Releasing oxygen into the atmosphereA number of billions of years ago there was no oxygen in the atmosphere. In fact oxygen was a poison to everything that lived. At some point in this distant past, a bacteria managed to absorb a smaller cell that used chlorophyll to photosynthesise. This became a symbiosis relationship where the bacteria protected the little cell, while the little cell fed the bacteria.
This successful harmony resulted in a large number of photosynthesising cells that take water (H2O - dihydrogen oxide) and used sunlight as the power source to extract the hydrogen for food and release the oxygen. The released oxygen found it's way into the atmosphere. This is the exact chemical reaction that continues today in all plants and algae.
Ok, for those on Study Island the answer is the free oxygen bubbled up from the oceans.
U.S. public schools have always made an effort to stay current with modern science. Since Darwin published his "Origin of the Species" in England in 1859, it can be assumed that public schools in the U.S. began teaching the principles of evolution soon afterward--perhaps within a decade or so. For example, the Nashville "Christian Advocate" (a Tennessee newspaper) was decrying the theory as a threat to faith as early as 1880. Evolutionary sciences met widespread resistance in the fundamentalist segments of the U.S., of course, leading to the infamous Scopes trial in Tennesee of 1925. It should be noted, however, that even though the fundamentalist cause was largely discredited during this event, evolution per se was more avoided than taught in public-school science classes (for fear of arousing the controversy again) until about the 1960s. It varies state by state. The word evolution is banned in some states and the words "change over time" must be used. Dover, Pennsylvania is having to deal with criticisms chicken butt
When DNA coils into a chromosome, it keeps its shape with the help of associated proteins. Where the DNA is very tightly coiled, and the proteins very condensed, that region appears as a dark band (G-band). G-bands are not very rich in genes, which means that it's ok that the DNA is tightly coiled, because it does not have to be very accessible anyway. Note that these bands are only visible when certain types of dye are used. Other types of dye will case gene-rich regions to stain darker and gene-poor regions to stain more brightly. See also: Chromosomes: Organization and Function By Adrian Thomas Sumner, 10.2.2
Your body uses the least amount of liquid available to flush the toxins and bacteria from your body. This means that your body is trying to conserve the little water in your body already and has to flush out just the same amount of "bad" stuff using a little amount of water. The little amount of water becomes very concentrated with the toxins and bacteria because it can not disperse the toxins over large amounts of water.
Petal shaped areas that work like gills.