Originally posted by PinkFloyd
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But I'll answer it anyway. I'm probably one of the most qualified people here to answer it.
Okay, so our story starts out around, ooo, shall we start 2 billion years ago (bya). Okay, so we start with oxygenic bacteria in the oceans. There was, up until this point pretty much no O2 in the atmosphere - all O2 formed by bacteria was used up oxidising iron. We can look at rocks of that age, and we see a lot of iron, yet no signs of free oxygen in the environment. But, about 2 billion years ago, the balance tipped, and the O2 concentration in the atmosphere started to creep up. Around 1 - 1.5 billion years ago the first eukaryotic cell formed. Now, this won't mean much to most people, but eukaryotic cells are what all "higher" organisms are made of, as opposed to bacteria. Eukaryotes (meaning true nucleus, or literally true nut, if you want to get technical) has their DNA enclosed in a nucleus, where as bacteria do not. Eukaryotes also contain organelles, such as mitochondria, chloroplasts (in plants and algae) and the nucleus itself may qualify (**Technical note** it is double membraned, which suggests it was formed by the incorporation of one bacteria into another, as per Lynne Marguilis' Endosymbiotic theory. Others, such as ribosomes or vacuoles, or the Golgi complex almost certainly do not fall into the category, since most are only single membrane invaginations of the plasma membrane). The atmosphere would be about 1% O2 by this point. This is important, since O2 has to diffuse into the cells, and with a low external O2 concentration this would be slow. As the external O2 concentration increases, so too can cell size. Now, you may be wondering why a photosynthetic cell would need external O2, and the obvious answer is that they wouldn't. However, the cells in question are not photosynthetic, they are effectively herbivores, feeding off the other cells in the environment. As the O2 concentration went up, they would increase in size, whilst the ancestral
Mitochondrion bacteria would remain small, since it requires a large surface area to volume ratio to absorb CO2 from the water (CO2 diffuses in water at only at about 1 / 10,000 the rate it does in air). Predatory cells get bigger, prey stays the same size. For some reason, probably to do with organelles having useful functions, such as giving off oxygen or being particularly good at releasing energy using O2 (chloroplasts and mitochondria, respectively), some of the cells retained their organelles. Of course, there was no "decision" to do so, yet those which were less voracious at digesting their prey were rewarded, and were more successfull, outcompeting their more voracious competitors.
O2 levels continued to rise. Around 800 million years ago, they reached about 2% (remember, CO2 is about 30% of the earth's atmosphere at that time). The next stage of evolution was upon us - cells started to clump together. This can be seen today in, for example, sponges. Although sponges are anatomically simple, they have several advantages over single cells in the medium. For example, living together offers protection from predators. Likewise, in modern sponges, they can channel and move water through them, increasing the efficiency of food acquisition.
Sponges, are collonies of cells, often groups of genetically identical or similar cells, derived from the same ancestor (s). The next stage is to turn that into a geneticaly individual organism, and we see that in the polyps and cnidaria. These are full individuals, with some hydrozoa pretty well know, as jellyfish! These have generations, yet reproduce asexually (generally, although some reproduce sexually - but I want to tackle sexual reproduction later). Cnidarian anatomy is starting to become more conventionally "animal-like" (they are animals though). They have a mouth at one end, and a primitive stomach. There is little differentiation of tissues, except for specialised nematocysts, which are very cool, and demand googling in their own right.
Okay, next we have the Cambrian explosion. This is a period of 10 - 50 million years were all the major phyla of animals came into being. It happenned around the time that we got to around 4% oxygen in the atmosphere, and calcium could complex with carbonate to give shells, which suggests the pH wasn't too low. Many types of animals evolved then, one of which, the chordata, which would evolve from jawless to jawed fishes, would eventually give rise to amphibians, reptiles, mammals and ultimately us. However, I'm getting ahead of myself.
Chordates are essentially comprised in a hollow tube, like we are, with a mouth at one end and an anus at the other. Primitive chordates were little more than that. However, they did possess a notochord, a partially flexible rod, running along the back. We are chordates, and have an embryonic (very small) notochord, which we get rid of in the womb. For chordates locomotion was a good idea, it allowed them to be on the move, which helped them to avoid predators, and also helped them in food collecting. Chordates have bilateral symmetry (you can cut us in half with (near enough) mirror image left and right sides), which allowed the development of strongg swim muscles - yummy sashimi nowadays. Jaws would come in handy, as would teeth, and those evolved over time, not in response to any will, but simply because those individuals which had structures which could be used as a jaw or teeth, no matter how inefficiently, had an advantage over those which didn't.
Okay, so we got to fish. But now, it's after 1am, and I'm going to sleep. I'll pick up the rest tomorrow. Today.
24 May '08 19:21
Accepting Science, evolution and believing in g...
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