25 million years ago. ), cloven-hoofed animals are relatively primitive ankles.
As you can imagine, hippopotamus and cetaceans are existing members of a branch differentiated from other cloven-hoofed animals before Eocene. This hypothesis means that the progressive traits of some cloven-hoofed animals evolved more than once: in the Carboniferous animal-hippopotamus branch (after whale differentiation), they evolved independently in other cloven-hoofed animals.
■ The conclusions are divergent and of high significance.
However, there are still many problems to be understood about the evolution of whales, and the conclusions of the above two research papers are still different. Taiweisen and others think that the two hypotheses of paleontologists and molecular biologists are incorrect, that is, whales are neither sisters of China animals nor hippos, but form a sisterhood relationship with the whole cloven-hoofed animals including hippos. On the other hand, although gingerich and others agree that whales are related to cloven-hoofed animals, they think that whales are descendants of cloven-hoofed animals. According to the principle of cladistic taxonomy, the whale is a member of cloven-hoofed animals, and has a sister relationship with the extinct group of hippos and Carboniferous animals, so hippos are close relatives of whales. This conclusion supports the hypothesis of molecular biologists.
Obviously, the thorough settlement of the debate will depend on the discovery of more fossil materials, and some related questions still lack answers, such as the exact ancestor of hippopotamus and the origin of cloven-hoofed animals themselves. The final answer may also come from the same fossil origin of the above two papers.
Although further discovery and research are needed, Deng Tao believes that the discovery of Tavison and gingerich has greatly promoted the research on the evolution of early cetaceans, especially that Tavison connected the missing link in the evolutionary chain of aquatic cetaceans and their land ancestors, just as Archaeopteryx discovered in that year played an important role in the early evolution of birds. He said that only a few fossils, such as the one discovered this time, revealed the connection between the two groups of vertebrates with different forms but similar evolution. When the living environment changes greatly, such as from land to water, the re-adapted animals will always undergo significant morphological changes due to the high pressure of natural selection, and any similarity with the original ancestors may soon disappear. But there is no doubt that these new fossils prove the connection between modern whales and their land ancestors, which is why some critics think they can be compared with the most primitive bird archaeopteryx and early human Australopithecus and other famous "transitional types".