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How many species are in those phyla? If you look, you find that nearly all those 16 phyla have 2–5 species. That ranks as a moderate sized genus.The phyla in the Cambrian is an artifact of the classification system. The biological reality is species. We then group similar/related species together in a genus. Similar/related genera are grouped in a Family. Similar/related Families are grouped in a Class, and similar/related Classes are grouped in a Phyla.When the K-Pg mass extinction occurred, about 7 species of birds from a single Order survived. The other Orders went extinct. What happened next was “adaptive radiation”, where the 7 species rapidly split into new species, taking advantage of empty ecological niches. Because the Class Aves (birds) already existed, the new lineages were classed as Orders — the next ranking “down”. The same thing happened to mammals. Most of the Orders of mammals evolved after the K-Pg extinction.So, what do we do in classification for the first adaptive radiation following a mass extinction? This is back in time when there are no phyla. Instead, we have the Kingdom Animalia (animals) that has been reduced to very few species. Those species split and form new lineages. What are those lineages going to be in the classification system? What is the next classification “down” from Kingdom? Phyla!Before the Cambrian there was a very diverse group of animals called the “Eidacaran”. An introduction to the Ediacaran fauna ; Origin of Animals and the Cambrian Explosion These do not resemble modern phyla. One example is Tribarchidium. Tribrachidium - Wikipedia It was a tri-radial symmetry animal (animals today are bi-laterally symmetrical). But look at the classification. It is a single species but is a phylum!There was an “End-Eidcaran” mass extinction. End-Ediacaran extinction - Wikipedia that happened before the Cambrian. “This implies that the "explosion" did not represent animals "replacing" the incumbent organisms, and pushing them gradually to extinction; rather, the data are more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna”So, there are a few surviving species that radiate to fill vacant ecological niches. Lots of new species. However, because there are no existing phyla to put these new species into, we end up with the same situation we see with Tribrachidium — new phyla with 2–5 related species, usually grouped into a single genus.Most of those genera (phyla) then go extinct. The surviving species then continue to radiate and diversify. Since they are in a “phyla”, the new species are put in the next level down — Class. As diversification continues, diversification and new species within each Class are put in a new Order. And so on. So now, because those first 2–5 species in the Cambrian survived and produced thousands of descendant species, living phyla have hundreds/ thousands of species.:) But not always. Welcome to Phylum Ginkgophyta: Phylum Ginkgophyta Just one species.The incredulity expressed in the question comes from forgetting that species is the only biological reality and that higher taxa, including phyla, are groupings of species. When there are no intermediate groupings, phyla can look impressive. But don’t feel bad, even Stephen Jay Gould made the mistake.The “appeared” part comes partly from the adaptive radiation, where new species evolved rapidly, but partly because the oxygen content of the atmosphere had risen to the point that hard body parts (like shells) evolved. This meant fossil preservation was greatly enhanced.Some still existing phyla do have soft-bodied fossils in pre-Cambrian strata:1. http://news.nationalgeographic.com/news/2001/07/0719_crustacean.html Crustacean pre-Cambrian but not hard-shelled.. Schopf, JW. Solution to Darwin's dilemma: Discovery of the missing Precambrian record of life. PNAS 97: 6947-6953, 2000. Solution to Darwin's dilemma: Discovery of the missing Precambrian record of lifeBottom line, there is a pre-Cambrian evolutionary history of life, and data showing that some Cambrian phyla were present before they “appeared” in the Cambrian. In particular, there are soft bodied members of Phyla Crustacea

Nitrogen fixation refers to the conversion of atmospheric nitrogen gas into a form usable by plants and other organisms. Nitrogen fixation is conducted by a variety of bacteria, both as free-living organisms and in symbiotic association with plants. Because it is the principal source of the nitrogen in the soil, nitrogen that plants need to grow, nitrogen fixation is one of the most important biochemical processes on Earth.Living organisms need nitrogen because it is a part of the amino acids that make up proteins , and the nucleic acids that make up DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nitrogen within living organisms is eventually decomposed and converted to atmospheric nitrogen. This form, however, is highly stable and unreactive chemically, and is therefore not available for use by most organisms. Some species of bacteria, though, can convert N2 into NH3(ammonia) or other usable forms of nitrogen. These nitrogen-fixing bacteria include species of the genera Rhizobium, Anabaena, Azotobacter, and Clostridium, as well as others.

The question doesn’t make much sense.A part of a defined genus will not colllectively split and form a new genus. All members of a genus are descended from a single species. That species no longer exists, but if it did instead of all its daugther species, and it were closely enough related to the species in another genus, it might not even have its own genus.Genera are just labels we use to group together related species. As we learn more about their relationships and history, we change and reshuffle them to better fit what we know.Now, there’s evidence of species evolving into different species. It’s a gradient. But onece you get back enough in time, it’s no longer useful to consider the species at the extremes to be in the same genus; so we put the modern and most modern-like ancestors in the same genus, and the least modern-like ancestors in their own genus. For example, Pierolapithecus turning into (probably) Sahelanthropus turning into Ardipithecus turning into Australopithecus turning into Homo. And that’s just one lineage; the modern genera Pongo, Pan and Gorilla, among others, are also ultimetely descendants of Pierolapithecus.If what you are looking for is a speciation event that turned the new species different enough to be granted its own new genus… well, yes, it has happened, but, that I am aware of, only in plants. That’s what happens when 2 species of different genera hybridize and the resulting hybrids are fertile and stable, meaning they can produce hybrids with the same overall genetic make-up. It’s happened naturally several times, but in the 19th century a notable one was developed by man: triticale, a hybrid between wheat and rye. These fertile hybrids, being descended from 2 different genera, need a new one. You argue, though, that their parent species should just be classified within the same genus if they can produce fertile hybrids. And this happens sometimes too: mollies used to be iin the genus Mollienesia while guppies used to be in the genus Lebistes. But after finding that they can successfully (albeit rarely) produce (sterile) hybrid offspring, both genera were merged together into Poecilia.So, genera, and other clades above are just labels we use to sort species and show their relatedness. Other than that, they are rather arbitrary: by the standards used for most organisms, the species in the Pan and Homo genera aren’t different enough to be in different genera… but… who’s going to mix us perfectly rational humans with savage filthy beasts like chimpanzees, huh? No, we need our own genus! we’re not animals at all!