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brad89 wrote
Well, I guess here is my best place to ask. How exactly does a species grow something as complex as the brain? I have always wondered this. I have looked in books, but they say that 'it did', rather than 'here's how'. I looked for online examples, and Ebon Musings says, talking about the brain and the soul, that the "fires of evolution have been working for 4 million years to cook up the brain the way it is," but if life only follows chemical reactions, random ones at that, how does such a thing arise?
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While my input is not so technically elegant as that posted previously, a brain is a lot easier to explain than a wing, I think. It helped me get a grip on the possible sequence of events in the origin (genesis?) of brains by looking at the taxonomic hierarchy as a model of the evolutionary sequence. You can easily see a logical evolutionary progression from life forms such as bacteria (Monera kingdom) to true single celled organisms (Protista kingdom), to multicelled animals (Animalia kingdom). In animals, the cells differentiated to muscles and neurons. Among the simplest of the 15 animal phyla are those with a simple nerve net, like sea anemones and jelly fish. The nerve net (no brain and no spinal cord, just a reticulum of neurons) somewhat resembles the peripheral nervous system of humans, in that it consists of neurons that respond to environmental stimuli, but it also connects directly to other neurons that move muscles. Simple in-out messaging.
The next higher phyla include animals with more complex systems, including a specialized head, body and tail. The body in these phyla consists of a bilaterally symmetrical segmented nerve trunk. The segments are similarly organized body parts, each segment containing both a sensory input and a motor output. The human nervous system is also bilaterally symmetrical as well as segmentally organized (the dermatomes resemble flatworm trunk segments).
http://a248.e.akamai.net/7/248/430/2...res/f165_2.gif(Note that sacral segment S2 serves both the anus and the genitals... oh wait, that was another thread...)
http://www.student.loretto.org/zoolo...oussystems.gif (this one's the flatworm)
The next most complex set of phyla up the list, including clams, snails, squids (except for the far more highly evolved Solidsquid, of course), insects, and octopuses, have ganglia. Ganglia are small clusters of similarly functioning nerve cells that act like simple little brains. Humans have ‘em, clusters of them line the outside of the spinal column, they’re called the dorsal root ganglia, and they serve as sensory processing areas whose activation initiates spinal motor reflexes as well as sending sensory information to higher brain areas. The larger and more complex the ganglion, the more brain-like it becomes. For example, some insects have ganglia in their heads that are specialized and large enough to merit being called brains. (Unlike some theists, I'm afraid)
Adding still more complexity to the part of nervous system contained in the head is called encephalization. Chordates are animals that have both a brain and a spinal cord. The word chordate comes from the term “notochord” which is the embryonic precursor to the spinal cord. Here’s a kicker for the fundies: both the nervous systems of prechordates like drosophila melanogaster (fruit fly) and chordates like humans are formed under the direction of groups of genes called homeobox gene clusters. These gene clusters specify the organization of the nervous system of both insects and chordates, and so indicate a common ancestor in the lineages. Yep, Schrack’s cousin is a fruit fly and Carico has naught but a ganglion in her head.
In humans, various brain structures (often referred to as ganglia, by the old anatomical nomenclature, as in “basal ganglia”), are comprised of cells with similar cytoarchitecture (structure) and similar functions. There is no “sensible” design to the brain, instead there is both parallel and hierarchical organization, with some structures inhibiting the functions of other structures, and the majority of the brain is composed of interneurons, whose primary function is to integrate incoming sensory input and initiate outgoing motor signals.
It seems that wherever in evolution there were 2 cells to rub together, there was room for specialization of function, which conferred definite selective advantage to those 2 cooperating units of life. The more units that cooperated, the more specialized the functions could become, and the greater the potential for selective advantage, so on and so forth until viola! There are naked apes with big brains. And the xians say we got morality from jeebus! I say it’s in the genes, and it goes as far back as Monera.
I hope you found this helpful... I paraphrased liberally from Kolb & Whishaw's "Introduction to Brain and Behavior" physiological psychology text if you want to read more on the topic.