We do that because science textbooks seem to always be in a state of flux, changing with every new discovery (The Appendix of Vestigial Textbooks - 4).
One positive aspect of the continually changing scientific landscape (besides built-in "planned obsolescence") is that new discoveries often answer questions that were once parts of a perplexing mystery.
Today, we will talk about a discovery that is leading to bigger questions, rather than providing satisfying answers to a mystery:
Between ancient, fragmented specimens and convoluted phylogenetic analyses, early mammalian evolution has long been a topic of scientific debate. Now, two newly discovered Jurassic-era fossils seem to only complicate matters further, suggesting possible mammalian origins that differ by tens of millions of years.(Where Did Mammals Originate?, emphasis added). Those differing million-year figures are 24-39 million years apart (200 - 176 = 24; 200 - 161 = 39).
The fossils, presented today (August 7) in Nature, were excavated by two teams working independently in China. Both specimens are nearly complete skeletons of haramiyids—early mammal-like creatures noted for their jaws and teeth—and estimated to be more than 160 million years old. Though the skeletal artifacts suggest the animals may have been relatives, their skeletons show markedly different features.
“Mammalian phylogeny is complex,” said Guillermo Rougier, a professor of anatomical sciences and neurobiology at the University of Louisville, who was not involved in either study. “The events we are trying to resolve occurred in the distant past—pertinent fossils are generally small, difficult to find, and often woefully incomplete.”
However, he added, “the two skeletons just described are precisely the high-quality specimens that allow scientists to contrast and refine our basic understanding of the complex evolutionary patterns involved in the origin of mammals.”
Because of their rodent-like teeth, some researchers have linked haramiyids to multituberculates, a group of ancient mammals. Describing Arboroharamiya, a short-faced, tree-dwelling animal, Jin Meng from the American Museum of Natural History (AMNH) and his colleagues suggest that haramiyids are indeed related to multituberculates, suggesting that mammals may have originated in the late Triassic, more than 200 million years ago.
Meanwhile, a team led by the University of Chicago’s Zhe-Xi Luo depict Megaconus, a terrestrial haramiyid with a primitive jaw and ankle that does not appear related to multituberculates. Compared with the Arboroharamiya fossil, the Megaconus specimen is consistent with a much more recent origin of mammals—around 176 million to 161 million years ago.
We may not yet know when mammals originated, but we know that they got a big opportunity when a mass extinction took place about sixty five million years ago:
The Cretaceous-Tertiary [K–Pg boundary; a.k.a. K-T boundary] mass extinction, which wiped out the dinosaurs and more than half of species on Earth, was caused by an asteroid colliding with Earth and not massive volcanic activity, according to a comprehensive review of all the available evidence, published in the journal Science.(Science Daily). The dinosaurs took a big hit and became extinct, but scientists tell us that placental mammals were able to fit into niches that were thereby made available:
A panel of 41 international experts, including UK researchers from Imperial College London, the University of Cambridge, University College London and the Open University, reviewed 20 years' worth of research to determine the cause of the Cretaceous-Tertiary (KT) extinction, which happened around 65 million years ago. The extinction wiped out more than half of all species on the planet, including the dinosaurs, bird-like pterosaurs and large marine reptiles, clearing the way for mammals to become the dominant species on Earth.
The new review of the evidence shows that the extinction was caused by a massive asteroid slamming into Earth at Chicxulub (pronounced chick-shoo-loob) in Mexico. The asteroid, which was around 15 kilometres wide, is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima. It would have blasted material at high velocity into the atmosphere, triggering a chain of events that caused a global winter, wiping out much of life on Earth in a matter of days.
The new study compares fossils from the past 150 million years with living mammals. Its findings support the view that modern placental lineages first appear around 65 million years ago in the Northern Hemisphere.(National Geographic, emphasis added). That the dominant species became placental mammals may involve another recent discovery and another big question and mystery:
"This is related to the demise of the dinosaurs," said John Wible, the curator of mammals at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
"The dinosaurs go extinct, and these various sorts of niches that were occupied by the dinosaurs all of a sudden become open, and it was into those niches that opportunistic placentals evolved."
If not for a virus, none of us [placental human mammals] would ever be born.(Discover, emphasis added). This is an opportunity for the weekend rebel science hypothesis: what if the asteroid contained viral material that became activated once the catastrophe subsided, and began to spread to surviving mammals, giving rise to placental mammals?
