Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site uwmacc.UUCP Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!decvax!mcnc!philabs!cmcl2!seismo!uwvax!uwmacc!dubois From: dubois@uwmacc.UUCP (Paul DuBois) Newsgroups: net.origins Subject: Re: Beast of Burden (Simpson on Horses) Message-ID: <1025@uwmacc.UUCP> Date: Fri, 3-May-85 14:11:17 EDT Article-I.D.: uwmacc.1025 Posted: Fri May 3 14:11:17 1985 Date-Received: Mon, 6-May-85 01:07:29 EDT References: <948@uwmacc.UUCP> <491@cybvax0.UUCP> <977@uwmacc.UUCP> <510@cybvax0.UUCP> Distribution: net Organization: UW-Madison Primate Center Lines: 129 >>>> [Paul DuBois] >>>> I didn't forget. But just how likely does it seem to you that two >>>> independent lines of development from a smooth-brained ancestor with no >>>> convolutions, to two different animals with convolutions, will result >>>> in EXACTLY the same number of fissures, and that all 14 will be in >>>> corresponding locations? I confess skepticism (that healthy quality so >>>> highly touted). >>> [Mike Huybensz] >>> The neocortex of all mammals is convoluted. Which smooth-brained ancestor >>> are you talking about? And what makes you think it was smooth brained? >> [Paul DuBois] >> A reasonable question. The information comes from a study of >> endocranial casts obtained from fossil skulls. Early horses were >> smooth-brained. More information may be obtained from: >> Tilly Edinger, "Evolution of the Horse Brain". >> Geol Soc Amer Memoir, 25, 16 Feb 1948. > [Mike Huybensz] > I don't think endocranial casts will show convolutions and fissures of the > brain. Look at the inside of a human skull: it is smooth on top. > I may get around to checking this one out.... You must be awfully confident about it to make such a statement before seeing what you can find out. Here's some information in which you might be interested. G G Simpson, _Horses_, Natural History Library, Garden City NJ, 1961, pp235ff. "The brain itself, soft tissue that decays very rapidly, is not preserved in any fossil horse and study is really based on the cavity formerly occupied by the brain. This reflects most of the important features of the brain, although minor details may be obscure or lacking. Sometimes the cavity became filled with mud after the brain had decayed and as the skull was buried. Hardened into rock, this filling may provide an excellent natural cast of the brain cavity. In other cases the cavity can be cleaned out in the laboratory and a rubber or gelatin cast made which can be reproduced in plaster for study and permanent preservation. "The most remarkable feature of the story of the horse brain, thus established, is at the beginning. In spite of its antiquitiy and many distinctive peculiarities, eohippus is often thoght of as simply a little horse. Most people would have guessed that it had a horse brain, smaller and simpler than in _Equus_ probably, but still distinctive of the brainy horse family. This is not the case. Eohippus turns out to have one of the most primitive brains known in any mammal. There is nothing horse-like about its brain, which more closely resembles the brain of a reptile or of the most primitive mmmals, such as the oppossum, notorious for stupidity. "A primitive mammal brain, seen from the side, seems to consist mainly of three rather lumpy and irregular parts stretched out in a line one behind the other. In front is the smallest and simplest part, the paired olfactory bulbs, concerned with nerves coming from the nose and with the sense of smell. Next come the likewise paired cerebral hemispheres, which are very complex in structure and function even in the most primitive animals, but in which the lower parts are mainly concerned with sensory and more or less instinctive functions while the outer layers (cortex) of the upper parts are more involved in association and functions related to intelligence. The most posterior main segment of the brain is the cerebellum, which has a median, central part (vermis), and a complex of paired swellings on each side (mainly the cerebellar hemispheres of paramedian lobes). This segment too is extremely complex in structure and function, but it is little concerned, directly, with conscious or intelligent behavior and has to do mainly with motor and motor-sensory coordination of the various parts of the body. "The most important feature of brain evolution among mammals is the development of the part especially concerned in intelligence, the outer layers of the upper part of the cerebrum, called the neo-cortex. Expansion of the neocortex is clearly required for the evolution of intelligent behavior and is correlated with higher types of mental functioning. It reaches an extreme in man, and is also, but less, well developed in the more intelligent subhumans, especially apes and also dogs, horse, and other fairly intelligent mammals. This expansion takes place in two ways, by absolute or relative increase in the size of the cerebrum, and by wrinkling of its upper surface. Development of numerous fissures on this region increases the surface area, which is the essential point, even without any increase in the volume of the cerebrum. "In eohippus, neocortex expansion either in size or by fissuring had hardly begun, and this is the most striking and primitive feature of the brain of that animal. The cerebrum was already the largest part of the brain, as it is in reptiles and many lower animals, but it was much smaller, relative, especially, to the olfactory bulbs, than in later horses or other mentally progressive mammals. The upper part, the neocortex, was small, only slightly swollen, no higher and barely wider than the cerebellum. It was almost smooth. Only three small fissures or sulci cn be seen. In eohippus the cerebellum also was much simpler and less fissured than in later horses, or most other mammals. Another very primtive feature that there is a gap between cerebellum and cerebrum and an intermediate part of the brain (midbrain) could still be seen here when the brain was viewed from above. "Eohippus must have been an extraordinarily stupid animal. Edinger remarks: 'The brain connects eohippus only with the past. The ancestral brain, an unspecialized mammalian brain, was held over in the equid body. No feature of this brain signals the future.'" --- I trust you no longer think I am making wild statements. The comment about the distinction between the brain of eohippus from its descendants is noteworthy, particularly as the similarity between eohippus and (currently extant) hyrax has been pointed out by other authors. Perhaps eohippus was not a horse at all. I think that it would be of interest to know if the hyrax has a convoluted brain. This would give us a better basis to evaluate the relationship. If the hyrax does have a convoluted brain, then we would have some reason to dissociate it from eohippus. Note that I am presenting a testable hypothesis here. Very scientific and all. -- | Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- | "The presence of weeds in the garden is not explained by | saying that the gardener has not pulled them yet."