1) The first living organisms on the 1 pe earth are known as O Protozoa O Multi-cellular Animals
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Evolution and paleontology
Protists were a dominant form of life on Earth 1.5 billion years ago. While protozoans evolved early and have survived to the present day as unicellular organisms, they have undoubtedly undergone considerable evolutionary change. That many species must have become extinct as others appeared can be deduced from the limited fossil record of protozoans. Extinct fossil foraminiferan species, for example, number around 34,000, while there are only about 4,000 described living species.
Only a small number of protozoans, most of which are testate amoebae, have left fossil remains. The calcareous shells of the foraminiferans and calcium-secreting coccolithophores (a group of algae), for example, produced substantial geologic strata in the chalk formed during the Cretaceous Period (145.5 million to 65.5 million years ago) and the well-developed foram-limestones of the Paleozoic Era (542 million to 251 million years ago), Early Cretaceous Epoch (145.5 million to 99.6 million years ago), and Cenozoic Era (65.5 million years ago to the present). The fossil-forming radiolarians date to late Precambrian times, and the testate lobose amoeba Melanocyrillium dates to the late Precambrian geologic record of the Grand Canyon in northwestern Arizona, U.S. The testate amoeba Nebela is found in deposits from the Cretaceous Period.
The most abundant and important fossil protozoans are the foraminiferans. This entirely marine group is extremely important as stratigraphic markers in oil exploration. Because species have appeared and then become extinct frequently during geologic history and because they have fairly wide geographic distribution, particularly planktonic species, their value is in showing distinct phases in geologic history and, with specific species, in typifying particular beds of rock or strata. Foraminiferans are also important in the reconstruction of paleoceanographic circulation patterns.
The poor fossil record of protozoans has hampered attempts at unraveling the complexities of their evolution. Modern biochemical and electron microscopy techniques, however, are providing evidence for new affinities between groups and are elucidating possible evolutionary pathways. Comparisons of flagellar structures, mitochondria, and nuclear and plastid characteristics in conjunction with ribosomal RNA (ribonucleic acid) sequences are revealing the relationships of various taxa.
The ancestral eukaryote organism is thought to have been an amoeboid creature that relied on anaerobic or microaerophilic metabolism (microaerophilic organisms survive on only very small amounts of oxygen). The evolution of mitochondria (the centres of aerobic respiration in the cell) as organelles from endosymbiotic bacteria and the establishment of oxidative pathways allowed a more efficient cellular energy balance, which led the way to the evolution of an enormously diverse array of eukaryotic organisms. Some of the early amoeboid eukaryotes developed flagella to enhance their food-gathering abilities and to provide a more efficient mode of propulsion. The flagellates gradually evolved different ways of life, and their structures became modified accordingly. As phagotrophs that ingested bacteria for food, they in some cases came to establish symbiotic associations with photosynthetic species, and ultimately the endosymbionts became plastids within the cell. Some of the flagellates came to depend entirely on photosynthesis and to abandon heterotrophy completely, though many still retain both heterotrophic and autotrophic nutrition as mixotrophs. (Some present-day mixotrophs, however, may be only secondarily mixotrophic, having reestablished heterotrophy in conjunction with photosynthesis.)
A considerable number of protozoans became parasitic, a mode of life that evolved independently among the protozoans many times. Ciliates and amoebae became symbionts in the intestinal tracts of both vertebrates and invertebrates as a result of surviving the digestive enzymes of the predator. (Most present-day parasites among these protists are intestinal parasites.) Once inside the intestine of the host, they multiplied and gradually, through mutation and selection, came to rely on the resistant cyst as a means of survival and dispersal, losing the ability to survive in a free-living feeding form.
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