It resulted in the divergence of most modern metazoan phyla
Fossils that have been found in great abundance date back to the Cambrian Period, which began about 545 million years ago. Trilobites first show up in the fossil record over 500 million years ago, and fossil lovers have collected trilobites for centuries. Naturalist Edward Lhwyd published a description of a "flatfish" in 1679. In the Victorian era, a well-polished trilobite was mounted in a gold pin known as the Dudley Locust Brooch, now on display at the Natural History Museum, London. Trilobite fossils vary so much in size that some are best measured in millimeters while others are better measured in feet. Some sport spines, others have eyes on the ends of stalks. Yet these ancient water bugs remain recognizable as trilobites thanks to their shared body plan: horizontal segments spanning three lengthwise sections. No matter how big and fancy some of them got, they shared the same ancestral shape.
Discredited hypotheses for the Cambrian explosion - …
Despite the abundance of trilobite fossils, geologists couldn't guess at the true diversity of Cambrian life for decades after the Cambrian Period was first named. One of the richest sites for the period is the Burgess Shale in the Rocky Mountains of Canada's British Columbia. Other geologists had excavated there before him, but it was Smithsonian boss who made the site famous in the early 20th century. Episodes of extremely rapid fossil burial about a half a billion years ago preserved soft tissues, giving paleontologists unprecedented amounts of information about animal life from the Cambrian. Thousands of fossils have been excavated from the Walcott Quarry since 1909. Fossil sites with similar preservation and diversity have been found nearby (the Marble Canyon site about 40 kilometers away) and as far away as China. So many new body plans showed up in the fossil record that the period gained the nickname "Cambrian explosion." The term is somewhat controversial, and many paleontologists favor "diversification" or "radiation."
Fossils from the Cambrian Period are among the first to show evidence of animal traits that persist today: heads, eyes, gills and legs. And mouths. The advent of mouth-bearing organisms forever changed the makeup of marine life. Before the Cambrian Period, cyanobacteria (water-dwelling bacteria that use photosynthesis to make their own food) were so abundant and unmolested that they formed layer upon layer of slimy, bacteria-sized high rises. Animals with mouths found them very appealing. Stromatolites were widespread before the Cambrian Period. They have been rare ever since.
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And then there are genes that manage the big picture in a variety of organisms. First found in fruit flies, Hox genes regulate overall body plans for everything from bugs to birds. Although the genes differ between vertebrates and arthropods, they show remarkable similarities. They often occur together in a comprehensible order, in contrast with most other genes, and their order matters. Arguably grisly experiments with these genes show that moving them around creates fairly disgusting mutants, like flies with legs sprouting from their heads. Likewise, grafting mouse mouth tissue into a developing chick embryo demonstrates how Hox genes work across different animals; the resulting chicks hatch with teeth, though the teeth look dinosaurian.
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Thesudden appearance of the major animal phyla in the fossil record during theCambrian period is called Cambrian explosion (the mother of allmacroevolutionary events).
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Paleoanthropologists estimate that australopithecines evolved into early forms of the genus sometime between 3 million and 2.5 million years ago, but verifying this hypothesis has been complicated by a dearth of early fossils. A 2.3-million-year-old maxilla (upper jaw) long served as the oldest example of the genus, and the type specimen for , or "handy man," is younger still. A computerized reconstruction of the mandible (lower jaw) from the type specimen showed australopithecine characteristics along with traits of our own genus. And comparisons of the fossils from this period suggests "an evolutionary explosion at the dawn of our genus," according to .
General Rebuttal to the Theory of Evolution
[New] data belie the common idea that animal species can't hybridize or, if they do, will produce inferior or infertile offspring — think mules. Such reproductive isolation is part of the classic definition of a species. But many animals, it is now clear, violate that rule: Not only do they mate with related species, but hybrid descendants are fertile enough to contribute DNA back to a parental species — a process called introgression. . . . Biologists long ago accepted that microbes can swap DNA, and they are now coming to terms with rampant gene flow among more complex creatures. "A large percent of the genome is free to move around," notes Chris Jiggins, an evolutionary biologist at the University of Cambridge in the United Kingdom. This "really challenges our concept of what a species is."