Eomaia , that early forerunner of placentals, lived in Asia. If the Riches are right, we have to rethink how placentals traveled from Asia to the Southern Hemisphere. Rather than traveling down the Americas, Eomaia may have found an island-hopping shortcut to Australia. Or perhaps placentals were widespread much earlier than we think now, and there's just no record of them. They could even have originated in Gondwana and spread out from there. Placentals, suggest the Riches, might even have become extinct with the dinosaurs in Australia, making room for the marsupials to move in later.
Rich himself concedes, "Most radical ideas are wrong. It's wise to be wary of them—especially when they are your own. Even more radical to many paleontologists has been the marriage of plate tectonics evidence and the placental family tree proposed by evolutionary geneticist Mark Springer and his colleagues. Springer is part of a new generation of researchers who examine the strands of an animal's DNA rather than scraping dirt from fossils at a dig.
These molecular biologists read the sequences of genes in a living animal's DNA like an evolutionary history book. The scientists can then determine how closely these animals are related genetically and how long ago their ancestors diverged. Troubling as it is to many paleontologists, Springer's reading of mammals' genetic history fits remarkably well with what geologists now know about the breaking up and subsequent motion of ancient continents.
The oldest group of living placental mammals, according to Springer and his colleagues, arose in Africa just before the continent finished breaking away from the rest of Gondwana around million years ago. Springer calls these animals afrotheres. They include elephants, aardvarks, manatees, and hyraxes. When Africa floated off, it carried these animals away to evolve on their own for tens of millions of years. The fossil record for Africa from this period is almost blank. Nevertheless, Emmanuel Gheerbrant, a researcher for the National Center for Scientific Research in France, speculates that Africa "must have been a laboratory for some very peculiar animals.
One species Gheerbrant has discovered from this period in Africa is the oldest and most primitive known member of the elephant group, the proboscideans. The million-year-old fossil of Phosphatherium escuilliei was discovered in Morocco. It was the size of a fox, and although it lacked a trunk, it had many dental and cranial features strikingly similar to modern elephants. Paleontologists had long thought elephants were one of the younger modern groups, evolving from ungulates that originated in Asia.
The molecular origin and evolution of dim‐light vision in mammals
But Gheerbrant's fossil, like the genetic evidence, suggests that proboscideans are in fact one of the oldest of the modern ungulate mammals. Today hyraxes resemble guinea pigs. But 35 million years ago hyraxes took many forms. Some were the size of rhinoceroses; others had long legs like gazelles. Most mammals on the African ark began to disappear around 20 million years ago, after Africa came into contact with the rest of the world again.
But Africa wasn't the only ark. An ancient seaway split South America from Eurasia and North America for millions of years, and South America became home to what geneticist Springer calls xenarthrans, another group of placental mammals. South America's fossil record during its isolation is far better than Africa's, and includes such xenarthrans as sloths, armadillos, and anteaters. Springer's data, in other words, indicate that the most recent common ancestor of placental mammals is Gondwanan.
Contrary to more than a century of northern chauvinism, the northern continents have the youngest placental mammals. One group, the laurasiatheres, includes seals, cows, horses, whales, and hedgehogs. The other group, euarchontoglires, includes rodents, tree shrews, monkeys, and humans. These genetic findings reveal more than simply which came first. They also redefine relationships among placental mammals. For one, anatomists have always assumed that bats were in the same superorder as tree shrews, flying lemurs, and primates. But genetic data place bats with pigs, cows, cats, horses, and whales.
The data further show that these superorders of living mammals started to diversify much earlier than the fossil record suggests.
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What gets fossilized is a record of an animal's shape. But geneticists contend that genes in an organism's mitochondria, the parts of a cell that are used to trace and date lineages, can be evolving rapidly without changing what would be left behind in the fossil record.
Birds have a slow rate, yet they can evolve physically very rapidly. However surprising the claims of geneticists seem at first, paleontologists and DNA researchers are finding that their theories can be complementary. Some stunning new fossils have confirmed a previously controversial DNA finding about whales.
Most paleontologists long believed that whales and dolphins—or cetaceans—descended from an extinct line of carnivorous mammals that for unknown reasons became aquatic between 50 and 45 million years ago. At the time of these fossils' discovery, molecular biologists were maintaining that new DNA work indicated the cetaceans were actually aligned closely with artiodactyls, an order that includes even-toed ungulates such as pigs, camels, deer, and hippopotamuses.
Paleontologists first dismissed this unlikely connection because nothing in the fossil record supported it. Then in September two teams of fossil hunters published finds that backed up the claims of the biologists. A group led by Hans Thewissen of Northeastern Ohio Universities College of Medicine found two species of the earliest known whales in million-year-old deposits in Pakistan.
Both had ear bones unique to whales, but the legs and anklebones of artiodactyls. Almost simultaneously, a group from the University of Michigan led by Philip Gingerich announced similar fossils from Pakistan that had the same dual traits.
The evolutionary origin of jaw yaw in mammals
The evolutionary transition among major groups of mammals is rarely illustrated so clearly. And no other discoveries have linked fossils to DNA findings with such precision. Until 65 million years ago dinosaurs dominated the land. The oceans swarmed with huge sharks and voracious marine reptiles. The dinosaurs and other large predators occupied the richest and most obvious evolutionary niches, keeping mammals at the margins. Then an event occurred whose scale is still hard to comprehend.
An object six miles 9. That impact may have been one of many over the next several hundred thousand years, each adding to the destruction. The temperature reached degrees in parts of the world.
Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation
They suddenly found themselves in a world without large carnivores. Restraints were off. Within , years they were diversifying and growing bigger.
Still, the majority of mammals didn't get much larger than a pig until the Eocene epoch, which began about 55 million years ago. Then a rapid increase in global temperature encouraged the spread of forests around the world—even near both Poles.
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This abundance of rich vegetation opened yet more ecological niches for mammals to exploit. Mammal diversity soared. One of the newcomers in the fossil record was our own order, the primates. The earliest primates belonged to the lemur branch. Today lemurs are confined to the island of Madagascar, where one species made it from Africa perhaps 50 million years ago, probably on rafts of storm-tossed debris.