In 2000, a team of Boston scientists discovered a peculiar gene in the human genome. It encoded a protein made only by cells in the placenta. They called it syncytin.
The cells that made syncytin were located only where the placenta made contact with the uterus. They fuse together to create a single cellular layer, called the syncytiotrophoblast, which is essential to a fetus for drawing nutrients from its mother. The scientists discovered that in order to fuse together, the cells must first make syncytin.
What made syncytin peculiar was that it was not a human gene. It bore all the hallmarks of a gene from a virus.
It turned out that syncytin was not unique to humans. Chimpanzees had the same virus gene at the same spot in their genome. So did gorillas. So did monkeys. What’s more, the gene was strikingly similar from one species to the next. The best way to explain this pattern was that the virus that gave us syncytin infected a common ancestor of primates, and it carried out an important function that has been favored ever since by natural selection. Later, the French virologist Thierry Heidmann and his colleagues discovered a second version of syncytin in humans and other primates, and dubbed them syncytin 1 and syncytin 2. Both virus proteins seemed to be important to our well-being. In pre-eclampsia, which gives pregnant women dangerously high blood pressure, levels of both syncytin 1 and syncytin 2 drop dramatically. Syncytin 2 also performs another viral trick to help its human master: it helps tamp down the mother’s immune system so she doesn’t attack her baby as a hunk of foreign tissue.
In 2005, Heidmann and his colleagues realized that syncytins were not just for primates. While surveying the mouse genome, they discovered two syncytin genes (these known as A and B), which were also produced in the same part of the placenta. This discovery allowed the scientists to test once and for all how important syncytin was to mammals. They shut down the syncytin A gene in mouse embryos and discovered they died after about 11 days because they couldn’t form their syncytiotrophoblast. So clearly this virus mattered enormously to its permanent host.
The complete story will have to wait until scientists have searched every placental mammal for syncytins from viruses. But in the meantime there is something interesting to consider. Some mammals that scientists have yet to investigate, such as pigs and horses, don’t have the open layer of cells in their placenta like we do. Scientists have come up with all sorts of explanations for why that may be, mainly by looking for differences in the biology of each kind of mammals. But the answer may be simpler: the ancestors of pigs and horses might never have gotten sick with the right virus.
There are some scientists who have impressive arguments and evidence to support the cosmic virus aspects of such a hypothesis:
How did these early Earthly life forms arise? And did they include archae, bacteria, and eukaryotes? Joseph and colleagues (Joseph 2009a; Joseph and Schild 2010ab; Joseph and Wickramasinghe 2010) have detailed and reviewed a large volume of evidence suggesting life arrived here encased in the debris which formed the surface of this planet. By contrast, Russell and colleagues (Milner-White and Russell 2010; Nitschke and Russell 2010; Russell and Kanik 2010) have presented an impressive body of data indicating Earthly life (and even extraterrestrial life) may have been fashioned by the fortuitous mixture of the necessary chemicals within a watery thermal environment. Certainly early Earth was hot. Likewise, evidence of the earliest life was left in rocky formations bathed in water, i.e. banded iron formations consisting of alternating magnetite and quartz dated to 4.28 bya (O'Neil et al, 2008)(A Structure RE: The Corruption of Memes - 4). No matter how we look at it, the genetic material of viruses in placental mammals, which makes survival during the birth process possible, is quite interesting and mysterious.
Richard Hoover has discovered evidence of microfossils similar to Cyanobacteria, in freshly fractured slices of the interior surfaces of the Alais, Ivuna, and Orgueil CI1 carbonaceous meteorites. Based on Field Emission Scanning Electron Microscopy (FESEM) and other measures, Richard Hoover has concluded they are indigenous to these meteors and are similar to trichomic cyanobacteria and other trichomic prokaryotes such as filamentous sulfur bacteria. He concludes these fossilized bacteria are not Earthly contaminants but are the fossilized remains of living organisms which lived in the parent bodies of these meteors, e.g. comets, moons, and other astral bodies.
That placental mammals arose ~65mya, and a virus is involved, is related enough to look into further (cf. Clocks vs. Rocks).
Mammals replacing dinosaurs was a cosmic accident, in the sense of a random asteroid impact, but we placental mammals are not complaining much about that, whether we are cosmic creatures or not.
The next post in this series is here, the previous post in this series is here.