The more scientists examine life on Earth, the more they unravel complex webs of interaction. For example, leafcutter ants feed on a certain type of fungus, and scientists long thought that leafcutter ants kept their fungal gardens pest-free with careful weeding. Then a graduate student discovered that the gardens were under continual threat from a different kind of fungus, a parasitic fungus. He also discovered that the worker ants' abdomens were covered with a substance commonly used in antibiotics. What had long looked like a simple partnership between two species (ants and the fungus they ate) was a more complicated relationship between four species (ants, their fungal food, an annoying fungal pest, and the built-in antibiotic).
Neanderthals have a tortured history in anthropology, identified as diseased modern humans then identified as stooped and awkward because the scientist making that determination examined a Neanderthal riddled with arthritis. One of the early claims made about Neanderthals, that they buried their dead, has been alternately challenged and vindicated. Like Neanderthal fossils preserve evidence of people who must have been looked after by others. Finds associated with Neanderthal remains suggest symbolic behavior. Near Krapina, Croatia, 130,000-year-old eagle talons from a Neanderthal site bear cut marks and abrasion marks that might have resulted from their use in necklaces. Bruniquel Cave in France bears structures built from broken-off stalagmites, dated at abut 176,000 years old, when Neanderthals were the only hominins known to inhabit Europe, though the purpose of these structures is a mystery.
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Primate fossils aren't the only evidence of divergence between the ancestors of humans and apes. Scientists can also look at the rates of change in nuclear DNA, mitochondrial DNA, and even the DNA of our gut bacteria, which have a symbiotic relationship with their primate hosts and can speciate as we do. A 2016 study examined two types of gut bacteria: Bacteroidaceae and Bifidobacteriaceae, which colonize the digestive tracts of chimps, bonobos, gorillas and humans. Comparing these strains of bacteria in different hosts indicates that we split from gorillas about 15.6 million years ago (this puts the human-gorilla split slightly further back in time than what mitochondrial DNA indicates, but it's in line with nuclear DNA). Gut bacterial differences also indicate that we diverged from chimps around 5.3 million years ago (this is a little more recent than what nuclear DNA differences suggest, but it's in agreement with mitochondrial DNA).
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Biologists have divided life on Earth into three domains: archaea, bacteria, and eukaryotes (this last category includes us, as well as all other animals and plants). Lynn Margulis found that mitochondria — eukaryotic cells' power generators — sport genes more like those of bacteria than eukaryotes. Meanwhile, Carl Woese and his colleagues found that archaea (single-celled organisms that sometimes occur in extreme environments) share more genetic material with eukaryotes than bacteria. A study published in 2015 identified Lokiarchaeota, a type of archaea excavated from beneath the Atlantic Ocean that shares more genes with eukaryotes than any other known archaea. Lokiarchaeota, the researchers found, has the same genes that build compartments inside eukaryote cells, and construct and destroy eukaryote "skeletons." The newly discovered type of archaea also has genes indicating that it could gobble up microbes and perhaps even turn some of those microbes into organelles.
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One of the most important ways in which the rock record supports evolutionary theory is the succession of fossils in older versus newer rock layers. As far back as the 18th century, scholars realized that fossils in older layers differed more from modern life forms than fossils in newer layers. While many fossils from the Pleistocene Ice Age resemble organisms living today, far fewer fossils from the Age of Reptiles do. If you venture back in the rock record to the Precambrian (prior to roughly 550 million years ago), you'll find few fossils of multicellular organisms at all, though you will find some. So striking has this fossil succession been that when asked what would disprove evolution, 20th-century British scientist J.B.S. Haldane quipped, "Fossil rabbits in the Precambrian." No such bunnies have ever been found.
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Evolution of other organisms plays a role in human healthcare. Since humans learned about antibiotics, we've used them to fight infection, but we haven't always enjoyed the results we wanted. (staph infection) evolved a resistance to Penicillin in 1946, to Methicillin in 1961, to Vancomycin in 1986, and to Zyvox in 1999. Growing resistance of hospital "superbugs" to antibiotics spread across six European Union nations between October 2010 and March 2011. Meanwhile, the HIV virus evolves so quickly that multiple species can thrive in a single patient. During the 2013-2016 Ebola outbreak in West Africa, the disease claimed more than 11,000 lives. Besides human factors such as widespread travel and poor public health management, the outbreak might have owed some of its lethality to a mutation that scientists named GP-A82V. GP-A82V made the virus much more effective at infecting human cells. At the same time, virus strains with that mutation were less effective at infecting bats, its typical host population. The strain that evolved early in the 2013 epidemic was not only more likely to spread in the human population, but also much more likely to kill its victims. Scientists studying the mutation cautioned that another outbreak was probable.