Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!decwrl!pyramid!hplabs!tektronix!orca!tekecs!mikes From: mikes@tekecs.UUCP Newsgroups: net.bio Subject: Re: the Goal of evolution Message-ID: <7265@tekecs.UUCP> Date: Wed, 14-May-86 19:51:31 EDT Article-I.D.: tekecs.7265 Posted: Wed May 14 19:51:31 1986 Date-Received: Sat, 17-May-86 02:27:13 EDT References: <487@bcsaic.UUCP> <1002@cybvax0.UUCP> <32@sphinx.UChicago.UUCP> <1469@ecsvax.UUCP> <312@parcvax.Xerox.COM> Organization: Tektronix, Wilsonville OR Lines: 71 > What I'd like to know is: although the expansion of the brain (of our > ancestors) into the current size (of humans) is apparently the most > phenomenal (in terms of a relatively small number of generations) > change known, the change from generation to generation was no more > than a fingernail clipping in volume. How could that have made a > difference in terms of selection? This is one of those, we'll never > really know questions, I guess. The only thing I can think of is > changes in structure as well as volume, or something. > > - rene Partly structure versus simple volume, true. Also probably true that we'll never get beyond the stage of armchair (or computerchair :-) arguments of the evolutionary theory involved. Still, a possible scenario: The change in brain size was not smoothly distributed over the generations (at the risk of bringing up "punctuated evolution" arguments again), but occurred in leaps and bounds. Early primate brains evolved highly developed sensory-somatic portions of the cortex, the better to swing in trees with (as Carl Sagan said, each brachiation from tree to tree was a chance for evolution). Since no other animals occupied the niche of survival through intelligence (on the scale of the early primates), there was little direct competition and they flourished. When the forests drew back in the late Tertiary Period (I believe), the primates were forced out onto the now- familiar savannahs. There their developed sense of balance came in handy, as did many of their other evolved traits. (As a side note that I have never seen mentioned elsewhere, could it be that our facility for speech was enhanced by the evolution of a highly motile tongue in response to changing and diverse dietary needs?) Here again, the capacity to survive by being smarter than the wildebeeste increased the possibility of bearing also-smart children, who then had an enhanced chance of survival by being smart...etc. One hypothesis for the mechanism of this is that of "neural spread." That is, an animal with one portion of the brain that is highly developed will tend to evolve other portions of the brain that are increasingly developed. This would explain not only how we got our current brain size and complexity from those as found in tree shrews, but also how we got that far from the ancestor of the shark, which had a brain the size of bb. A long time ago, having multiple cells increased the chance and efficacy of survival, which led to differentiated cells that increased the chance and efficacy of survival, which eventually led to having more and better cells of a particular type being better for survival. One branch of animals evolved along the lines of having more neural cells, which became a self-feeding cycle. The cranium had to expand to hold these cells (those who were born with the increased brain size but without the larger cranium did not survive to breed), until it was limited by the size of the pelvis and birth canal. To continue increasing, the strategy of folding the surface up was exploited, eventually getting us where we are today: highly neotonus, with neural growth continuing well into childhood (c. 8 years) and neural change occuring perhaps even after that. Are our brains continuing to increase in size and complexity over evolutionary time periods? Who knows. Maybe we've disarmed most of the evolutionary mechanisms by having more assured breeding for those who want it, and maybe we're just expressing some new sort of evolutionary mechanism. Anyway, there's a lot I've left out here, for the sake of time and space (!). One other note: we do not, contrary to popular belief "not use 75% (or 90% or whatever) of our brains." This belief stems (erroneously) from studies that show much of our brain as "uncommitted" early in life, and from early memory studies done by Karl Lashley. Crud, the firmware guys are here...more later. -- Mike Sellers UUCP: {...your spinal column here...}!tektronix!tekecs!mikes "If anyone would accurately gauge his knowledge and learn how little he knows, let him try to teach something...the limitations of his knowledge will press him closely -- all the more closely, perhaps, the more he may have learned, unless he be content to pour old ignorance from vase to vase and call it learning." -- Scientific American, May 1886